go language book

8/22/2019 go Language book

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An Introduction to Programming in Go

Copyright 2012 by Caleb Doxsey

All rights reserved !o part o" this boo# may be

reproduced or transmitted in any "orm or by any

means$ electronic or mechanical$ including

photocopying$ recording$ or by any in"ormation storage

and retrieval system %ithout the %ritten permission o"

the author$ except %here permitted by la%

I&'!( )*+,1-*+.//+2.

Cover art by Abigail Doxsey Anderson

Portions o" this text are reproduced "rom %or# created

and shared by Google and used according to terms

described in the Creative Commons .0 Attribution

icense


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Contents

1 Getting Started 1

11 iles and olders 2

12 he erminal /

1. ext 3ditors )

1- Go ools 1.

2 Your First Program 15

21 4o% to 5ead a Go Program 1*

3 Types 23

.1 !umbers 2-

.2 &trings 2)

.. 'ooleans .1

4 Variables 35

-1 4o% to !ame a 6ariable .)-2 &cope -0

-. Constants -.

-- De"ining 7ultiple 6ariables --

-/ An 3xample Program -/

5 Control Structures 4

/1 or -+

/2 I" /1

/. &%itch //

! "rrays# Slices and $aps 5%

81 Arrays /+

82 &lices 8-

8. 7aps 8*

Functions !

*1 9our &econd unction **

*2 5eturning 7ultiple 6alues +2

*. 6ariadic unctions +2


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*- Closure +-

*/ 5ecursion +8

*8 De"er$ Panic : 5ecover ++

% Pointers &2

+1 he ; and : operators ).

+2 ne% )-

& Structs and 'nter(aces &

)1 &tructs )+

)2 7ethods 101

). Inter"aces 10-

1) Concurrency 1)%

101 Goroutines 10+

102 Channels 111

11 Pac*ages 12)

111 Creating Pac#ages 121

112 Documentation 12-

12 Testing 12

13 T+e Core Pac*ages 132

1.1 &trings 1.2

1.2 Input < =utput 1.-

1.. iles : olders 1./

1.- 3rrors 1-0

1./ Containers : &ort 1-1

1.8 4ashes : Cryptography 1--

1.* &ervers 1-*

1.+ Parsing Command ine Arguments 1//

1.) &ynchroni>ation Primitives 1/8

14 ,e-t Steps 15&

1-1 &tudy the 7asters 1/)

1-2 7a#e &omething 1801-. eam ?p 181


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1 Getting Started

Computer programming is the art$ cra"t and science o"

%riting programs %hich de"ine ho% computers operatehis boo# %ill teach you ho% to %rite computer pro,

grams using a programming language designed by

Google named Go

Go is a general purpose programming language %ith

advanced "eatures and a clean syntax 'ecause o" its

%ide availability on a variety o" plat"orms$ its robust

%ell,documented common library$ and its "ocus on

good so"t%are engineering principles$ Go is an ideal

language to learn as your "irst programming language

he process %e use to %rite so"t%are using Go @and

most programming languages is "airly straight"or,

%ard(

1 Gather reBuirements

2 ind a solution

. rite source code to implement the solution

1


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Getting Started 2

- Compile the source code into an executable

/ 5un and test the program to ma#e sure it %or#s

his process is iterative @meaning its done many

times and the steps usually overlap 'ut be"ore %e

%rite our "irst program in Go there are a "e% prereBui,

site concepts %e need to understand

1.1 Files and Folders

A "ile is a collection o" data stored as a unit %ith a

name 7odern operating systems @li#e indo%s or

7ac =& contain millions o" "iles %hich store a large

variety o" di""erent types o" in"ormation E everything

"rom text documents to executable programs to multi,

media "iles

All "iles are stored in the same %ay on a computer(

they all have a name$ a de"inite si>e @measured in

bytes and an associated type ypically the "ileFs typeis signi"ied by the "ileFs extension E the part o" the "ile

name that comes a"ter the last . or example a "ile

%ith the name hello.txthas the extension txt%hich

is used to represent textual data

olders @also called directories are used to group "ilestogether hey can also contain other "olders =n in,


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3 Getting Started

do%s "ile and "older paths @locations are represented

%ith the \ @bac#slash character$ "or example(C:\Users\john\example.txt example.txt is the "ile

name$ it is contained in the "older john$ %hich is itsel"

contained in the "older Users%hich is stored on drive C

@%hich represents the primary physical hard drive in

indo%s =n =& @and most other operating sys,

tems "ile and "older paths are represented %ith the /@"or%ard slash character$ "or example(

/Users/john/example.txt i#e on indo%s

example.txt is the "ile name$ it is contained in the

"older john$ %hich is in the "older Users ?nli#e in,

do%s$ =& does not speci"y a drive letter %here the "ile

is stored


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Getting Started 4

/indo0s

=n indo%s "iles and "olders can be bro%sed using

indo%s 3xplorer @accessible by double,clic#ing 7y

ComputerH or typing %ine(


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5 Getting Started

S

=n =& "iles and "olders can be bro%sed using inder

@accessible by clic#ing the inder icon E the "ace icon

in the lo%er le"t bar(

1.2 T+e Terminal

7ost o" the interactions %e have %ith computers today

are through sophisticated graphical user inter"aces

@G?Is e use #eyboards$ mice and touchscreens to

interact %ith visual buttons or other types o" controls

that are displayed on a screen

It %asnFt al%ays this %ay 'e"ore the G?I %e had the

terminal E a simpler textual inter"ace to the computer


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Getting Started 6

%here rather than manipulating buttons on a screen

%e issued commands and received replies e had aconversation %ith the computer

And although it might appear that most o" the comput,

ing %orld has le"t behind the terminal as a relic o" the

past$ the truth is that the terminal is still the "unda,

mental user inter"ace used by most programming lan,

guages on most computers he Go programming lan,

guage is no di""erent$ and so be"ore %e %rite a program

in Go %e need to have a rudimentary understanding o"

ho% a terminal %or#s

/indo0s

In indo%s the terminal @also #no%n as the command

line can be brought up by typing the %indo%s #ey r

@hold do%n the %indo%s #ey then press r$ typing

cmd.exeand hitting enter 9ou should see a blac# %in,

do% appear that loo#s li#e this(


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7 Getting Started

'y de"ault the command line starts in your home di,

rectory @In my case this is C:\Users\caleb 9ou issue

commands by typing them in and hitting enter ry

entering the command dir$ %hich lists the contents o"

a directory 9ou should see something li#e this(

C:\Users\caleb>dirVolume in drive C has no label.Volume Serial Number is B!"#!$"

ollo%ed by a list o" the "iles and "olders contained in

your home directory 9ou can change directories by us,ing the command cd or example you probably have a

"older called %es&top 9ou can see its contents by en,

tering cd %es&topand then entering dir o go bac# to

your home directory you can use the special directory

name ..@t%o periods next to each other( cd .. A sin,

gle period represents the current "older @#no%n as the%or#ing "older$ so cd .doesnFt do anything here are


