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

Initialise count to zero, then walk the list once. Inside the loop an if guards the increment: count only rises when v > threshold.

naive.py
Replay: real traced execution (multi-file project)
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)

  1. values ← [4, 7, 2, 9, 1, 8, 3, 6]

    1values = [4, 7, 2, 9, 1, 8, 3, 6]2threshold = 5
    values this step[4, 7, 2, 9, 1, 8, 3, 6]values

  2. threshold ← 5

    1values = [4, 7, 2, 9, 1, 8, 3, 6]2threshold = 53count = 0
    values this step5threshold

  3. count ← 0

    2threshold = 53count = 04for v in values:
    values this step0count

  4. v ← 4

    3count = 04for v in values:5    if v > threshold:
    values this step4v

  5. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  6. v ← 7

    3count = 04for v in values:5    if v > threshold:
    values this step4 → 7v

  7. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  8. count ← 1

    5    if v > threshold:6        count = count + 17print('RESULT:', count)
    values this step0 → 1count

  9. v ← 2

    3count = 04for v in values:5    if v > threshold:
    values this step7 → 2v

  10. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  11. v ← 9

    3count = 04for v in values:5    if v > threshold:
    values this step2 → 9v

  12. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  13. count ← 2

    5    if v > threshold:6        count = count + 17print('RESULT:', count)
    values this step1 → 2count

  14. v ← 1

    3count = 04for v in values:5    if v > threshold:
    values this step9 → 1v

  15. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  16. v ← 8

    3count = 04for v in values:5    if v > threshold:
    values this step1 → 8v

  17. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  18. count ← 3

    5    if v > threshold:6        count = count + 17print('RESULT:', count)
    values this step2 → 3count

  19. v ← 3

    3count = 04for v in values:5    if v > threshold:
    values this step8 → 3v

  20. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  21. v ← 6

    3count = 04for v in values:5    if v > threshold:
    values this step3 → 6v

  22. if v > threshold:

    4for v in values:5    if v > threshold:6        count = count + 1

  23. count ← 4

    5    if v > threshold:6        count = count + 17print('RESULT:', count)
    values this step3 → 4count

  24. for v in values:

    3count = 04for v in values:5    if v > threshold:

  25. stdout ← RESULT: 4

    6        count = count + 17print('RESULT:', count)
    values this stepRESULT: 4stdout

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.

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.