Lists and Records

Count Matching

Count how many values in a list satisfy a condition — here, readings above a threshold. Incrementing a counter inside a conditional is the simplest aggregate pattern after sum and mean.

By hand

The Pythonic way

sum(1 for v in values if v > threshold) is a generator expression that produces 1 for each qualifying element; sum adds them up. No explicit loop or counter variable is needed.

naive.py 
values = [4, 7, 2, 9, 1, 8, 3, 6]
threshold = 5
count = 0
for v in values:
    if v > threshold:
        count = count + 1
print('RESULT:', count)
library.py 
values = [4, 7, 2, 9, 1, 8, 3, 6]
threshold = 5
count = sum(1 for v in values if v > threshold)
print('RESULT:', count)

RESULT: 4

Implementation notes

  • Every if v > threshold: event is zero-delta (condition evaluation changes no variables). Matching values are distinguished by the count = count + 1 event that immediately follows; non-matching values jump straight to the next for-header.
  • The generator expression inside sum(...) runs in its own scope, so its loop variable does not appear in the trace.