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Getting Started 8

a lot more commands you can use$ but this should be

enough to get you started

S

In =& the terminal can be reached by going to inder

J Applications J ?tilities J erminal 9ou should

see a %indo% li#e this(

'y de"ault the terminal starts in your home directory

@In my case this is /Users/caleb 9ou issue commands

by typing them in and hitting enter ry entering the

command ls$ %hich lists the contents o" a directory

9ou should see something li#e this(


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9 Getting Started

caleb#min:' caleb( ls

%es&top %o)nloads *ovies +ictures%ocuments ,ibrar- *usic +ublic

hese are the "iles and "olders contained in your home

directory @in this case there are no "iles 9ou can

change directories using the cdcommand or example

you probably have a "older called %es&top 9ou can seeits contents by entering cd %es&topand then entering

ls o go bac# to your home directory you can use the

special directory name .. @t%o periods next to each

other( cd .. A single period represents the current

"older @#no%n as the %or#ing "older$ so cd .doesnFt do

anything here are a lot more commands you can use$

but this should be enough to get you started

1.3 Te-t ditors

he primary tool programmers use to %rite so"t%are is

a text editor ext editors are similar to %ord process,

ing programs @7icroso"t ord$ =pen =""ice$ K but un,

li#e such programs they donFt do any "ormatting$ @!o

bold$ italic$ K instead they operate only on plain text

'oth =& and indo%s come %ith text editors but

they are highly limited and I recommend installing a

better one

o ma#e the installation o" this so"t%are easier an in,


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Getting Started 10

staller is available at the boo#Fs %ebsite(

http://))).olan#boo&.com/ his installer %ill in,stall the Go tool suite$ setup environmental variables

and install a text editor

/indo0s

or %indo%s the installer %ill install the &cite text ed,

itor 9ou can open it by going to &tart J All Programs

J Go J &cite 9ou should see something li#e this(

he text editor contains a large %hite text area %here

text can be entered o the le"t o" this text area you

can see the line numbers At the bottom o" the %indo%


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11 Getting Started

is a status bar %hich displays in"ormation about the

"ile and your current location in it @right no% it saysthat %e are on line 1$ column 1$ text is being inserted

normally$ and %e are using %indo%s,style ne%lines

9ou can open "iles by going to ile J =pen and bro%s,

ing to your desired "ile iles can be saved by going to

ile J &ave or ile J &ave As

As you %or# in a text editor it is use"ul to learn #ey,

board shortcuts he menus list the shortcuts to their

right 4ere are a "e% o" the most common(

Ctrl & E save the current "ile

Ctrl E cut the currently selected text @remove it

and put it in your clipboard so it can be pasted later

Ctrl C E copy the currently selected text

Ctrl 6 E paste the text currently in the clipboard

?se the arro% #eys to navigate$ 4ome to go to the

beginning o" the line and 3nd to go to the end o" the

line

4old do%n shi"t %hile using the arro% #eys @or

4ome and 3nd to select text %ithout using the

mouse

Ctrl E brings up a "ind in "ile dialog that you can

use to search the contents o" a "ile


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Getting Started 12

S

or =& the installer installs the ext rangler text

editor(

i#e &cite on indo%s ext rangler contains a large

%hite area %here text is entered iles can be opened

by going to ile J =pen iles can be saved by going to

ile J &ave or ile J &ave As 4ere are some use"ul

#eyboard shortcuts( @Command is the #ey


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13 Getting Started

Command & E save the current "ile

Command E cut the currently selected text @re,move it and put it in your clipboard so it can be

pasted later

Command C E copy the currently selected text

Command 6 E paste the text currently in the clip,

board

?se the arro% #eys to navigate Command E brings up a "ind in "ile dialog that

you can use to search the contents o" a "ile

1.4 Go Tools

Go is a compiled programming language$ %hich means

source code @the code you %rite is translated into a

language that your computer can understand here,

"ore be"ore %e can %rite a Go program$ %e need the Go

compiler

he installer %ill setup Go "or you automatically e

%ill be using version 1 o" the language @7ore in"orma,

tion can be "ound at http://))).olan.or

etFs ma#e sure everything is %or#ing =pen up a ter,

minal and type the "ollo%ing(

o version


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Getting Started 14

9ou should see the "ollo%ing(

o version o$..

9our version number may be slightly di""erent I" you

get an error about the command not being recogni>ed

try restarting your computer

he Go tool suite is made up o" several di""erent com,

mands and sub,commands A list o" those commands is

available by typing(

o help

e %ill see ho% they are used in subseBuent chapters


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2 Your First Program

raditionally the "irst program you %rite in any pro,

gramming language is called a 4ello orldH programE a program that simply outputs 0ello 1orldto your

terminal etFs %rite one using Go

irst create a ne% "older %here %e can store our pro,

gram he installer you used in chapter 1 created a

"older in your home directory named 2o Create a

"older named '/2o/src/olan#boo&/chapter @here

'means your home directory rom the terminal you

can do this by entering the "ollo%ing commands(

m&dir 2o/src/olan#boo&m&dir 2o/src/olan#boo&/chapter

?sing your text editor type in the "ollo%ing(

15


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Your First Program 16

pac&ae main

import 34mt3

// this is a comment

4unc main56 7 4mt.+rintln530ello 1orld368

7a#e sure your "ile is identical to %hat is sho%n here

and save it as main.o in the "older %e Lust created

=pen up a ne% terminal and type in the "ollo%ing(

cd 2o/src/olan#boo&/chapter

o run main.o

9ou should see 0ello 1orlddisplayed in your termi,

nal he o runcommand ta#es the subseBuent "iles

@separated by spaces$ compiles them into an exe,

cutable saved in a temporary directory and then runs

the program I" you didnFt see 0ello 1orld displayed

you may have made a mista#e %hen typing in the pro,

gram he Go compiler %ill give you hints about %here

the mista#e lies i#e most compilers$ the Go compiler

is extremely pedantic and has no tolerance "or mis,

ta#es


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2.1 o0 to ead a Go Program

etFs loo# at this program in more detail Go programs

are read top to bottom$ le"t to right @li#e a boo# he

"irst line says this(

pac&ae main

his is #no% as a pac#age declarationH 3very Go pro,

gram must start %ith a pac#age declaration Pac#ages

are GoFs %ay o" organi>ing and reusing code here are

t%o types o" Go programs( executables and libraries

3xecutable applications are the #inds o" programs that

%e can run directly "rom the terminal @in indo%sthey end %ith .exe ibraries are collections o" code

that %e pac#age together so that %e can use them in

other programs e %ill explore libraries in more de,

tail later$ "or no% Lust ma#e sure to include this line in

any program you %rite

he next line is a blan# line Computers represent

ne%lines %ith a special character @or several charac,

ters !e%lines$ spaces and tabs are #no%n as %hite,

space @because you canFt see them Go mostly doesnFt

care about %hitespace$ %e use it to ma#e programs

easier to read @9ou could remove this line and the pro,

gram %ould behave in exactly the same %ay


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hen %e see this(

import 34mt3

he import#ey%ord is ho% %e include code "rom other

pac#ages to use %ith our program he 4mt pac#age

@shorthand "or "ormat implements "ormatting "or in,

put and output Given %hat %e Lust learned about

pac#ages %hat do you thin# the 4mt pac#ageFs "iles

%ould contain at the top o" themM

!otice that 4mtabove is surrounded by double Buotes

he use o" double Buotes li#e this is #no%n as a string

literalH %hich is a type o" expressionH In Go stringsrepresent a seBuence o" characters @letters$ numbers$

symbols$ K o" a de"inite length &trings are described

in more detail in the next chapter$ but "or no% the im,

portant thing to #eep in mind is that an opening 3

character must eventually be "ollo%ed by another 3

character and anything in bet%een the t%o is includedin the string @he 3character itsel" is not part o" the

string

he line that starts %ith //is #no%n as a comment

Comments are ignored by the Go compiler and are

there "or your o%n sa#e @or %hoever pic#s up thesource code "or your program Go supports t%o di""er,


