Arrays and Iteration

Reverse Array In Place (Two Pointers)

Walk two indices toward each other from the ends of the array, swapping at each step. Stops when the indices meet or cross. Demonstrates the two-pointer pattern with the smallest possible state.

Algorithm

Canonical input (1, 2, 3, 4, 5, 6, 7) (odd length, middle element stays put) yields three swap frames and reverses to [7, 6, 5, 4, 3, 2, 1].

two pointers `$left` starts at index `0`, `$right` starts at `scalar(@arr) - 1`. Each loop iteration swaps `$arr[$left]` and `$arr[$right]` and moves the pointers toward each other.

Basic Implementation

basic.pl
Replay: real traced execution (multi-file project)
use strict; use warnings;
my @arr = (1, 2, 3, 4, 5, 6, 7);
my $left = 0;
my $right = scalar(@arr) - 1;
while ($left < $right) {
	my $tmp = $arr[$left];
	$arr[$left] = $arr[$right];
	$arr[$right] = $tmp;
	$left = $left + 1;
	$right = $right - 1;
}
print "[" . join(", ", @arr) . "]\n";

  1. arr ← [1, 2, 3, 4, 5, 6, 7]

    1use strict; use warnings;2my @arr = (1, 2, 3, 4, 5, 6, 7);3my $left = 0;
    values this step[1, 2, 3, 4, 5, 6, 7]arr

  2. left ← 0

    2my @arr = (1, 2, 3, 4, 5, 6, 7);3my $left = 0;4my $right = scalar(@arr) - 1;
    values this step0left[1, 2, 3, 4, 5, 6, 7]arr

  3. right ← 6

    3my $left = 0;4my $right = scalar(@arr) - 1;5while ($left < $right) {
    values this step6right0left

  4. arr ← [7, 2, 3, 4, 5, 6, 1]

    6my $tmp = $arr[$left];7$arr[$left] = $arr[$right];8$arr[$right] = $tmp;
    values this step[1, 2, 3, 4, 5, 6, 7] → [7, 2, 3, 4, 5, 6, 1]arr0left6right

  5. left ← 1

    8$arr[$right] = $tmp;9$left = $left + 1;10$right = $right - 1;
    values this step0 → 1left

  6. right ← 5

    9	$left = $left + 1;10	$right = $right - 1;11}
    values this step6 → 5right

  7. arr ← [7, 6, 3, 4, 5, 2, 1]

    6my $tmp = $arr[$left];7$arr[$left] = $arr[$right];8$arr[$right] = $tmp;
    values this step[7, 2, 3, 4, 5, 6, 1] → [7, 6, 3, 4, 5, 2, 1]arr1left5right

  8. left ← 2

    8$arr[$right] = $tmp;9$left = $left + 1;10$right = $right - 1;
    values this step1 → 2left

  9. right ← 4

    9	$left = $left + 1;10	$right = $right - 1;11}
    values this step5 → 4right

  10. arr ← [7, 6, 5, 4, 3, 2, 1]

    6my $tmp = $arr[$left];7$arr[$left] = $arr[$right];8$arr[$right] = $tmp;
    values this step[7, 6, 3, 4, 5, 2, 1] → [7, 6, 5, 4, 3, 2, 1]arr2left4right

  11. left ← 3

    8$arr[$right] = $tmp;9$left = $left + 1;10$right = $right - 1;
    values this step2 → 3left

  12. right ← 3

    9	$left = $left + 1;10	$right = $right - 1;11}
    values this step4 → 3right

  13. while ($left < $right)

    4my $right = scalar(@arr) - 1;5while ($left < $right) {6	my $tmp = $arr[$left];
    values this step[7, 6, 5, 4, 3, 2, 1]arr3left3right

Complexity

  • Time: O(n)
  • Space: O(1)

Implementation notes

  • Perl: explicit three-line my $tmp = $arr[$left]; $arr[$left] = $arr[$right]; $arr[$right] = $tmp swap keeps the move visible. The stdlib reverse @arr would hide the lesson entirely (and returns a new list rather than mutating in place), and @arr[$left, $right] = @arr[$right, $left] (list-slice assignment) would collapse the swap into a single frame.
  • $left = 0 and $right = scalar(@arr) - 1 use plain integer indices; the $left < $right guard handles the meet-in-the-middle exit honestly for the odd-length canonical input.
  • The replay distinguishes swap frames from pointer-advance frames so the viewer can see $left and $right converge.