Introduction to Python Programming. CSE 110: Introduction to Computer Science

Transcription

1 Introduction to Python Programming CSE 110: Introduction to Computer Science

2 Announcements Labs begin on Wednesday Labs will meet in CS 2129, NOT the CS SINC site! Homework 1 is due in class on Friday Check Blackboard for your Academic Honesty form check-off

3 What is Python? Python is a programming language A notation for describing algorithms Used to instruct a computer to do something Python is a high-level language It s much more readable than 1s and 0s Python was named for Monty Python, not the snake!

4 Why Python? Python is interpreted Get immediate feedback as you program Python has simple syntax It has very clear rules for writing code Enforces good programming style Python is free and runs everywhere

5 Getting Started Download Python from the Python Home Page: Python programs can be written live in the Python interpreter, or they can be written in a text editor and then executed as scripts We ll do both this semester The IDLE program lets us work both ways

6 Our First Program Traditionally, the first program anyone writes in a new language is Hello, world! In Python, this can be done in one line: print Hello, world!

7 Commands and Expressions print is a Python command It tells Python to do something All Python commands are lowercase Hello, world! is an expression It has a value In this case, it s a string (a sequence of characters)

8 More Expressions Numbers are also expressions 5 is a integer expression is a floating-point expression Expressions can consist of other expressions 2 * 9 is an expression Python also has Boolean expressions True and False are equivalent to 1 and 0

9 Arithmetic in Python + = addition - = subtraction * = multiplication / = division % = remainder (modulus) ** = exponentiation (raising to a power)

10 Division in Python Dividing two floating-point numbers works as you would expect 4.5 / 3 gives you 1.5 Dividing two integers gives you the integer part of the division (the quotient) 6 / 4 gives you 1, not 1.5! Use modulus (%) to get the remainder 6 / 4 gives you 2 (the remainder)

11 More on Printing print moves to the next line when done We can combine print statements Use commas to separate values: print 2, squared is, 2*2 Spaces are added between elements

12 Comments Comments are used to describe code Should tell why something is being done, not how it s being done (we can see that) Comments provide a record for the author and others who read the code later Python comments begin with a # and run to the end of the line

13 Variables Variables are names that refer to values Storing a value in a variable means you don t have to rewrite it every time To create a variable in Python, just assign it a value number_of_guesses = 5 Now, we can use number_of_guesses

14 Basic Input We can read from the keyboard using a command called raw_input() Put a string in the parentheses to serve as a prompt Save the result in a variable response = raw_input( What s your name? )

15 Loops in Python Loops are used to repeat groups of statements Python has two kinds of loops: for and while for loops execute a fixed number of times while loops execute a variable number of times

16 for Loops Basic syntax for <var> in <sequence> : statements to be repeated Note the indentation! Python uses indentation (spaces) to indicate that several statements are part of a group (block)

17 Sequences A sequence is just a collection of values Ex. characters in a string A for loop sets its loop variable to each value in turn The loop executes one time for each value in the sequence

18 Sequences of Integers Python uses the range() command to generate a list of integer values List does not include supplied end value range() takes 1, 2, or 3 arguments range(3) produces [0, 1, 2] range(1, 5) produces [1, 2, 3, 4] range(1,6,2) produces [1,3,5]

19 for Loop Example # Print the characters in a string for i in This is an ex-parrot : print i

20 Another Example # Print the values 1-9, all on the same line for q in range(1, 10): print q, # Print the next value in the range

21 Putting It All Together We can combine variables and loops: # A simple countdown max = 10 for i in range(max, 0, -1) : # count down by 1s print i

22 Next Time while loops conditional (if) statements Read through Chapter 4 of Dawson