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ent styles o" comments( //comments in %hich all the

text bet%een the //and the end o" the line is part o"the comment and /9 9/comments %here everything

bet%een the 9s is part o" the comment @And may in,

clude multiple lines

A"ter this you see a "unction declaration(

4unc main56 7 4mt.+rintln530ello 1orld368

unctions are the building bloc#s o" a Go program

hey have inputs$ outputs and a series o" steps called

statements %hich are executed in order All "unctions

start %ith the #ey%ord 4unc"ollo%ed by the name o"

the "unction @mainin this case$ a list o" >ero or more

parametersH surrounded by parentheses$ an optional

return type and a bodyH %hich is surrounded by curly

braces his "unction has no parameters$ doesnFt re,

turn anything and has only one statement he name

mainis special because itFs the "unction that gets called

%hen you execute the program

he "inal piece o" our program is this line(

4mt.+rintln530ello 1orld36


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his statement is made o" three components irst %e

access another "unction inside o" the 4mt pac#agecalled +rintln @thatFs the 4mt.+rintln piece$ +rintln

means Print ine hen %e create a ne% string that

contains 0ello 1orldand invo#e @also #no%n as call or

execute that "unction %ith the string as the "irst and

only argument

At this point %eFve already seen a lot o" ne% terminol,

ogy and you may be a bit over%helmed &ometimes its

help"ul to deliberately read your program out loud

=ne reading o" the program %e Lust %rote might go

li#e this(

Create a ne% executable program$ %hich re"erences

the 4mt library and contains one "unction called

main hat "unction ta#es no arguments$ doesnFt re,

turn anything and does the "ollo%ing( Access the

+rintln "unction contained inside o" the 4mt pac#,

age and invo#e it using one argument E the string

0ello 1orld

he +rintln "unction does the real %or# in this pro,

gram 9ou can "ind out more about it by typing the "ol,

lo%ing in your terminal(

odoc 4mt +rintln


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Among other things you should see this(

+rintln 4ormats usin the de4ault 4ormats 4orits operands and )rites to standard output.Spaces are al)a-s added bet)een operands and ane)line is appended. t returns the number o4b-tes )ritten and an- )rite error encountered.

Go is a very %ell documented programming language

but this documentation can be di""icult to understand

unless you are already "amiliar %ith programming lan,

guages !evertheless the odoccommand is extremely

use"ul and a good place to start %henever you have a

Buestion

'ac# to the "unction at hand$ this documentation is

telling you that the +rintln "unction %ill send %hat,

ever you give to it to standard output E a name "or the

output o" the terminal you are %or#ing in his "unc,

tion is %hat causes 0ello 1orldto be displayed

In the next chapter %e %ill explore ho% Go stores and

represents things li#e 0ello 1orldby learning about

types


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Problems

1 hat is %hitespaceM

2 hat is a commentM hat are the t%o %ays o"

%riting a commentM

. =ur program began %ith pac&ae main hat

%ould the "iles in the 4mtpac#age begin %ithM

- e used the +rintln"unction de"ined in the 4mt

pac#age I" %e %anted to use the ;xit "unction

"rom the ospac#age %hat %ould %e need to doM

/ 7odi"y the program %e %rote so that instead o"

printing 0ello 1orld it prints 0ello< m- name

is"ollo%ed by your name


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3 Types

In the last chapter %e used the data type string to

store 0ello 1orld Data types categori>e a set o" re,lated values$ describe the operations that can be done

on them and de"ine the %ay they are stored &ince

types can be a di""icult concept to grasp %e %ill loo# at

them "rom a couple di""erent perspectives be"ore %e

see ho% they are implemented in Go

Philosophers sometimes ma#e a distinction bet%een

types and to#ens or example suppose you have a dog

named 7ax 7ax is the to#en @a particular instance or

member and dog is the type @the general concept

DogH or dognessH describes a set o" properties that all

dogs have in common Although oversimplistic %e

might reason li#e this( All dogs have - legs$ 7ax is a

dog$ there"ore 7ax has - legs ypes in programming

languages %or# in a similar %ay( All strings have a

length$ x is a string$ there"ore x has a length

In mathematics %e o"ten tal# about sets or example(

@the set o" all real numbers or @the set o" all natu,ral numbers 3ach member o" these sets shares prop,

23


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Types 24

erties %ith all the other members o" the set or exam,

ple all natural numbers are associative( "or all natu,ral numbers a$ b$ and c$ a @b c N @a b c and a O

@b O c N @a O b O cH In this %ay sets are similar to

types in programming languages since all the values o"

a particular type share certain properties

Go is a statically typed programming language his

means that variables al%ays have a speci"ic type and

that type cannot change &tatic typing may seem cum,

bersome at "irst 9ouFll spend a large amount o" your

time Lust trying to "ix your program so that it "inally

compiles 'ut types help us reason about %hat our pro,

gram is doing and catch a %ide variety o" common mis,

ta#es

Go comes %ith several built,in data types %hich %e

%ill no% loo# at in more detail

3.1 ,umbers

Go has several di""erent types to represent numbers

Generally %e split numbers into t%o di""erent #inds(

integers and "loating,point numbers

'ntegers

Integers E li#e their mathematical counterpart E are


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25 Types

numbers %ithout a decimal component @K$ ,.$ ,2$ ,1$

0$ 1$ K ?nli#e the base,10 decimal system %e use torepresent numbers$ computers use a base,2 binary sys,

tem

=ur system is made up o" 10 di""erent digits =nce

%eFve exhausted our available digits %e represent

larger numbers by using 2 @then .$ -$ /$ K digits put

next to each other or example the number a"ter ) is

10$ the number a"ter )) is 100 and so on Computers

do the same$ but they only have 2 digits instead o" 10

&o counting loo#s li#e this( 0$ 1$ 10$ 11$ 100$ 101$ 110$

111 and so on he other di""erence bet%een the num,

ber system %e use and the one computers use is that

all o" the integer types have a de"inite si>e hey only

have room "or a certain number o" digits &o a - bit in,

teger might loo# li#e this( 0000$ 0001$ 0010$ 0011$

0100 3ventually %e run out o" space and most com,

puters Lust %rap around to the beginning @hich can

result in some very strange behavior

GoFs integer types are( uint=$ uint$$ uint?$ uint@$

int=$ int$$ int?and int@.+$ 18$ .2 and 8- tell us

ho% many bits each o" the types use uintmeans un,

signed integerH %hile intmeans signed integerH ?n,

signed integers only contain positive numbers @or

>ero In addition there t%o alias types( b-te%hich isthe same as uint= and rune %hich is the same as


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Types 26

int? 'ytes are an extremely common unit o" mea,

surement used on computers @1 byte N + bits$ 102-bytes N 1 #ilobyte$ 102- #ilobytes N 1 megabyte$ K

and there"ore GoFs b-tedata type is o"ten used in the

de"inition o" other types here are also . machine de,

pendent integer types( uint$ intand uintptr hey are

machine dependent because their si>e depends on the

type o" architecture you are using

Generally i" you are %or#ing %ith integers you should

Lust use the inttype

Floating Point ,umbers

loating point numbers are numbers that contain a

decimal component @real numbers @12.-$ 12.-$

0000012.-$ 12.-0000 heir actual representation on

a computer is "airly complicated and not really neces,

sary in order to #no% ho% to use them &o "or no% %e

need only #eep the "ollo%ing in mind(

1 loating point numbers are inexact =ccasion,

ally it is not possible to represent a number or

example computing $.$ # .AA results in

.$= E A number extremely

close to %hat %e %ould expect$ but not exactly

the same


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27 Types

2 i#e integers "loating point numbers have a cer,

tain si>e @.2 bit or 8- bit ?sing a larger si>ed"loating point number increases itFs precision

