Reversing an array in javascript?

Question

Im working through this problem in a javascript book to reverse an array, and I just dont know why this code isnt working. Can someone please explain? This codes supposed to swap the values, but it does nothing to the array.

let nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];
function reverseArrayInPlace(array) {
 for (let start = 0, end = array.length - 1; start < array.length, end >= 0; start++, end--) {
 let x = array[start];
 array[start] = array[end];
 array[end] = x;
 }
 return array;
}
reverseArrayInPlace(nums);
console.log(nums);

• Robert Harvey: I wrote this program for my assignment and it doesn’t work. What do I do now?
• Alexander Nied: If you're simply doing this for an exercise, so be it, but if this is something critical for a production application, an array.reverse() method does exist.

Answer 1

You need just to check if start is smaller then end. The other check is superfluous, because before end reaches zero, the loop stops.

function reverseArrayInPlace(array) {
 for (let start = 0, end = array.length - 1; start < end; start++, end--) {
 let x = array[start];
 array[start] = array[end];
 array[end] = x;
 }
 return array;
}
let nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];
reverseArrayInPlace(nums);
console.log(nums);

Answer 2

This code swaps each pair of elements twice: once when start < end and once when start > end. In your if statement, you want to change the break condition to start < end and not start < array.length.

let nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];
function reverseArrayInPlace(array) {
 for (let start = 0, end = array.length - 1; start < end; start++, end--) {
 let x = array[start];
 array[start] = array[end];
 array[end] = x;
 }
 return array;
}
reverseArrayInPlace(nums);
console.log(nums);

• Alexander Nied: I moved your code into a snippet-- it does not appear to be working as intended as the array is still in the same order when logged.
• Andrew Mascillaro: I've fixed the code so it properly reverses the array now

Answer 3

As long as the values in the array are integers, you can perform a bit-wise exclusive or (XOR) without the need a a temporary variable. You could also do this with one control variable i.e. i. The time complexity will be O(n/2).

let nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];
function reverseIntArrayInPlace(arr) {
 for (let i = 0; i < Math.floor(arr.length / 2); i++) {
 arr[i] = arr[i] ^ arr[arr.length - 1 - i];
 arr[arr.length - 1 - i] = arr[i] ^ arr[arr.length - 1 - i];
 arr[i] = arr[i] ^ arr[arr.length - 1 - i];
 }
 return arr;
}
console.log(reverseIntArrayInPlace(nums));

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If you want to swap other types, then you will need a temporary variable.

let nums = [1, 2, 3, 4, 5, 6, 7, 8, 9];
function reverseArrayInPlace(arr) {
 for (let i = 0; i < Math.floor(arr.length / 2); i++) {
 const tmp = arr[i];
 arr[i] = arr[arr.length - 1 - i];
 arr[arr.length - 1 - i] = tmp;
 }
 return arr;
}
console.log(reverseArrayInPlace(nums));

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