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. Run it: within a second or two the page stops it for printing too much. A stuck loop that prints nothing runs until you press Stop. Either way, look for what the loop was supposed to be changing, fix it, and run it again.

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;

Link to original

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).

Link to original

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;

Link to original