@ho% many digits it can represent

. In addition to numbers there are several other

values %hich can be represented( not a num,

berH @NaN$ "or things li#e / and positive and

negative in"inity @and D

Go has t%o "loating point types( 4loat? and 4loat@

@also o"ten re"erred to as single precision and double

precision respectively as %ell as t%o additional types

"or representing complex numbers @numbers %ith

imaginary parts( complex@and complex$= Generally

%e should stic# %ith 4loat@%hen %or#ing %ith "loat,

ing point numbers

-ample

etFs %rite an example program using numbers irstcreate a "older called chapter?and ma#e a main.o"ile

containing the "ollo%ing(


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Types 28

pac&ae main

import 34mt3

4unc main56 7 4mt.+rintln53$ $ E3< $ $68

I" you run the program and you should see this(

( o run main.o$ $ E

!otice that this program is very similar to the pro,

gram %e %rote in chapter 2 It contains the same pac#,age line$ the same import line$ the same "unction dec,

laration and uses the same +rintln"unction his time

instead o" printing the string 0ello 1orld%e print the

string $ $ E"ollo%ed by the result o" the expression

$ $ his expression is made up o" three parts( the

numeric literal $@%hich is o" type int$ the operator@%hich represents addition and another numeric lit,

eral $ etFs try the same thing using "loating point

numbers(

4mt.+rintln53$ $ E3< $. $.6

!otice that %e use the .to tell Go that this is a "loat,


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29 Types

ing point number instead o" an integer 5unning this

program %ill give you the same result as be"ore

In addition to addition Go has several other operators(

addition

# subtraction

9 multiplication

/ division

F remainder

3.2 Strings

As %e sa% in chapter 2 a string is a seBuence o" char,

acters %ith a de"inite length used to represent text Go

strings are made up o" individual bytes$ usually one

"or each character @Characters "rom other languages

li#e Chinese are represented by more than one byte

&tring literals can be created using double Buotes

30ello 1orld3or bac# tic#s G0ello 1orldG he di""er,

ence bet%een these is that double Buoted strings can,

not contain ne%lines and they allo% special escape se,

Buences or example \ngets replaced %ith a ne%line

and \tgets replaced %ith a tab character


34/165

Types 30

&everal common operations on strings include "inding

the length o" a string( len530ello 1orld36$ accessingan individual character in the string( 30ello

1orld3H$I$ and concatenating t%o strings together(

30ello 3 31orld3 etFs modi"y the program %e cre,

ated earlier to test these out(

pac&ae main

import 34mt3

4unc main56 7 4mt.+rintln5len530ello 1orld366 4mt.+rintln530ello 1orld3H$I6 4mt.+rintln530ello 3 31orld36

8

A "e% things to notice(

1 A space is also considered a character$ so the

stringFs length is 11 not 10 and the . rdline has

30ello 3instead o" 30ello3

2 &trings are indexedH starting at 0 not 1 H$I

gives you the 2ndelement not the 1st Also notice

that you see $$instead o" e%hen you run this

program his is because the character is repre,

sented by a byte @remember a byte is an

integer


35/165

31 Types

=ne %ay to thin# about indexing %ould be tosho% it li#e this instead( 30ello 1orld3$ 9ouFd

read that as he string 4ello orld sub 1$H

he string 4ello orld at 1H or he second

character o" the string 4ello orldH

. Concatenation uses the same symbol as addi,

tion he Go compiler "igures out %hat to do

based on the types o" the arguments &ince both

sides o" the are strings the compiler assumes

you mean concatenation and not addition @Ad,

dition is meaningless "or strings

3.3 6ooleans

A boolean value @named a"ter George 'oole is a spe,

cial 1 bit integer type used to represent true and "alse

@or on and o"" hree logical operators are used %ith

boolean values(

JJ and

KK or

L not

4ere is an example program sho%ing ho% they can be


36/165

Types 32

used(

4unc main56 7 4mt.+rintln5true JJ true6 4mt.+rintln5true JJ 4alse6 4mt.+rintln5true KK true6 4mt.+rintln5true KK 4alse6 4mt.+rintln5Ltrue68

5unning this program should give you(

( o run main.otrue4alse

truetrue4alse

e usually use truth tables to de"ine ho% these opera,

tors %or#(

-pression Value

true JJ true true

true JJ 4alse 4alse

4alse JJ true 4alse

4alse JJ 4alse 4alse


37/165

33 Types

-pression Value

true KK true true

true KK 4alse true

4alse KK true true

4alse KK 4alse 4alse

-pression Value

Ltrue 4alse

L4alse true

hese are the simplest types included %ith Go and

"orm the "oundation "rom %hich all later types arebuilt


38/165

Types 34

Problems

1 4o% are integers stored on a computerM

2 e #no% that @in base 10 the largest 1 digit

number is ) and the largest 2 digit number is

)) Given that in binary the largest 2 digit num,

ber is 11 @.$ the largest . digit number is 111

@* and the largest - digit number is 1111 @1/

%hatFs the largest + digit numberM @int( 101,1 N

) and 102,1 N ))

. Although overpo%ered "or the tas# you can use

Go as a calculator rite a program that com,

putes ?$?" M @@"$and prints it to the ter,

minal @?se the 9operator "or multiplication

- hat is a stringM 4o% do you "ind its lengthM

/ hatFs the value o" the expression 5true JJ

4alse6 KK 54alse JJ true6 KK L54alse JJ4alse6M


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4 Variables

?p until no% %e have only seen programs that use lit,

eral values @numbers$ strings$ etc but such programsarenFt particularly use"ul o ma#e truly use"ul pro,

grams %e need to learn t%o ne% concepts( variables

and control "lo% statements his chapter %ill explore

variables in more detail

A variable is a storage location$ %ith a speci"ic type

and an associated name etFs change the program %e

%rote in chapter 2 so that it uses a variable(

pac&ae main

import 34mt3

4unc main56 7 var x strin E 30ello 1orld3 4mt.+rintln5x68

!otice that the string literal "rom the original program

still appears in this program$ but rather than send itdirectly to the +rintln"unction %e assign it to a vari,

35


40/165

!aria"#es 36

able instead 6ariables in Go are created by "irst using

the var#ey%ord$ then speci"ying the variable name @x$the type @strin and "inally assigning a value to the

variable @0ello 1orld he last step is optional so an

alternative %ay o" %riting the program %ould be li#e

this(

pac&ae main

import 34mt3

4unc main56 7 var x strin x E 30ello 1orld3 4mt.+rintln5x6

8

6ariables in Go are similar to variables in algebra but

there are some subtle di""erences(

irst %hen %e see the Esymbol %e have a tendency to

read that as x eBuals the string 4ello orldH hereFsnothing %rong %ith reading our program that %ay$ but

itFs better to read it as x ta#es the string 4ello orldH

or x is assigned the string 4ello orldH his distinc,

tion is important because @as their name %ould sug,

gest variables can change their value throughout the

li"etime o" a program ry running the "ollo%ing(


41/165

37 !aria"#es

pac&ae main

import 34mt3

4unc main56 7 var x strin x E 34irst3 4mt.+rintln5x6 x E 3second3

4mt.+rintln5x68

In "act you can even do this(

var x strinx E 34irst 34mt.+rintln5x6x E x 3second34mt.+rintln5x6

his program %ould be nonsense i" you read it li#e an

algebraic theorem 'ut it ma#es sense i" you are care,

"ul to read the program as a list o" commands hen%e see x E x 3second3%e should read it as assign

the concatenation o" the value o" the variable x and the

string literal second to the variable xH he right side

o" the Eis done "irst and the result is then assigned to

the le"t side o" the E

he x E x -"orm is so common in programming that


42/165

!aria"#es 38

Go has a special assignment statement( E e could

have %ritten x E x 3second3as x E 3second3and it%ould have done the same thing @=ther operators can

be used the same %ay

Another di""erence bet%een Go and algebra is that %e

use a di""erent symbol "or eBuality( EE @%o eBual

signs next to each other EEis an operator li#e and itreturns a boolean or example(

