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);

I wrote this program for my assignment and it doesn’t work. What do I do now? — Robert Harvey
– Robert Harvey, Commented Aug 10, 2020 at 20:02
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. — Alexander Nied
– Alexander Nied, Commented Aug 10, 2020 at 20:03

Nina Scholz · Accepted Answer · 2020-08-10 20:09:20Z

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);

answered Aug 10, 2020 at 20:09

Nina Scholz

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Comments

Andrew Mascillaro · Accepted Answer · 2020-08-10 20:19:18Z

0

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);

edited Aug 10, 2020 at 20:19

answered Aug 10, 2020 at 20:03

Andrew Mascillaro

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2 Comments

Alexander Nied Over a year ago

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 Over a year ago

I've fixed the code so it properly reverses the array now

Mr. Polywhirl · Accepted Answer · 2020-08-10 20:28:15Z

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));

.as-console-wrapper { top: 0; max-height: 100% !important; }

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));

.as-console-wrapper { top: 0; max-height: 100% !important; }

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