L3.2: Introduction to while loop

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Learning outcomes

• write correct while loop syntax
• explain execution flow line by line
• avoid infinite-loop mistakes
• solve simple counting tasks with while

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Syntax

while condition:
    # loop body

• loop runs while condition is True
• stops when condition becomes False

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Execution flow

1. evaluate condition
2. if True, run body
3. update loop variable
4. return to step 1
5. if False, exit loop

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Counting example

i = 1
while i <= 5:
    print(i)
    i += 1

1
2
3
4
5

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Dry-run table

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Reverse counting example

i = 5
while i >= 1:
    print(i)
    i -= 1

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Infinite loop trap

i = 1
while i <= 5:
    print(i)

• i never changes, condition always True

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while with boolean input loop

ans = "yes"
while ans == "yes":
    print("Running task...")
    ans = input("Continue? yes/no: ").strip().lower()

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Exam hints and traps

• update must move variable toward stop condition
• indentation errors can move update outside loop
• while True needs explicit break inside

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Practice

1. Print even numbers from 2 to 20.
2. Print numbers from 10 to 1.
3. Find error:

i = 1
while i < 5:
print(i)
    i += 1

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Answer key

i = 2
while i <= 20:
    print(i)
    i += 2

i = 10
while i >= 1:
    print(i)
    i -= 1

3. Indentation error (print(i) must be indented in loop block).