var x strin E 3hello3 var - strin E 3)orld3 4mt.+rintln5x EE -6

his program should print 4alsebecause hellois not

the same as )orld =n the other hand(

var x strin E 3hello3 var - strin E 3hello3 4mt.+rintln5x EE -6

his %ill print truebecause the t%o strings are the

same

&ince creating a ne% variable %ith a starting value is

so common Go also supports a shorter statement(


43/165

39 !aria"#es

x :E 30ello 1orld3

!otice the :be"ore the Eand that no type %as speci,

"ied he type is not necessary because the Go compiler

is able to in"er the type based on the literal value you

assign the variable @&ince you are assigning a string

literal$ x is given the type strin he compiler can

also do in"erence %ith the varstatement(

var x E 30ello 1orld3

he same thing %or#s "or other types(

x :E " 4mt.+rintln5x6

Generally you should use this shorter "orm %henever

possible

4.1 o0 to ,ame a Variable

!aming a variable properly is an important part o"

so"t%are development !ames must start %ith a letter

and may contain letters$ numbers or the @underscore

symbol he Go compiler doesnFt care %hat you name a

variable so the name is meant "or your @and others


44/165

!aria"#es 40

bene"it Pic# names %hich clearly describe the vari,

ableFs purpose &uppose %e had the "ollo%ing(

x :E 3*ax3 4mt.+rintln53*- doOs name is3< x6

In this case x is not a very good name "or a variable A

better name %ould be(

name :E 3*ax3 4mt.+rintln53*- doOs name is3< name6

or even(

dosName :E 3*ax3 4mt.+rintln53*- doOs name is3< dosName6

In this last case %e use a special %ay to represent mul,

tiple %ords in a variable name #no%n as lo%er camel

case @also #no% as mixed case$ bumpy caps$ camel

bac# or hump bac# he "irst letter o" the "irst %ord is

lo%ercase$ the "irst letter o" the subseBuent %ords is

uppercase and all the other letters are lo%ercase

4.2 Scope

Going bac# to the program %e sa% at the beginning o"


45/165

41 !aria"#es

the chapter(

pac&ae main

import 34mt3

4unc main56 7 var x strin E 30ello 1orld3 4mt.+rintln5x6

8

Another %ay o" %riting this program %ould be li#e

this(

pac&ae main

import 34mt3

var x strin E 30ello 1orld3

4unc main56 7 4mt.+rintln5x68

!otice that %e moved the variable outside o" the main

"unction his means that other "unctions can access

this variable(


46/165

!aria"#es 42

var x strin E 30ello 1orld3

4unc main56 7 4mt.+rintln5x68

4unc 456 7 4mt.+rintln5x68

he 4"unction no% has access to the xvariable !o%

suppose that %e %rote this instead(

4unc main56 7 var x strin E 30ello 1orld3

4mt.+rintln5x68

4unc 456 7 4mt.+rintln5x68

I" you run this program you should see an error(

.\main.o:$$: unde4ined: x

he compiler is telling you that the xvariable inside o"

the 4"unction doesnFt exist It only exists inside o" the

main"unction he range o" places %here you are al,lo%ed to use xis called the s$opeo" the variable Ac,


47/165

43 !aria"#es

cording to the language speci"ication Go is lexically

scoped using bloc#sH 'asically this means that thevariable exists %ithin the nearest curly braces 7 8 @a

bloc# including any nested curly braces @bloc#s$ but

not outside o" them &cope can be a little con"using at

"irst as %e see more Go examples it should become

more clear

4.3 Constants

Go also has support "or constants Constants are basi,

cally variables %hose values cannot be changed later

hey are created in the same %ay you create variables

but instead o" using the var#ey%ord %e use the const

#ey%ord(

pac&ae main

import 34mt3

4unc main56 7

const x strin E 30ello 1orld3 4mt.+rintln5x68

his(


48/165

!aria"#es 44

const x strin E 30ello 1orld3

x E 3Some other strin3

5esults in a compile,time error(

.\main.o:P: cannot assin to x

Constants are a good %ay to reuse common values in a

program %ithout %riting them out each time or ex,

ample +iin the mathpac#age is de"ined as a constant

4.4 7e(ining $ultiple Variables

Go also has another shorthand %hen you need to de,

"ine multiple variables(

var 5 a E " b E $

c E $"6

?se the #ey%ord var @or const "ollo%ed by parenthe,

ses %ith each variable on its o%n line


49/165

45 !aria"#es

4.5 "n -ample Program

4ereFs an example program %hich ta#es in a number

entered by the user and doubles it(

pac&ae main

import 34mt3

4unc main56 74mt.+rint53;nter a number: 36var input [email protected]< Jinput6

output :E input 9

4mt.+rintln5output68

e use another "unction "rom the 4mtpac#age to read

the user input @Scan4 Jinput%ill be explained in a

later chapter$ "or no% all %e need to #no% is that Scan4

"ills input %ith the number %e enter


50/165

!aria"#es 46

Problems

1 hat are t%o %ays to create a ne% variableM

2 hat is the value o" x a"ter running(

x :E "Q x E $M

. hat is scope and ho% do you determine the

scope o" a variable in GoM

- hat is the di""erence bet%een varand constM

/ ?sing the example program as a starting point$

%rite a program that converts "rom ahrenheit

into Celsius @C E 5! # ?6 9 "/A

8 rite another program that converts "rom "eet

into meters @$ 4t E .?@= m


51/165

5 Control Structures

!o% that %e #no% ho% to use variables itFs time to

start %riting some use"ul programs irst letFs %rite aprogram that counts to 10$ starting "rom 1$ %ith each

number on its o%n line ?sing %hat %eFve learned so

"ar %e could %rite this(

pac&ae main

import 34mt3

4unc main56 7 4mt.+rintln5$6 4mt.+rintln56 4mt.+rintln5?6 4mt.+rintln5@6 4mt.+rintln5"6

4mt.+rintln56 4mt.+rintln5P6 4mt.+rintln5=6 4mt.+rintln5A6 4mt.+rintln5$68

=r this(

47


52/165

%ontro# Stru$tures 48

pac&ae main

import 34mt3

4unc main56 7 4mt.+rintln5G$?@"

P=A$G68

'ut both o" these programs are pretty tedious to %rite

hat %e need is a %ay o" doing something multiple

times

5.1 For

he 4orstatement allo%s us to repeat a list o" state,

ments @a bloc# multiple times 5e%riting our previous

program using a 4orstatement loo#s li#e this(


53/165

49 %ontro# Stru$tures

pac&ae main

import 34mt3

4unc main56 7 i :E $ 4or i RE $ 7 4mt.+rintln5i6 i E i $

88

irst %e create a variable called ithat %e use to store

the number %e %ant to print hen %e create a 4or

loop by using the #ey%ord 4or$ providing a conditional

expression %hich is either true or 4alse and "inally

supplying a bloc# to execute he "or loop %or#s li#e

this(

1 e evaluate @run the expression i RE $@i less

than or eBual to 10H I" this evaluates to true

then %e run the statements inside o" the bloc#

=ther%ise %e Lump to the next line o" our pro,

gram a"ter the bloc# @in this case there is noth,

ing a"ter the "or loop so %e exit the program

2 A"ter %e run the statements inside o" the bloc#

%e loop bac# to the beginning o" the "or state,

ment and repeat step 1


54/165


he i E i $ line is extremely important$ because

%ithout it i RE $%ould al%ays evaluate to trueandour program %ould never stop @hen this happens

this is re"erred to as an in"inite loop

As an exercise lets %al# through the program li#e a

computer %ould(

Create a variable named i%ith the value 1

Is i RE $M 9es

Print i

&et ito i $@ino% eBuals 2

Is i RE $M 9es

Print i &et ito i $@ino% eBuals .

