Repetition

The Hundred-Line Problem was not a trick. Everybody genuinely started copying and pasting the same print line, and everybody stopped somewhere around the twelfth copy, because by then the mistake was obvious: the program does not contain a hundred ideas. It contains one idea, a hundred times. A loop is how you say that.

Counting laps in advance

for day in range(1, 8):
    print(f"Day {day}: warm-up, drills, scrimmage")

Day 1: warm-up, drills, scrimmage
Day 2: warm-up, drills, scrimmage
Day 3: warm-up, drills, scrimmage
Day 4: warm-up, drills, scrimmage
Day 5: warm-up, drills, scrimmage
Day 6: warm-up, drills, scrimmage
Day 7: warm-up, drills, scrimmage

range(1, 8) produces 1 up to but not including 8 — seven numbers. That exclusive ending is deliberate (it makes range(len(items)) line up with list positions) and it is also the most reliable source of off-by-one bugs in the language. When a loop runs one time too few, suspect range before you suspect anything else.

Use for when the number of repetitions is knowable before the loop starts: every day of a week, every mark in a list, every line in a file.

Repeating until something changes

answer = ""
while answer != "yes":
    answer = input("Is the form in yet? ")

while re-checks its condition before every pass and keeps going while it is True. You cannot know in advance how many times somebody will mistype a number, so validating input is while territory — that is exactly how Looping Programs refuses to start until it has a sensible number of days.

The loop that never ends

A while loop whose condition can never become False will run until you stop it. The usual cause is forgetting to change the variable the condition depends on:

count = 3
while count > 0:
    print(count)

count is never reduced, so this prints 3 forever. Press Ctrl + C in the terminal to interrupt it — then look for what the loop was supposed to be changing.

The accumulator pattern

Most useful loops are not printing; they are building up an answer in a variable that lives outside the loop.

minutes = [45, 0, 60, 30, 0, 90, 25]
 
total = 0
for session in minutes:
    total = total + session
 
print(f"Total: {total} minutes over {len(minutes)} days")

Total: 250 minutes over 7 days

Three lines carry the whole pattern: start the accumulator at a value that means “nothing yet”, update it once per pass, and use it after the loop. Change the starting value and the update, and the same skeleton finds a highest value, counts how many items match a condition, or builds a sentence. It is the reason a pile of numbers can become something a person can act on.

Loops inside loops

Once you have met Lists, loops start containing loops — one pass per week on the outside, one pass per day on the inside:

weeks = []
weeks.append([45, 0, 60, 30, 0, 90, 25])
weeks.append([30, 30, 60, 0, 45, 60, 0])
 
for week_number in range(len(weeks)):
    total = 0
    for minutes in weeks[week_number]:
        total = total + minutes
    print(f"Week {week_number + 1}: {total} minutes")

Week 1: 250 minutes
Week 2: 225 minutes

Notice where total = 0 sits. Inside the outer loop, it resets each week, which is what you want. Move it above the outer loop and you get a running total across all weeks — also a legitimate program, just not this one. Indentation is the whole difference, and tracing it by hand is faster than guessing; see Trace It.

Get the reps in Loops Practice, then read a complete loop-driven program in Looping Programs.

Curriculum connection

A2.2

use sequence, selection, and repetition control structures to create programming solutions;

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A2.3

write algorithms with nested structures (e.g., to count elements in an array, calculate a total, find highest or lowest value, or perform a linear search).

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B3.1

design simple algorithms (e.g., add data to a sorted array, delete a datum from the middle of an array) according to specifications;

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