Introduction to While Loops

Iteration is the act of repeating a mathematical or computational process. Each repeated step in iteration is also called an iteration.

While Loops

While loops are our very first iteration statements that we can incorporate into our program. It behaves very similar to an if statement; however, a while loop will repeat its code block when its condition evaluates to True.

# While Loop Format

while __boolean_condition__:

    your code here

# end of while loop

We can use while loops to complete repetitive tasks, generate ranges of values, and express algorithms.

When a while loop reaches the end of its code block, it will always re-evaluate its Boolean condition.

• If the condition is True, it will execute its code block again

• If the condition is False, it will ignore the code block and continue on with the flow of the program.

Example: Printing Factors of a Number

Code

# Example Program 1:
# Factors of a Number

num = 12
divisor = 1

while divisor <= num:
    if num % divisor == 0:
        print(divisor, 'is a factor of', num)

    divisor += 1
# end of while

Output

1 is a factor of 12
2 is a factor of 12
3 is a factor of 12
4 is a factor of 12
6 is a factor of 12
12 is a factor of 12

Code Explanation:

• We only repeat the while loop’s code block if divisor is less or equal to num.

• During every iteration, we are checking a condition

  • If the condition is True, we output the factor

• To make sure we exit the loop eventually, we are modifying divisor variable so that the while loop’s condition will evaluate to False

Python Specific: While … Else Structure

There is no elif for while loops, but we can add an else statement to make sure to execute certain code block when the while loop’s condition evaluates to False.

Code

# Example 2: while loop's condition is False right at the beginning

num = 3

while num % 2 == 0:
    print(num)
    num //= 2
else:
    print('We have reached an odd number.')

Output

We have reached an odd number.

Examine the while loop’s condition.

• Currently num is an odd number; therefore, num % 2 == 0 will never be True.

• In this situation, we go into the else statement to execute the code block

Code Example

# Example 3: Else code block executing after the while loop condition becomes False

num = 10

while num > 0:
    print(num)
    num -= 1
else:
    print('We have reached the end.')

Output

10
9
8
7
6
5
4
3
2
1
We have reached the en

• The code above has a while loop that starts with its condition evaluating to True.

• This allows the while loop to execute its code block until the condition evaluates to False.

• Once the condition becomes False, we execute the else code block as well.