K

&et ito i $@ino% eBuals 11

Is i RE $M !o

!othing le"t to do$ so exit

=ther programming languages have a lot o" di""erent

types o" loops @%hile$ do$ until$ "oreach$ K but Go only

has one that can be used in a variety o" di""erent %ays

he previous program could also have been %ritten

li#e this(


55/165


4unc main56 7

4or i :E $Q i RE $Q i 7 4mt.+rintln5i6 88

!o% the conditional expression also contains t%o other

statements %ith semicolons bet%een them irst %e

have the variable initiali>ation$ then %e have the con,

dition to chec# each time and "inally %e incrementH

the variable @adding 1 to a variable is so common that

%e have a special operator( &imilarly subtracting 1

can be done %ith ##

e %ill see additional %ays o" using the "or loop inlater chapters

5.2 '(

etFs modi"y the program %e Lust %rote so that instead

o" Lust printing the numbers 1,10 on each line it alsospeci"ies %hether or not the number is even or odd

i#e this(


56/165


$ odd

even? odd@ even" odd evenP odd= evenA odd

$ even

irst %e need a %ay o" determining %hether or not a

number is even or odd An easy %ay to tell is to divide

the number by 2 I" you have nothing le"t over then the

number is even$ other%ise itFs odd &o ho% do %e "ind

the remainder a"ter division in GoM e use the Foper,ator $ F eBuals $$ F eBuals $ ? F eBuals $and

so on

!ext %e need a %ay o" choosing to do di""erent things

based on a condition or that %e use the i4 state,

ment(

i4 i F EE 7 // even8 else 7 // odd8

An i4statement is similar to a 4orstatement in that it


57/165


has a condition "ollo%ed by a bloc# I" statements also

have an optional elsepart I" the condition evaluatesto truethen the bloc# a"ter the condition is run$ other,

%ise either the bloc# is s#ipped or i" the elsebloc# is

present that bloc# is run

I" statements can also have else i4parts(

i4 i F EE 7 // divisible b- 8 else i4 i F ? EE 7 // divisible b- ?8 else i4 i F @ EE 7 // divisible b- @8

he conditions are chec#ed top do%n and the "irst one

to result in true %ill have its associated bloc# exe,

cuted !one o" the other bloc#s %ill execute$ even i"

their conditions also pass @&o "or example the number

+ is divisible by both - and 2$ but the // divisible b-

@bloc# %ill never execute because the // divisible b-

bloc# is done "irst

Putting it all together %e have(


58/165


4unc main56 7

4or i :E $Q i RE $Q i 7 i4 i F EE 7

4mt.+rintln5i< 3even36 8 else 7 4mt.+rintln5i< 3odd36 8 88

etFs %al# through this program(

Create a variable io" type intand give it the value $

Is iless than or eBual to $M 9es( Lump to the bloc#

Is the remainder o" iQ eBual to M !o( Lump to the

elsebloc#

Print i"ollo%ed by odd

Increment i@the statement a"ter the condition

Is iless than or eBual to $M 9es( Lump to the bloc#

Is the remainder o" iQ eBual to M 9es( Lump to the

i4bloc#

Print i"ollo%ed by even

K

he remainder operator$ %hile rarely seen outside o"

elementary school$ turns out to be really use"ul %hen

programming 9ouFll see it turn up every%here "rom

>ebra striping tables to partitioning data sets


59/165


5.3 S0itc+

&uppose %e %anted to %rite a program that printed

the 3nglish names "or numbers ?sing %hat %eFve

learned so "ar %e might start by doing this(

i4 i EE 74mt.+rintln53ero36

8 else i4 i EE $ 7 4mt.+rintln53Tne368 else i4 i EE 7 4mt.+rintln53)o368 else i4 i EE ? 7 4mt.+rintln53hree368 else i4 i EE @ 7

4mt.+rintln53!our36

8 else i4 i EE " 7 4mt.+rintln53!ive368

&ince %riting a program in this %ay %ould be pretty

tedious Go provides another statement to ma#e this

easier( the s)itchstatement e can re%rite our pro,gram to loo# li#e this(


60/165


s)itch i 7

case : 4mt.+rintln53ero36case $: 4mt.+rintln53Tne36case : 4mt.+rintln53)o36case ?: 4mt.+rintln53hree36case @: 4mt.+rintln53!our36case ": 4mt.+rintln53!ive36de4ault: 4mt.+rintln53Un&no)n Number368

A s%itch statement starts %ith the #ey%ord s)itch"ol,

lo%ed by an expression @in this case i and then a se,

ries o" cases he value o" the expression is compared

to the expression "ollo%ing each case#ey%ord I" they

are eBuivalent then the statement@s "ollo%ing the :is

executed

i#e an i" statement each case is chec#ed top do%n

and the "irst one to succeed is chosen A s%itch also

supports a de"ault case %hich %ill happen i" none o"

the cases matches the value @Rind o" li#e the else in

an i" statement

hese are the main control "lo% statements Additional

statements %ill be explored in later chapters


61/165


Problems

1 hat does the "ollo%ing program print(

i :E $i4 i > $ 7 4mt.+rintln53Bi368 else 7

4mt.+rintln53Small368

2 rite a program that prints out all the numbers

evenly divisible by . bet%een 1 and 100 @.$ 8$ )$

etc

. rite a program that prints the numbers "rom 1

to 100 'ut "or multiples o" three print Si>>S in,

stead o" the number and "or the multiples o" "ive

print S'u>>S or numbers %hich are multiples

o" both three and "ive print Si>>'u>>S


62/165

! "rrays# Slices and $aps

In chapter . %e learned about GoFs basic types In this

chapter %e %ill loo# at three more built,in types( ar,rays$ slices and maps

!.1 "rrays

An array is a numbered seBuence o" elements o" a sin,

gle type %ith a "ixed length In Go they loo# li#e this(

var x H"Iint

x is an example o" an array %hich is composed o" /

ints ry running the "ollo%ing program(

58


63/165

59 &rrays' S#i$es and (aps

pac&ae main

import 34mt3

4unc main56 7var x H"IintxH@I E $4mt.+rintln5x6

8

9ou should see this(

H $I

xH@I E $should be read set the /thelement o" the

array x to 100H It might seem strange that xH@Irepre,

sents the /thelement instead o" the -thbut li#e strings$

arrays are indexed starting "rom 0 Arrays are ac,

cessed in a similar %ay e could change

4mt.+rintln5x6to 4mt.+rintln5xH@I6and %e %ould get

100

4ereFs an example program that uses arrays(


64/165

&rrays' S#i$es and (aps 60

4unc main56 7var x H"I4loat@xHI E A=xH$I E A?xHI E PPxH?I E =xH@I E =?

var total 4loat@ E 4or i :E Q i R "Q i 7

total E xHiI84mt.+rintln5total / "6

8

his program computes the average o" a series o" test

scores I" you run it you should see =. etFs %al#

through the program(

irst %e create an array o" length / to hold our test

scores$ then %e "ill up each element %ith a grade

!ext %e setup a "or loop to compute the total score

inally %e divide the total score by the number o" el,

ements to "ind the average

his program %or#s$ but Go provides some "eatures %e

can use to improve it irst these 2 parts( i R " and

total / "should thro% up a red "lag "or us &ay %e


65/165


changed the number o" grades "rom / to 8 e %ould

also need to change both o" these parts It %ould bebetter to use the length o" the array instead(

var total 4loat@ E 4or i :E Q i R len5x6Q i 7

total E xHiI8

4mt.+rintln5total / len5x66

Go ahead and ma#e these changes and run the pro,

gram 9ou should get an error(

( o run tmp.o

command#line#aruments.\tmp.o:$A: invalid operation: total / "5mismatched t-pes 4loat@ and int6

he issue here is that len5x6and totalhave di""erent

types totalis a 4loat@%hile len5x6is an int &o %e

need to convert len5x6into a 4loat@(

4mt.+rintln5total / 4loat@5len5x666

his is an example o" a type conversion In general to

convert bet%een types you use the type name li#e a

"unction


66/165


Another change to the program %e can ma#e is to use

a special "orm o" the "or loop(

var total 4loat@ E 4or i< value :E rane x 7

total E value84mt.+rintln5total / 4loat@5len5x666

In this "or loop irepresents the current position in the

array and valueis the same as xHiI e use the #ey,

%ord rane "ollo%ed by the name o" the variable %e

%ant to loop over

5unning this program %ill result in another error(

( o run tmp.o command#line#aruments.\tmp.o:$: i declared and not used

he Go compiler %onFt allo% you to create variablesthat you never use &ince %e donFt use iinside o" our

loop %e need to change it to this(


67/165


var total 4loat@ E

4or < value :E rane x 7total E value

84mt.+rintln5total / 4loat@5len5x666

A single @underscore is used to tell the compiler that

%e donFt need this @In this case %e donFt need the iter,

ator variable

Go also provides a shorter syntax "or creating arrays(

x :E H"I4loat@7 A=< A?< PP< =< =? 8

e no longer need to speci"y the type because Go can

"igure it out &ometimes arrays li#e this can get too

long to "it on one line$ so Go allo%s you to brea# it up

li#e this(

x :E H"I4loat@7A=ero value "or the value type

@%hich "or strings is the empty string Although %ecould chec# "or the >ero value in a condition

