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 arr=(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].

Basic Implementation

basic.sh

#!/usr/bin/env bash
set -euo pipefail
arr=(1 2 3 4 5 6 7)
left=0
right=$(( ${#arr[@]} - 1 ))
while [ "$left" -lt "$right" ]; do
	tmp=${arr[left]}
	arr[left]=${arr[right]}
	arr[right]=$tmp
	left=$((left + 1))
	right=$((right - 1))
done
printf '['
sep=''
for v in "${arr[@]}"; do
	printf '%s%d' "$sep" "$v"
	sep=', '
done
printf ']\n'

Complexity

Time: O(n)
Space: O(1)

Implementation notes

Bash: explicit three-line tmp=${arr[left]}; arr[left]=${arr[right]}; arr[right]=$tmp swap keeps the move visible. The shell has no built-in reverse for arrays; spawning printf '%s\n' "${arr[@]}" | tac would push the work to another process and return a stream instead of mutating the array.
left=0 and right=$(( ${#arr[@]} - 1 )) use plain integer indices; ${#arr[@]} returns the fixed length of the canonical array.
The replay distinguishes swap frames from pointer-advance frames so the viewer can see left and right converge.

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