A quick Ruby Tutorial

COMP313

Source: Programming Ruby, The Pragmatic Programmers’ Guide by Dave Thomas, Chad

Fowler, and Andy Hunt

Ruby

• Invented by Yukihiro Matsumoto, “Matz”

• 1995

• Fully object-oriented

• alternative to Perl or Python

How to run

• Command line: ruby <file>

• Interactive: irb

• Resources: – see web page– man ruby

Simple method example

def sum (n1, n2) n1 + n2end

sum( 3 , 4) => 7sum(“cat”, “dog”) => “catdog”

load “fact.rb”fact(10) => 3628800

executable shell script

#!/usr/bin/ruby

print “hello world\n”

or better: #!/usr/bin/env ruby

make file executable: chmod +x file.rb

Method calls

"gin joint".length → 9

"Rick".index("c") → 2

-1942.abs → 1942

Strings: ‘as\n’ => “as\\n”

x = 3

y = “x is #{x}” => “x is 3”

Another method definition

def say_goodnight(name)

"Good night, #{name}"

end

puts say_goodnight('Ma')

produces: Good night, Ma

Name conventions/rules

• local var: start with lowercase or _• global var: start with $• instance var: start with @• class var: start with @@• class names, constant: start uppercasefollowing: any letter/digit/_ multiwords

either _ (instance) or mixed case (class), methods may end in ?,!,=

Naming examples

• local: name, fish_and_chips, _26

• global: $debug, $_, $plan9

• instance: @name, @point_1

• class: @@total, @@SINGLE,

• constant/class: PI, MyClass, FeetPerMile

Arrays and Hashes

• indexed collections, grow as needed• array: index/key is 0-based integer• hash: index/key is any object

a = [ 1, 2, “3”, “hello”]

a[0] => 1 a[2] => “3”

a[5] => nil (ala null in Java, but proper Object)

a[6] = 1

a => [1, 2, “3”, “hello”, nil, nil, 1]

Hash examples

inst = { “a” => 1, “b” => 2 }

inst[“a”] => 1

inst[“c”] => nil

inst = Hash.new(0) #explicit new with default 0 instead of nil for empty slots

inst[“a”] => 0

inst[“a”] += 1

inst[“a”] => 1

Control: if example

if count > 10

puts "Try again"

elsif tries == 3

puts "You lose"

else

puts "Enter a number"

end

Control: while example

while weight < 100 and num_pallets <= 30

pallet = next_pallet()

weight += pallet.weight

num_pallets += 1

end

nil is treated as false

while line = gets

puts line.downcase

end

Statement modifiers

• useful for single statement if or while• similar to Perl• statement followed by condition:

puts "Danger" if radiation > 3000

square = 2

square = square*square while square < 1000

builtin support for Regular expressions

/Perl|Python/ matches Perl or Python/ab*c/ matches one a, zero or more bs and one c/ab+c/ matches one a, one or more bs and one c/\s/ matches any white space/\d/ matches any digit/\w/ characters in typical words/./ any character (more later)

more on regexps

if line =~ /Perl|Python/

puts "Scripting language mentioned: #{line}"

end

line.sub(/Perl/, 'Ruby') # replace first 'Perl' with 'Ruby'

line.gsub(/Python/, 'Ruby') # replace every 'Python' with 'Ruby’

# replace all occurances of both ‘Perl’ and ‘Python’ with ‘Ruby’

line.gsub(/Perl|Python/, 'Ruby')

Code blocks and yield

def call_block puts "Start of method" yield yield puts "End of method" end

call_block { puts "In the block" } produces: Start of method In the block In the block End of method

parameters for yield

def call_block yield("hello",2)end

then

call_block { | s, n | puts s*n, "\n" } prints hellohello

Code blocks for iteration

animals = %w( ant bee cat dog elk ) # create an array

# shortcut for animals = {“ant”,”bee”,”cat”,”dog”,”elk”}

animals.each {|animal| puts animal } # iterate produces: ant

bee

cat

dog

elk

Implement “each” with “yield”

# within class Array...

def each

for every element # <-- not valid Ruby

yield(element)

end

end

More iterations

[ 'cat', 'dog', 'horse' ].each {|name| print name, " " }

5.times { print "*" }

3.upto(6) {|i| print i }

('a'..'e').each {|char| print char } [1,2,3].find { |x| x > 1}

(1...10).find_all { |x| x < 3}

I/O

• puts and print, and C-like printf:printf("Number: %5.2f,\nString: %s\n", 1.23,

"hello") #produces: Number: 1.23, String: hello #inputline = gets print line

leaving the Perl legacy behind

while gets if /Ruby/ print end end

ARGF.each {|line| print line if line =~ /Ruby/ }

print ARGF.grep(/Ruby/)

Classes

class Song def initialize(name, artist, duration) @name = name @artist = artist @duration = duration end end

song = Song.new("Bicylops", "Fleck", 260)

override to_s

s.to_s => "#<Song:0x2282d0>"

class Song def to_s "Song: #@name--#@artist (#@duration)" endend

s.to_s => "Song: Bicylops--Fleck (260 )”

Some subclass

class KaraokeSong < Song def initialize(name, artist, duration, lyrics) super(name, artist, duration) @lyrics = lyrics end endsong = KaraokeSong.new("My Way", "Sinatra",

225, "And now, the...") song.to_s → "Song: My Way--Sinatra (225)"

supply to_s for subclass

class KaraokeSong < Song

# Format as a string by appending lyrics to parent's to_s value.

def to_s

super + " [#@lyrics]"

end

end

song.to_s → "Song: My Way--Sinatra (225) [And now, the...]"

Accessors

class Song

def name

@name

end

end

s.name => “My Way”

# simpler way to achieve the same

class Song

attr_reader :name, :artist, :duration

end