CSCE 110 Programming I Basics of Python: Variables, Expressions, and Input/Output

CSCE 110 — Programming I

Basics of Python: Variables, Expressions, and

Input/Output

Dr. Tiffani L. Williams

Department of Computer Science and Engineering Texas A&M University

Python

I Python was developed in 1989 by Guido van Rossum in the Netherlands.

I Python was released for public distribution in early 1991. I How did Python begin?

I _{van Rossum was having a hard time getting the job done with}

the existing tools available.

I _{He envisioned that there was an easier way to get things done.}

I While Python has been around for over 15 years, Python is still relatively new to general software development. However, it has a lot of a lot of support from the community and new users are finding themselves programming in Python (and loving it) everyday.

Be an Explorer

I Since you are learning a new language, it is very important that you experiment with the material.

I Be curious about trying different things.

I Don’t be afraid to make a mistake or crash your computer. That’s how we learn best.

Python Distribution Used in this Course

I In my opinion, the easiest way to get Python (and the most popular modules) is to use the Enthought Python Distribution (EPD).

I _{It’s free for academic use!}

I _{It’s available for the major operating systems: Windows, Mac,}

and Linux

I _{It’s what’s installed on the lab machines.}

I _{Here’s the URL if you want to install it on your personal}

computer.

http://www.enthought.com/products/edudownload.php I _{At the time of this writing, EPD uses version 2.7 of Python.}

We will not cover any nuances associated with Python 3.x in this course.

WingWare IDE 101

I _{Now that we have a Python distribution, we need a way to} type in our code.

I _{We will use WingWare IDE 101 in this course.}

I _{It’s free for academic use!}

I _{It’s available for the major operating systems: Windows, Mac,}

and Linux

I _{It’s what’s installed on the lab machines.}

I _{Here’s the URL if you want to install it on your personal}

computer.

http://wingware.com/downloads/wingide-101/4.0.3-1/ binaries

Now, we are ready to start programming!

I We will take it slow and easy in the beginning. I want you to get comfortable getting acquainted with the basics.

I _{Following along in class is not enough.}

I _{You MUST type in the programs on your computer and see}

how they work.

I _{You MUST type in the programs so that if you make a mistake}

you can learn how to make the appropriate correction.

I _{You MUST type in the programs because programming}

Interactive Execution in the

Python Shell

“Hello, World!”

I Python’s print statement is the tool for displaying program output to your users.

I Type the following in the command-line editor.

Using Python as a Calculator

>‌>‌> 3 + 4 >‌>‌> 15 / 3 >‌>‌> 12 * 10 + 4

Operators

I Mathematical operators: +, -, *, /, %, ** I _{addition (+)} I _{subtraction (-)} I _{multiplication (*)} I _{division (/)} I _{modulus or remainder (%)} I _{exponenentiation (**)} I Comparision operators: <, <=, >, >=, ==, !=

I _{strictly less than (<)} I _{less than or equal to (<=)} I _{strictly greater than (>)} I _{greater than or equal to (>=)} I _{equal to (==)}

I _{not equal to (!=)}

Variables and Assignment

>‌>‌> counter = 0 >‌>‌> miles = 1000.0 >‌>‌> name = ’Bob’ >‌>‌> counter = counter + 1 >‌>‌> kilometers = 1.609 * miles >‌>‌> print miles 1000.0 >‌>‌> print counter 1 >‌>‌> print kilometers 1609.0

Variables

I Variables are the set of valid strings that are allowed as names in a computer language such as Python.

I The rules for forming Python variables are:

I _{First character must be a letter or underscore (_)}

I _{Any additional characters can be alphanumeric or underscore} I _{Case-sensitive}

I No variables can begin with a number.

I No symbols other than alphanumerics or underscores are ever allowed.

I No variable can be the same as keywords, which form the foundation of the language.

Keywords

and as assert break

class continue def del

elif else except exec

finally for from global

if import in is

lambda not or pass

print raise return try

Numbers

I Python supports five basic numerical types.

I _{int(signed integers)} I _{long(long integers)} I _{bool(Boolean values)}

I _{float(floating point real numbers)} I _{complex(complex numbers)}

I Here are some examples.

I _{int: 100 200 -437}

I _{long: -84140l 299556678883L} I _{bool: True False}

I _{float: 3.456 -33.55 34.1}

Strings

I Strings are a contiguous set of characters in between quotation marks.

I _{Strings are immutable sequences.}

I _{A program can refer to elements or subsequences of strings.}

However, strings cannot be modified in place.

I Python allows for either pairs of single or double quotes. Triple quotes (three consecutive single or double quotes) can be used to escape special characters.

I Subsets of strings can be taken using the index ( [ ] ) and slice ( [ : ] ) operators, which work with indexes starting at 0 in the beginning of the string and working their way from -1 at the end.

I _{Slice ([]) gives the character from the given index or location} I _{Range slice ([x:y]) gives the characters starting at index x}

and ending at index y -1

I The plus (+ ) sign is the string concatenation operator. I The asterisk ( * ) is the repetition operator.

String Examples

>‌>‌> string = "Texas A&M University" >‌>‌> string[0] ’T’ >‌>‌> string[2:5] ’xas’ >‌>‌> string[4:] ’s A&M University’ >‌>‌> string[:6] ’Texas ’ >‌>‌> string * 2

’Texas A&M UniversityTexas A&M University’ >‌>‌> string = ’It\’s a girl!’

>‌>‌> string "It’s a girl!"

User Input

1. Type the following text directly into your editor.

>‌>‌> name = raw_input("Please enter your name: ") >‌>‌> print "Hello", name, "- good to see you!"

2. Type the above text into a file called hello.py and save it as hello.py. Afterward, run the program.

3. Discuss the difference between the two different approaches for entering Python programs.

Write the Following Python Programs

1. Full name greeting. Write a program that asks for a person’s first name, then middle, and then last. Afterwards, it should greet the person using their full name.

2. Guessing game? Write a program that asks a person to guess a number. Add 1 to the guessed number, and then suggest that their guess was close to being correct.

3. Guessing game (version 2)? Write a program that asks a person to guess a number. Tell the user that their guess was off by a random amount.