Lists and Records

Slice Select

Extract the first N and last N elements of a list by building two new lists with index loops. The trace shows each append growing the output lists one element at a time, making the index arithmetic visible.

By hand

Two separate loops, each running n times. The first loop picks values[i] directly. The second computes the start offset as len(values) - n and adds i to step forward from there.

naive.py
Replay: real traced execution (multi-file project)
values = [10, 20, 30, 40, 50, 60, 70, 80]
n = 3
first_n = []
last_n = []
for i in range(n):
    first_n.append(values[i])
for i in range(n):
    last_n.append(values[len(values) - n + i])
print('RESULT:', (first_n, last_n))

  1. values ← [10, 20, 30, 40, 50, 60, 70, 80]

    1values = [10, 20, 30, 40, 50, 60, 70, 80]2n = 3
    values this step[10, 20, 30, 40, 50, 60, 70, 80]values

  2. n ← 3

    1values = [10, 20, 30, 40, 50, 60, 70, 80]2n = 33first_n = []
    values this step3n

  3. first_n ← []

    2n = 33first_n = []4last_n = []
    values this step[]first_n

  4. last_n ← []

    3first_n = []4last_n = []5for i in range(n):
    values this step[]last_n

  5. i ← 0

    4last_n = []5for i in range(n):6    first_n.append(values[i])
    values this step0i

  6. first_n ← [10]

    5for i in range(n):6    first_n.append(values[i])7for i in range(n):
    values this step[] → [10]first_n

  7. i ← 1

    4last_n = []5for i in range(n):6    first_n.append(values[i])
    values this step0 → 1i

  8. first_n ← [10, 20]

    5for i in range(n):6    first_n.append(values[i])7for i in range(n):
    values this step[10] → [10, 20]first_n

  9. i ← 2

    4last_n = []5for i in range(n):6    first_n.append(values[i])
    values this step1 → 2i

  10. first_n ← [10, 20, 30]

    5for i in range(n):6    first_n.append(values[i])7for i in range(n):
    values this step[10, 20] → [10, 20, 30]first_n

  11. for i in range(n):

    4last_n = []5for i in range(n):6    first_n.append(values[i])

  12. i ← 0

    6    first_n.append(values[i])7for i in range(n):8    last_n.append(values[len(values) - n + i])
    values this step2 → 0i

  13. last_n ← [60]

    7for i in range(n):8    last_n.append(values[len(values) - n + i])9print('RESULT:', (first_n, last_n))
    values this step[] → [60]last_n

  14. i ← 1

    6    first_n.append(values[i])7for i in range(n):8    last_n.append(values[len(values) - n + i])
    values this step0 → 1i

  15. last_n ← [60, 70]

    7for i in range(n):8    last_n.append(values[len(values) - n + i])9print('RESULT:', (first_n, last_n))
    values this step[60] → [60, 70]last_n

  16. i ← 2

    6    first_n.append(values[i])7for i in range(n):8    last_n.append(values[len(values) - n + i])
    values this step1 → 2i

  17. last_n ← [60, 70, 80]

    7for i in range(n):8    last_n.append(values[len(values) - n + i])9print('RESULT:', (first_n, last_n))
    values this step[60, 70] → [60, 70, 80]last_n

  18. for i in range(n):

    6    first_n.append(values[i])7for i in range(n):8    last_n.append(values[len(values) - n + i])

  19. stdout ← RESULT: ([10, 20, 30], [60, 70, 80])

    8    last_n.append(values[len(values) - n + i])9print('RESULT:', (first_n, last_n))
    values this stepRESULT: ([10, 20, 30], [60, 70, 80])stdout

The Pythonic way

Slice syntax values[:n] and values[-n:] express the same selections in one token each. Negative indices count from the end, so no length arithmetic is needed.

library.py 
values = [10, 20, 30, 40, 50, 60, 70, 80]
n = 3
first_n = values[:n]
last_n = values[-n:]
print('RESULT:', (first_n, last_n))

RESULT: ([10, 20, 30], [60, 70, 80])

Implementation notes

  • first_n and last_n grow visibly in the trace: [] → [10] → [10, 20] → [10, 20, 30] and [] → [60] → [60, 70] → [60, 70, 80].
  • values[-n:] is equivalent to values[len(values)-n:]; Python resolves the negative index internally, which is exactly what the naive loop computes by hand.
  • The two loops share the variable i; its value resets to 0 at the start of the second loop, which is visible in the trace.