The ‘While’ Loop

The while statement executes a block of code repeatedly as long as the control condition of the loop is true. Unlike the for loop, which walks a known sequence, a while loop keeps going purely on the strength of its condition.

How a while loop executes

• The control condition is evaluated before any statement inside the loop runs.
• After each iteration, the condition is tested again; the loop continues while it remains true.
• When the condition becomes false, the body is not executed and control transfers to the statement immediately following the body of the while loop.
• If the condition is false at the very start, the body is not executed even once.
• The statements inside the body must ensure the condition eventually becomes false; otherwise the loop becomes an infinite loop — a logical error in the program.

The flowchart of the while loop is shown in Figure 6.5: initialisation, then the test expression, then (if true) the body, looping back to the test; when the test is false, control moves to the statements following the loop.

Syntax

while test_condition:
    body of while

The body of the loop is indented with respect to the while statement.

Counting with a while loop

The first five natural numbers, printed with a control variable that is initialised, tested and incremented explicitly:

# Print first 5 natural numbers using a while loop
n = 1
while n <= 5:
    print(n)
    n += 1

1
2
3
4
5

Trace the pattern: n starts at 1 (initialisation), n <= 5 is the test, and n += 1 inside the body is what guarantees the condition eventually turns false. Remove that increment and this becomes an infinite loop.

Finding the factors of a number

A while loop works naturally when candidate values are tried one after another. A factor of a whole number divides it exactly (remainder 0 with the % operator). Since 1 divides every number and every number divides itself, those two factors can be printed without testing; only the candidates from 2 up to half the number need checking — no factor other than the number itself can exceed its half:

# Find the factors of a whole number
number = int(input("Enter a number to find its factor: "))
print(1, end=' ')            # 1 is a factor of every number
trial = 2
while trial <= number / 2:
    if number % trial == 0:
        # end=' ' makes print use a blank space as the delimiter,
        # so the next value appears on the same line
        print(trial, end=' ')
    trial += 1 …