20. Loop patterns

Almost every loop is one of four shapes. Name them and you stop inventing each loop from scratch — you reach for the one that fits. The four are accumulate, count, search, and flag, each doing one small job. The examples use a range of numbers, but the shapes work on anything you can loop over.

1. Accumulate — build up a result

Hold a variable and change it every turn. The running total is the classic case:

sum = 0
for i in range(1, 11):
    sum = sum + i
print(sum)       # 55

The shape is always: set a start value before the loop, update it inside, use it after. It also builds strings (start "", glue on each piece) and finds a maximum (start low, raise it whenever you see something bigger).

2. Count — how many match?

A counter is an accumulator that goes up by one, but only when a test passes. "How many numbers from 1 to 30 divide evenly by 3?"

hits = 0
for i in range(1, 31):
    if i % 3 == 0:
        hits = hits + 1
print(hits)      # 10

The if inside the loop is the test; hits = hits + 1 is the tally.

3. Search — find the first match and stop

When you only need the first value that fits, break out the moment you find it — no point continuing:

first = None
for i in range(1, 101):
    if i * i > 50:
        first = i
        break
print(first)     # 8   (because 8 × 8 = 64, the first square over 50)

first starts as None. If nothing matched, it stays None — a handy "not found" answer you can test with if.

4. Flag — did any turn match?

Sometimes you want a yes/no, not the value: "is any number from 1 to 20 divisible by 7?" Keep a boolean that starts False and flips to True the moment you see a match:

found = False
for i in range(1, 21):
    if i % 7 == 0:
        found = True
print(found)     # True   (7 and 14 are in range)

If you only care whether one exists, break when the flag flips — that turns it into a search. Flag and search are cousins.

Python bonus: continue

Python has a continue statement. Where break exits the loop entirely, continue skips the rest of the current iteration and jumps straight to the next one:

for i in range(1, 11):
    if i % 2 == 0:
        continue       # skip even numbers
    print(i)           # only odd numbers print: 1, 3, 5, 7, 9

continue is useful when you want to skip certain values without nesting a deeper if. The same result can always be written without it, but continue can make the intent clearer.

Mixing the patterns

Real programs combine these. One pass can accumulate a total, count how many fit a rule, and flag whether something special appeared, all at once:

total = 0
evens = 0
has_multiple_of_5 = False

for i in range(1, 11):
    total = total + i                        # accumulate
    if i % 2 == 0:
        evens = evens + 1                    # count
    if i % 5 == 0:
        has_multiple_of_5 = True             # flag

print("total", total)                        # 55
print("evens", evens)                        # 5
print("multiple of 5?", has_multiple_of_5)  # True

One loop, three patterns, three answers.

Homework

Homework files are in exercises/20/homework/.

Stuck or finished? Open the homework solutions page.