@elementsH3Un3I EE 33 Go provides a better %ay(

name< o& :E elementsH3Un3I4mt.+rintln5name< o&6

Accessing an element o" a map can return t%o values

instead o" Lust one he "irst value is the result o" the

loo#up$ the second tells us %hether or not the loo#up

%as success"ul In Go %e o"ten see code li#e this(

i4 name< o& :E elementsH3Un3IQ o& 74mt.+rintln5name< o&6

8

irst %e try to get the value "rom the map$ then i" itFs

success"ul %e run the code inside o" the bloc#

i#e %e sa% %ith arrays there is also a shorter %ay to

create maps(


76/165


elements :E mapHstrinIstrin7

303: 30-droen3e a map$ etc

or an exceptional condition that thereFs no easy %ay to

recover "rom @4ence the name panicH


95/165

91 Fun$tions

Problems

1 sumis a "unction %hich ta#es a slice o" numbers

and adds them together hat %ould its "unc,

tion signature loo# li#e in GoM

2 rite a "unction %hich ta#es an integer and

halves it and returns true i" it %as even or "alse

i" it %as odd or example hal45$6should return

5< 4alse6 and hal456 should return 5$ed domain speci"ic #no%ledge @"or

example( cryptography It is beyond the scope o" this

boo# to explain these underlying technologies

13.1 Strings

Go includes a large number o" "unctions to %or# %ith

strings in the strinspac#age(

132


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133 Te %ore Pa$+ages

pac&ae main

import 534mt33strins3

6

4unc main56 74mt.+rintln5

// truestrins.Contains53test3< 3es36ed and supports

both the eaderand 1riterinter"aces 9ou can convert

it into a HIb-teby calling bu4.B-tes56 I" you only need

to read "rom a string you can also use the

strins.Ne)eader "unction %hich is more e""icient

than using a bu""er

13.3 Files 8 Folders

o open a "ile in Go use the Tpen"unction "rom the os

pac#age 4ere is an example o" ho% to read the con,tents o" a "ile and display them on the terminal(


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Te %ore Pa$+ages 136

pac&ae main

import 534mt33os3

6

4unc main56 74ile< err :E os.Tpen53test.txt36

i4 err LE nil 7// handle the error herereturn

8de4er 4ile.Close56

// et the 4ile siZestat< err :E 4ile.Stat56i4 err LE nil 7

return8

// read the 4ilebs :E ma&e5HIb-te< stat.SiZe566< err E 4ile.ead5bs6i4 err LE nil 7

return

8

str :E strin5bs64mt.+rintln5str6

8

e use de4er 4ile.Close56right a"ter opening the "ile

to ma#e sure the "ile is closed as soon as the "unction


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completes 5eading "iles is very common$ so thereFs a

shorter %ay to do this(

pac&ae main

import 534mt33io/ioutil3

6

4unc main56 7bs< err :E ioutil.ead!ile53test.txt36i4 err LE nil 7

return8str :E strin5bs64mt.+rintln5str6

8

4ere is ho% %e can create a "ile(


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pac&ae main

import 53os3

6

4unc main56 74ile< err :E os.Create53test.txt36i4 err LE nil 7

// handle the error herereturn8de4er 4ile.Close56

4ile.1riteStrin53test368

o get the contents o" a directory %e use the same

os.Tpen"unction but give it a directory path instead o"

a "ile name hen %e call the eaddirmethod(


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pac&ae main

import 534mt33os3

6

4unc main56 7dir< err :E os.Tpen53.36

i4 err LE nil 7return8de4er dir.Close56

4ilen4os< err :E dir.eaddir5#$6i4 err LE nil 7

return8

4or < 4i :E rane 4ilen4os 74mt.+rintln54i.Name566

88

="ten %e %ant to recursively %al# a "older @read the

"olderFs contents$ all the sub,"olders$ all the sub,sub,

"olders$ K o ma#e this easier thereFs a 1al&"unction

provided in the path/4ilepathpac#age(


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pac&ae main

import 534mt33os33path/4ilepath3

6

4unc main56 7

4ilepath.1al&53.3< 4unc5path strin< in4oos.!ilen4o< err error6 error 74mt.+rintln5path6return nil

868

he "unction you pass to 1al& is called "or every "ile

and "older in the root "older @in this case .

13.4 rrors

Go has a built,in type "or errors that %e have already

seen @the errortype e can create our o%n errors by

using the Ne)"unction in the errorspac#age(


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pac&ae main

import 3errors3

4unc main56 7err :E errors.Ne)53error messae36

8

13.5 Containers 8 Sort

In addition to lists and maps Go has several more col,

lections available underneath the container pac#age

eFll ta#e a loo# at the container/listpac#age as an

example

=ist

he container/list pac#age implements a dou,

bly,lin#ed list A lin#ed list is a type o" data structure

that loo#s li#e this(

3ach node o" the list contains a value @1$ 2$ or . in this

case and a pointer to the next node &ince this is a

doubly,lin#ed list each node %ill also have pointers tothe previous node his list could be created by this


146/165


program(

pac&ae main

import 534mt3 Q 3container/list36

4unc main56 7var x list.,istx.+ushBac&5$6

x.+ushBac&56x.+ushBac&5?6

4or e :E x.!ront56Q e LE nilQ eEe.Next56 74mt.+rintln5e.Value.5int66

88

he >ero value "or a ,istis an empty list @a 9,istcan

also be created using list.Ne) 6alues are appended

to the list using +ushBac& e loop over each item in

the list by getting the "irst element$ and "ollo%ing all

the lin#s until %e reach nil

Sort

he sort pac#age contains "unctions "or sorting arbi,

trary data here are several prede"ined sorting "unc,

tions @"or slices o" ints and "loats 4ereFs an example

"or ho% to sort your o%n data(


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pac&ae main

import 534mt3 Q 3sort36

t-pe +erson struct 7Name strinWe int

8

t-pe B-Name HI+erson

4unc 5this B-Name6 ,en56 int 7return len5this6

84unc 5this B-Name6 ,ess5i< j int6 bool 7

return thisHiI.Name R thisHjI.Name84unc 5this B-Name6 S)ap5i< j int6 7

thisHiI< thisHjI E thisHjI< thisHiI8

4unc main56 7&ids :E HI+erson7

73]ill3


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%e %ant to sort e then de"ine the . methods

