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}.

Basic Implementation

basic.lua

local arr = {1, 2, 3, 4, 5, 6, 7}
local left = 1
local right = #arr
while left < right do
	local tmp = arr[left]
	arr[left] = arr[right]
	arr[right] = tmp
	left = left + 1
	right = right - 1
end
io.write("[")
for i = 1, #arr do
	if i > 1 then io.write(", ") end
	io.write(tostring(arr[i]))
end
io.write("]\n")

Complexity

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

Implementation notes

Lua: explicit three-line local tmp = arr[left]; arr[left] = arr[right]; arr[right] = tmp swap keeps the move visible. Lua has no stdlib reverse for tables, but a for loop with arr[i], arr[#arr - i + 1] = arr[#arr - i + 1], arr[i] (parallel assignment) would collapse the swap into a single frame.
left = 1 and right = #arr use plain integer indices; the # length operator 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 `1`, `right` starts at `#arr`. Each loop iteration swaps `arr[left]` and `arr[right]` and moves the pointers toward each other.