Last modified on 2 March 2014, at 13:18

Non-Programmer's Tutorial for Python 2.6/Decisions

If statementEdit

As always I believe I should start each chapter with a warm-up typing exercise, so here is a short program to compute the absolute value of a number:

n = input("Number? ")
if n < 0:
   print "The absolute value of", n, "is", -n
else:
   print "The absolute value of", n, "is", n

Here is the output from the two times that I ran this program:

Number? -34
The absolute value of -34 is 34
Number? 1
The absolute value of 1 is 1

So what does the computer do when it sees this piece of code? First it prompts the user for a number with the statement "n = input("Number? ")". Next it reads the line "if n < 0:". If n is less than zero Python runs the line "print "The absolute value of", n, "is", -n". Otherwise it runs the line "print "The absolute value of", n, "is", n".

More formally Python looks at whether the expression n < 0 is true or false. An if statement is followed by an indented block of statements that are run when the expression is true. Optionally after the if statement is an else statement and another indented block of statements. This second block of statements is run if the expression is false.

There are a number of different tests that an expression can have. Here is a table of all of them:

operator function
< less than
<= less than or equal to
> greater than
>= greater than or equal to
== equal
!= not equal
<> another way to say not equal (old style, not recommended)

Another feature of the if command is the elif statement. It stands for else if and means if the original if statement is false but the elif part is true, then do the elif part. And if neither the if or elif expressions are true, then do what's in the else block. Here's an example:

a = 0
while a < 10:
    a = a + 1
    if a > 5:
        print a, ">", 5
    elif a <= 7:
        print a, "<=", 7
    else:
        print "Neither test was true"

and the output:

1 <= 7
2 <= 7
3 <= 7
4 <= 7
5 <= 7
6 > 5
7 > 5
8 > 5
9 > 5
10 > 5

Notice how the elif a <= 7 is only tested when the if statement fails to be true. There can be more than one elif expression, allowing multiple tests to be done in a single if statement.

ExamplesEdit

# This Program Demonstrates the use of the == operator
# using numbers
print 5 == 6
# Using variables
x = 5
y = 8
print x == y

And the output

False
False

High_low.py

# Plays the guessing game higher or lower 
 
# This should actually be something that is semi random like the
# last digits of the time or something else, but that will have to
# wait till a later chapter.  (Extra Credit, modify it to be random
# after the Modules chapter)
number = 78
guess = 0
 
while guess != number: 
    guess = input("Guess a number: ")
    if guess > number:
        print "Too high"
    elif guess < number:
        print "Too low"
 
print "Just right"

Sample run:

Guess a number: 100
Too high
Guess a number: 50
Too low
Guess a number: 75
Too low
Guess a number: 87
Too high
Guess a number: 81
Too high
Guess a number: 78
Just right

even.py

# Asks for a number.
# Prints if it is even or odd
 
number = input("Tell me a number: ")
if number % 2 == 0:
    print number, "is even."
elif number % 2 == 1:
    print number, "is odd."
else:
    print number, "is very strange."

Sample runs:

Tell me a number: 3
3 is odd.
Tell me a number: 2
2 is even.
Tell me a number: 3.14159
3.14159 is very strange.

average1.py

# keeps asking for numbers until 0 is entered.
# Prints the average value.
 
count = 0
sum = 0.0
number = 1 # set to something that will not exit the while loop immediately.
 
print "Enter 0 to exit the loop"
 
while number != 0:
    number = input("Enter a number: ")
    if number != 0:
        count = count + 1
        sum = sum + number
 
print "The average was:", sum / count

Sample runs:

Enter 0 to exit the loop
Enter a number: 3
Enter a number: 5
Enter a number: 0
The average was: 4.0
Enter 0 to exit the loop
Enter a number: 1
Enter a number: 4
Enter a number: 3
Enter a number: 0
The average was: 2.66666666667

average2.py

# keeps asking for numbers until count numbers have been entered.
# Prints the average value.
 
sum = 0.0
 
print "This program will take several numbers then average them"
count = input("How many numbers would you like to average: ")
current_count = 0
 
while current_count < count:
    current_count = current_count + 1
    print "Number", current_count
    number = input("Enter a number: ")
    sum = sum + number
 
print "The average was:", sum / count

Sample runs:

This program will take several numbers then average them
How many numbers would you like to average: 2
Number 1
Enter a number: 3
Number 2
Enter a number: 5
The average was: 4.0
This program will take several numbers then average them
How many numbers would you like to average: 3
Number 1
Enter a number: 1
Number 2
Enter a number: 4
Number 3
Enter a number: 3
The average was: 2.66666666667

ExercisesEdit

  1. Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated." Note that the program does not have to quit asking for the number before it is guessed, it just has to print this after the number is guessed.
  2. Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."
  3. Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name."
Solution

Modify the higher or lower program from this section to keep track of how many times the user has entered the wrong number. If it is more than 3 times, print "That must have been complicated."

number = 42
guess = 0
count = 0
while guess != number:
    count = count + 1
    guess = input('Guess a number: ')
    if guess > number:
        print 'Too high'
    elif guess < number:
        print 'Too low'
    else:
        print 'Just right'
        break
    if count > 2:
        print 'That must have been complicated.'
        break

Write a program that asks for two numbers. If the sum of the numbers is greater than 100, print "That is a big number."

number1 = input('1st number: ')
number2 = input('2nd number: ')
if number1 + number2 > 100:
    print 'That is a big number.'

Write a program that asks the user their name, if they enter your name say "That is a nice name", if they enter "John Cleese" or "Michael Palin", tell them how you feel about them ;), otherwise tell them "You have a nice name."

name = raw_input('Your name: ')
if name == 'Ada':
    print 'That is a nice name.'
elif name == 'John Cleese' or name == 'Michael Palin':
    print '... some funny text.'
else:
    print 'You have a nice name.'