&orting our list o" people is then as easy as casting the

list into our ne% type e could also sort by age by do,

ing this(

t-pe B-We HI+erson

4unc 5this B-We6 ,en56 int 7return len5this684unc 5this B-We6 ,ess5i< j int6 bool 7

return thisHiI.We R thisHjI.We84unc 5this B-We6 S)ap5i< j int6 7

thisHiI< thisHjI E thisHjI< thisHiI8

13.! as+es 8 Cryptograp+y

A hash "unction ta#es a set o" data and reduces it to a

smaller "ixed si>e 4ashes are "reBuently used in pro,

gramming "or everything "rom loo#ing up data to eas,

ily detecting changes 4ash "unctions in Go are bro#en

into t%o categories( cryptographic and non,crypto,

graphic

he non,cryptographic hash "unctions can be "ound

underneath the hash pac#age and include adler?$

crc?$ crc@and 4nv 4ereFs an example using crc?(


149/165


pac&ae main

import 534mt33hash/crc?3

6

4unc main56 7h :E crc?.Ne);;;56

h.1rite5HIb-te53test366v :E h.Sum?564mt.+rintln5v6

8

he crc? hash obLect implements the 1riter inter,

"ace$ so %e can %rite bytes to it li#e any other 1riter

=nce %eFve %ritten everything %e %ant %e callSum?56to return a uint? A common use "or crc?is

to compare t%o "iles I" the Sum?value "or both "iles is

the same$ itFs highly li#ely @though not 100V certain

that the "iles are the same I" the values are di""erent

then the "iles are de"initely not the same(


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pac&ae main

import 534mt33hash/crc?33io/ioutil3

6

4unc et0ash54ilename strin6 5uint?< error6 7

bs< err :E ioutil.ead!ile53test$.txt36i4 err LE nil 7

return < err8h :E crc?.Ne);;;56h.1rite5bs6return h.Sum?56< nil

8

4unc main56 7h$< err :E et0ash53test$.txt36i4 err LE nil 7

return8h< err :E et0ash53test.txt36i4 err LE nil 7

return84mt.+rintln5h$< h< h$ EE h6

8

Cryptographic hash "unctions are similar to their non,

cryptographic counterparts$ but they have the added

property o" being hard to reverse Given the crypto,

graphic hash o" a set o" data$ itFs extremely di""icult to


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determine %hat made the hash hese hashes are o",

ten used in security applications

=ne common cryptographic hash "unction is #no%n as

&4A,1 4ereFs ho% it is used(

pac&ae main

import 534mt33cr-pto/sha$3

6

4unc main56 7h :E sha$.Ne)56h.1rite5HIb-te53test366

bs :E h.Sum5HIb-te7864mt.+rintln5bs6

8

his example is very similar to the crc?one$ because

both crc? and sha$ implement the hash.0ash inter,

"ace he main di""erence is that %hereas crc? com,putes a .2 bit hash$ sha$ computes a 180 bit hash

here is no native type to represent a 180 bit number$

so %e use a slice o" 20 bytes instead

13. Ser9ers

riting net%or# servers in Go is very easy e %ill


152/165


"irst ta#e a loo# at ho% to create a CP server(

pac&ae main

import 53encodin/ob334mt33net3

6

4unc server56 7// listen on a portln< err :E net.,isten53tcp3< 3:AAAA36i4 err LE nil 7

4mt.+rintln5err6return

8

4or 7// accept a connectionc< err :E ln.Wccept56i4 err LE nil 7

4mt.+rintln5err6continue

8// handle the connectiono handleServerConnection5c6

88


153/165


4unc handleServerConnection5c net.Conn6 7

// receive the messaevar ms strinerr :E ob.Ne)%ecoder5c6.%ecode5Jms6i4 err LE nil 7

4mt.+rintln5err68 else 7

4mt.+rintln53eceived3< ms68

c.Close568

4unc client56 7// connect to the serverc< err :E net.%ial53tcp3< 3$P...$:AAAA36i4 err LE nil 7

4mt.+rintln5err6

return8

// send the messaems :E 30ello 1orld34mt.+rintln53Sendin3< ms6err E ob.Ne);ncoder5c6.;ncode5ms6i4 err LE nil 7

4mt.+rintln5err68

c.Close568


154/165


4unc main56 7

o server56o client56

var input strin4mt.Scanln5Jinput6

8

his example uses the encodin/ob pac#age %hichma#es it easy to encode Go values so that other Go

programs @or the same Go program in this case can

read them Additional encodings are available in pac#,

ages underneath encodin@li#e encodin/json as %ell

as in .rd party pac#ages @"or example %e could use

labix.or/v/mo/bson "or bson support

TTP

4P servers are even easier to setup and use(


155/165


pac&ae main

import 53net/http3 Q 3io36

4unc hello5res http.esponse1riter< re[9http.e[uest6 7

res.0eader56.Set53Content#-pe3

Rtitle>0ello 1orldR/title>R/head>Rbod->

0ello 1orldLR/bod->

R/html>Gation in Go is through goroutines and channels as

discussed in chapter 10 4o%ever Go does provide

more traditional multithreading routines in the s-nc

and s-nc/atomicpac#ages

$ute-es

A mutex @mutal exclusive loc# loc#s a section o" code

to a single thread at a time and is used to protect

shared resources "rom non,atomic operations 4ere is

an example o" a mutex(


161/165


pac&ae main

import 534mt33s-nc33time3

64unc main56 7

m :E ne)5s-nc.*utex6

4or i :E Q i R $Q i 7o 4unc5i int6 7

m.,oc&564mt.+rintln5i< 3start36time.Sleep5time.Second64mt.+rintln5i< 3end36m.Unloc&56

85i6

8

var input strin4mt.Scanln5Jinput6

8

hen the mutex @m is loc#ed any other attempt to loc#

it %ill bloc# until it is unloc#ed Great care should be

ta#en %hen using mutexes or the synchroni>ation

primitives provided in the s-nc/atomicpac#age

raditional multithreaded programming is di""icult

itFs easy to ma#e mista#es and those mista#es are

hard to "ind$ since they may depend on a very speci"ic$relatively rare$ and di""icult to reproduce set o" circum,


162/165


stances =ne o" GoFs biggest strengths is that the con,

currency "eatures it provides are much easier to under,stand and use properly than threads and loc#s


163/165

14 ,e-t Steps

e no% have all the in"ormation %e need to %rite

most Go programs 'ut it %ould be dangerous to con,clude that there"ore %e are competent programmers

Programming is as much a cra"t as it is Lust having

#no%ledge his chapter %ill provide you %ith some

suggestions about ho% best to master the cra"t o" pro,

gramming

14.1 Study t+e $asters

Part o" becoming a good artist or %riter is studying the

%or#s o" the masters ItFs no di""erent %ith program,

ming =ne o" the best %ays to become a s#illed pro,

grammer is to study the source code produced by oth,

ers Go is %ell suited to this tas# since the source code

"or the entire proLect is "reely available

or example %e might ta#e a loo# at the source code to

the io/ioutillibrary available at(

http(


164/165

,e-t Steps 160

5ead the code slo%ly and deliberately ry to under,

stand every line and read the supplied comments orexample in the ead!ile method thereFs a comment

that says this(

// tOs a ood but not certain bet that !ilen4o// )ill tell us exactl- ho) much to read< so// letOs tr- it but be prepared 4or the ans)er

// to be )ron.

his method probably started out simpler than %hat it

became so this is a great example o" ho% programs can

evolve a"ter testing and %hy itFs important to supply

comments %ith those changes All o" the source code

"or all o" the pac#ages is available at(

http(


165/165

161 ,e-t Steps

14.3 Team >p

7ost real,%orld so"t%are proLects are done in teams$

there"ore learning ho% to %or# in a team is crucial I"

you can$ "ind a "riend E maybe a classmate E and team

up on a proLect earn ho% to divide a proLect into

pieces you can both %or# on simultaneously

Another option is to %or# on an open source proLect

ind a .rdparty library$ %rite some code @perhaps "ix a

bug$ and submit it to the maintainer Go has a gro%,

ing community %hich can be reached via the mailing

list @http(

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