Writing Simple Programs

Chapter Two

# convert.py# A program to convert Celsius temps to Fahrenheit# by: Susan Computewell

def main(): celsius = input("What is the Celsius temperature? ") fahrenheit = 9.0 / 5.0 * celsius + 32 print "The temperature is", fahrenheit, "degrees Fahrenheit."

main()

Note that indentation defines statements in main()

Change the convert.py program so it converts dollars to Euros.

Find the current rate online Work in pairs

All names are called identifiers. Python rules: all identifier smust begin with

a letter or underscore (_) Identifiers are case sensitive Choose identifiers that describe what is

being named Reserved words cannot be used as

identifiers

and 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 while yield

Expressions are code fragments which manipulate data

When you “hard code” data or characters into your code, you are creating “literals”

Name Errors are produced when Python can’t find a value associated with an identifer

Spaces don’t matter inside expressions. Arithmetic operators: + - * / ** (the last

one is exponentiation)

Using the Python interpreter, create a variable called subtotal equal to 118.45

Print subtotal Write an expression which adds a tax of 8%

to subtotal Ask the interpreter to print a variable called

item_cost

The print command displays output on the screen

Try these examples: print by itself produces a blank line Separating values with a comma means you

can print multiple values : print 8, 9, 10 You can print expressions: print (8 * 8) / 2 You can also print string literals (characters

between quotations which the programmer hard codes): print “The area of a circle is “, 4 * 4 * 3.14

<variable> = <expr> Example: euro = dollar * 1.39 Assignment is ALWAYS right to left. In other

words, the value on the right is “loaded into” the variable on the left.

You can assign a new value to an existing variable. newVar = 21 newVar = 55

When you assign a new value to a variable, the old value is still in memory, just without reference.

newVar

21

newVar

21

55

Python makes getting user input really easy. The reserved word input gets data from the user and loads it into a variable.

dollars = input(“Please enter the dollar amount you want converted to euros: ”)

Python evaluates the prompt and displays it on screen. Python pauses and waits for user input.

Python allows you to simultaneously assign several variables—it’s a shortcut

tax, subtotal, total = .08, 0, 0 Simultaneous assignment can use

expressions as well as literals:sale_price, total = unit_price * .75, subtotal * .08

# avg2.py# A simple program to average two exam scores. # Illustrates use of multiple input.

def main(): print "This program computes the average of two exam scores." print

score1, score2 = input("Enter two scores separated by a comma: ") average = (score1 + score2) / 2.0

print "The average of the scores is:", average

main()

A definite loop just repeats a number of actions (or statements) a definite number of times. Python knows when to stop the loop (because you tell it so)

This is also called iteration, and a definite loop is an iterative loop or iterative control structure

For <var> in <sequence>:<expr><expr>…

for i in range(10):x = 3.9 * x * (1-x)print x

range(10) is a built-in Python command which controls the number of times the loop is executed. It will start at 0 and quit after 9 (always by integers)

Loop index; changes value each time the body statements are executedBody

The command range(10) is really a convenient shorthand for a list of values: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

The definite loop changes the value of the loop index to match the progression of values in the sequence.

For x in [7, 14, 21, 28]:print x

And the screen result will be:7142128

Have the Python Interpreter print the squares of the odd numbers between 0 and 20, using a definite loop

# futval.py# A program to compute the value of an investment# carried 10 years into the future

def main(): print "This program calculates the future value of a 10-year investment."

principal = input("Enter the initial principal: ") apr = input("Enter the annual interest rate: ")

for i in range(10): principal = principal * (1 + apr)

print "The value in 10 years is:", principal

main()

Alter the futval.py program so that it displays the value of the investment each year

Alter the futval.py program so that the user controls the number of years the investment will accrue