Python Loop Patterns

Loop patterns are reusable loop shapes for common jobs.

Accumulator: build a total

Total starts at zero. Each price is added to it.

prices = [10, 20, 15]
total = 0
for price in prices:
    total += price
print(total)

Counter: count the matches

Passes starts at zero. It grows only when a score passes.

scores = [70, 45, 90, 50]
passes = 0
for score in scores:
    if score >= 50:
        passes += 1
print(passes)

Collector: gather the chosen items

Evens starts as an empty list. Each even number joins it with +.

numbers = [1, 2, 3, 4]
evens = []
for number in numbers:
    if number % 2 == 0:
        evens = evens + [number]
print(evens)

Best-so-far: keep the winner

Hottest starts with the first value. A warmer value takes its place.

temperatures = [18, 24, 21, 27]
hottest = temperatures[0]
for temperature in temperatures:
    if temperature > hottest:
        hottest = temperature
print(hottest)

Example

prices = [24, 9, 42, 15, 31]

total = 0
for price in prices:
    total += price
print("Total:", total)

count = 0
for price in prices:
    count += 1
print("Count:", count)

largest = prices[0]
for price in prices:
    if price > largest:
        largest = price
print("Most expensive:", largest)

Key points

Pattern: a loop shape you can reuse for many jobs
Accumulator: a value that grows step by step, such as a total
Counter: a value that counts how many items match
Collector: a value that gathers chosen items into one result
Best-so-far: a value that keeps the best item seen so far

In this example, prices holds five prices. The first loop adds each price into total. It prints a total of 121. The second loop counts one for each price. It prints a count of 5. The third loop starts largest with the first price, 24. A price takes its place only when it is bigger. The largest price found is 42, so that prints last.

Note

For best-so-far, start with the first item, not zero.

Zero would wrongly win if every value were negative.

Try it Yourself

Edit the code, then press Run.

Running is simulated · Python Playground

prices = [24, 9, 42, 15, 31]

total = 0
for price in prices:
    total += price
print("Total:", total)

count = 0
for price in prices:
    count += 1
print("Count:", count)

largest = prices[0]
for price in prices:
    if price > largest:
        largest = price
print("Most expensive:", largest)

Exercise

You need the highest score in a list. Which pattern fits best?

Accumulator Counter Best-so-far Collector