L3.10: Tutorial on for loop and difference between while loop and for loop

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Learning outcomes

• choose between while and for correctly
• convert loop logic from one form to the other
• explain control and readability tradeoffs
• debug loop boundary and update issues

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Core comparison table

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Same task in both styles

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Task: print 1 to 5

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

for i in range(1, 6):
    print(i)

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Converting while -> for

i = 2
while i <= 10:
    print(i)
    i += 2

for i in range(2, 11, 2):
    print(i)

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Converting for -> while

for i in range(3, 12, 3):
    print(i)

i = 3
while i < 12:
    print(i)
    i += 3

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Decision rule (exam-safe)

• iterating over string/list/tuple/set/dict
• running fixed number of times

• loop depends on runtime condition (e.g., keep asking until valid input)
• stopping point is event-based, not count-based

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Typical exam traps

• writing range(1, n) when n must be included
• forgetting update in while
• using while where iterable traversal is clearer with for

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Mini debug exercise

i = 1
while i < 10:
    if i % 2 == 0:
        continue
    print(i)
    i += 1

• infinite loop when i becomes 2 (update skipped by continue)

i = 1
while i < 10:
    if i % 2 == 0:
        i += 1
        continue
    print(i)
    i += 1

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Practice

1. Write sum of 1..n in both while and for styles.
2. Print each character of "college" using for and then while.
3. Convert this while loop to for loop:

i = 10
while i >= 0:
    print(i)
    i -= 2

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Answer snippets

for i in range(10, -1, -2):
    print(i)