Reverse Array In Place (Two Pointers)

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

Algorithm

The canonical input [1, 2, 3, 4, 5, 6, 7] reverses to [7, 6, 5, 4, 3, 2, 1] after three swaps. The middle element at index 4 (1-based) is untouched because the pointers meet there.

Basic Implementation

basic.f90

program array_reverse
    implicit none
    integer :: arr(7) = [1, 2, 3, 4, 5, 6, 7]
    integer :: left, right, tmp
    left = 1
    right = 7
    do while (left < right)
        tmp = arr(left)
        arr(left) = arr(right)
        arr(right) = tmp
        left = left + 1
        right = right - 1
    end do
    print '(*(I0,1X))', arr
end program array_reverse

Complexity

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

Implementation notes

Fortran: use the explicit tmp = arr(left); arr(left) = arr(right); arr(right) = tmp triple. Avoid arr = arr(7:1:-1); it would hide the step-by-step pointer walk the lesson is teaching.
Replay highlights both left and right per frame plus the new array contents after each swap, matching the lesson spec.

two pointers Indices walk toward each other and swap.