think python chapter 1

8/10/2019 Think Python Chapter 1

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Think Python, Chapter 1

CS 1400: Fundamentals of Computing Prof. Cindy Marling 1

Think Python:How to Think Like a Computer Scientist

Allen B. Downey

http://www.greenteapress.com/thinkpython/thinkpython.pdf


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Chapter 1: The Way of the Program

This book aims to teach you to think like a computer scientist These thought processes draw from mathematics, engineering, and

natural sciences

Problem solving is the most important skill

Computer scientists must formulate problems, find creativesolutions to problems, and express these solutions clearly

Writing computer programs requires problem solving Programming is a useful practical skill

Programming helps people become better problem solvers



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The Python Programming Language

Python is a high-level programming language Most computer programs are written in high-level

programming languages and then converted into low-level forms that computers can execute

This conversion is accomplished by other computerprograms, interpretersand compilers

Python is an interpreted language An interpreter reads a Python program and does

what the program says

It processes the program a little at a time,executing the lines of the program as it readsthem



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More on Python

Languages can be interpreted, like Python, or compiled A compiler reads and converts a whole program before it starts running

Interpreters allow more flexibility, while compilers optimize speed

Python can be used in interactive mode or in script mode

In interactive mode, you type Python programs directly into theinterpreter, and the interpreter shows the results

>>> 1 + 1

2 The interpreter displays a prompt, >>>, to show that it is ready to

process what you type



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Pythons Script Mode

When programs grow longer than a few lines, it is handy to keep themin files, called scripts

You can open a script in the interpreter to use it without typing it all in

Our Python environment, IDLE, has a Filedrop-down menu, whichallows you to Opena Python script, much as you would open a file inMicrosoft Word (or the software package of your choice)

It has a Rundrop-down menu, with a Run Module option, that tells theinterpreter to execute the script that you opened

This will become Extremely Clear shortly!



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What is a Program?

Aprogramis a sequence of instructions that specifies how to perform acomputation

In other words, when you write a program, you tell the computerwhat to do

The computer can do mathematical computations, computationsthat create and display presentation slides (like these),

computations that play music, plot maps, or whatever else you canfigure out how to tell it to do!



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Basic Instructions in Computer Programs

Different programming languages have different details, but the basicinstructions are similar in all of them. These are:

Input: Get data from a computer keyboard, file, or other device

Output: Display data on a screen, or send it to a file or device

Math: Perform operations like addition and subtraction

Conditional Execution: Check for certain conditions and execute the

code that applies to the current condition Example: If severe weather is expected, display an alert on a cell

phone. (But dont display alerts on bright, sunny days.)

Repetition: Perform some instruction repeatedly, usually with somevariation

Example: Send a welcome message to every new freshman, but

vary the email address for each message



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What is Debugging?

Debuggingis the process of finding and fixing errors in programs It would be great if we all wrote error-free code, but even the bestprogrammers run up against bugs!

The first computer bug was an actual moth, found in a US Navycomputer by Admiral Grace Hoppers team, in 1947. This moth is in theSmithsonian Institutions National Museum of American History.



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Three Kinds of Bugs

Syntax Errors are mistakes in the form of the program

These are like spelling or punctuation mistakes in essays

The interpreter will display an error message instead of runningyour program

Runtime Errors are exceptions that occur after the program starts

running The syntax was OK, but perhaps the program cant access a

resource it needs, like a file or a database

The program will start running, but it will be unable to finish andwill crash

Semantic Errors occur when the program gives the wrong answer or

does something it shouldnt do The computer does what you say, not what you mean

The program will run to completion, but you will not get theresults you want



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The Debugging Process

Debugging is something like detective work Look for clues to figure out why the program doesnt work

Debugging is something like experimental science

Develop a hypothesis about what went wrong, modify the program,and try again

If the program still doesnt work, come up with a new hypothesis,

and start again Debugging may seem very different from other intellectual pursuits

It can bring out strong emotions

It is natural for people get frustrated, angry, sad, or embarrassed

The computer may seem like an opponent, rather than a tool

When this happens: Try to stay calm (Take a break, if you have to)

Practice your problem-solving skills

Experience satisfaction once bugs are found and fixed



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Formal Languages vs. Natural Languages

Natural languages are the ones people naturally speak, like English andChinese

Formal languages are designed by people for specific purposes, likenotations for expressing mathematical formulas or representations ofthe chemical structure of molecules

Programming languages are formal languages designed to express

computations Formal languages have strict rules about syntax

3 + 3 = 6 and H20 are syntactically valid

3 + = @ 6 and 2HO are not

Syntaxincludes tokens(basic elements) and structure(arrangement)

3, +, =, and 6 are valid mathematical tokens, but @ is not

2HO has invalid structure, because subscripts must come after, notbefore, the names of elements in chemical formulas

Figuring out the structure of a statement, in a formal language, or asentence, in a natural language, is calledparsing



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More on Language Differences

People who grew up speaking natural languages (all of us!) may havetrouble adjusting to formal languages

Here are some differences to keep in mind

Ambiguity: Natural language has ambiguity, which people deal withthrough context. Example: Ill get it. Ill understand it? Ill obtain it?Ill answer it? In a formal language, a statement must have only one

meaning. Redundancy: To make up for ambiguity, natural languages mayemploy redundancy, or be verbose. Formal languages are moreconcise. It takes longer to read and understand a program than thesame amount of text in a natural language.

Literalness: Natural languages use idiom and metaphor. Examples:Raining cats and dogs; Costs an arm and a leg. Formal languagesmean exactly what they say.



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The First Program

Traditionally, the first program you write in any language is the Hello,World! program

This program writes the words Hello, World! to the screen

Here it is in Python:

>>> print 'Hello, World!'

Hello, World!



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About the Glossary

There is a glossary of important vocabulary terms at the end of eachchapter in Think Python

Be sure to review the glossary!

A common vocabulary helps us to communicate more effectively

Terms from the glossary will appear on quizzes and exams


think python chapter 1

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