While Statements

Note

Source: Adapted from the C# edition (while/whilestatements.rst). Tea-cooling and countdown examples are Python translations of the original. The while True / break idiom replaces C#’s do-while.

We have seen that programs can branch with if statements and call functions. The last essential control structure is repetition — executing a block of code over and over. The simplest loop in Python is the while loop.

If you heard someone say “While your tea is too hot, add a chip of ice,” you would understand intuitively: test, act, test again, act again, and stop when the condition is no longer true. Python’s syntax works the same way:

while condition:
    statement(s)

The condition is tested before each iteration. If it is True, the body executes; then the condition is tested again. When the condition becomes False, the loop exits and execution continues after the loop.

The Cooling Tea Example

Suppose tea starts at 115 °F and we want it at 112 °F; each chip of ice lowers the temperature by one degree:

temperature = 115
while temperature > 112:
    print(f"Temperature: {temperature}°F — adding ice.")
    temperature -= 1
print(f"Tea is ready at {temperature}°F.")

Output:

Temperature: 115°F — adding ice.
Temperature: 114°F — adding ice.
Temperature: 113°F — adding ice.
Tea is ready at 112°F.

Tracing a Loop

To understand a loop, trace through it by tracking the variable before each test:

temperature	condition > 112	Action
115	True	body
114	True	body
113	True	body
112	False	exit

Countdown Example

count = 5
while count > 0:
    print(count)
    count -= 1
print("Blastoff!")

Output:

5
4
3
2
1
Blastoff!

Infinite Loops

If the condition never becomes False, the loop runs forever. This is almost always a bug:

# BUG: count never changes
count = 5
while count > 0:
    print(count)   # loops forever

Press Ctrl-C to interrupt a runaway loop in the terminal.

while True and break

Python has no do-while statement. The idiomatic replacement is while True: with an explicit break to exit when done:

while True:
    answer = input("Type 'yes' to continue: ")
    if answer == "yes":
        break
    print("Please type 'yes'.")
print("Continuing...")

break immediately exits the innermost loop.

continue

continue skips the rest of the current iteration and jumps back to test the condition:

i = 0
while i < 10:
    i += 1
    if i % 2 == 0:
        continue      # skip even numbers
    print(i)          # prints 1 3 5 7 9

Loop Planning Rubric

When designing a while loop, answer these questions:

1. What changes each iteration? Identify the loop-driving variable(s).

2. What is the initial value? Set it before the loop.

3. What is the continuation condition? The loop runs while this is True.

4. How does the body update the variable? Ensure the condition eventually becomes False.

5. What happens after the loop? Handle any cleanup or output.