Loops

Computational Thinking made a promise: anything that repeats is a loop waiting to be written. You have already been one — executing the same steps over and over in Human Robot. Copy-pasting a line five times works too, until “five” becomes “five hundred”, or “until they get it right”. Loops let a program repeat without you repeating.

for — when you can count the laps

for number in range(1, 6):
    print(number, "times 7 is", number * 7)

for runs its block once per item — here, once for each number from 1 through 5. Use it when the number of repetitions is knowable before the loop starts: every question in a quiz, every line in a file.

while — when you cannot

answer = ""
while answer != "yes":
    answer = input("Are we there yet? ")
print("Finally.")

while keeps going as long as its condition stays true. You cannot know in advance how many tries a player will need in Guess My Number — that is while territory. The danger is a condition that never turns false: the infinite loop. When a program hangs, that is the first suspect.

Choosing between them

One question settles it: do I know the lap count before the loop begins? Known count → for. Repeat until something changes → while.

Off by one

The most famous loop bug is being wrong by exactly one lap.

Self-check: how many lines does this print?

for count in range(3, 7):
    print(count)

Four — 3, 4, 5, 6. range includes its first number and stops before its second. Fence-post errors like this are why Predict the Output exists: it is cheaper to be wrong out loud than in code.

When loops start feeling natural, Loops Practice has the reps — and The Quiz Machine is where they begin earning their keep.

Curriculum connection

C1.5

identify and explain situations in which conditional and repeating structures are required

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C2.4

write programs that include sequential, selection, and repeating events

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