JS: Reverse an Array but Reverse the Original Array Only --> Error: running with no output

Question

I have following problem:

// Reverse Array

Write a function that accepts an array and reverses that array in place. The behavior should mimic the behavior of the native .reverse() array method. However, your reverse function should accept the array to operate on as an argument, rather than being invoked as a method on that array.

Do not use the native .reverse() method in your own implementation.

I tried the following code:

let myArray = [1, 2, 3, 4];


function reverse(myArray) {

  let newArray = []; 

  // pop all of elements from roginal array, and store in new array

  for (i=myArray.length-1; i>=0; i--){
    newArray.push(myArray[i])

    console.log(newArray)
  }

  while (newArray.length){

    myArray.unshift(newArray)
  }


  return myArray; 
}


reverse(myArray);
console.log(myArray) // expected output is [4, 3, 2, 1]

My code just keeps running and no console.log output is produced. Notice I want the reverse done to the input array argument.

What am I doing wrong? Also, what does while (newArray.length) mean / what is it doing conceptually?

You've missed an important part of the requirement: "...and reverses that array in place" — Andreas
– Andreas, Commented Mar 30, 2019 at 16:34
and the reason you code "just keeps running" is your infinite loop - nothing inside that loop while (newArray.length) changes the length of newArray — Robin Zigmond
– Robin Zigmond, Commented Mar 30, 2019 at 16:36
To reverse an array in place you just swap the first and last element, and then the second and second-to-last element, etc, stopping when you get half way. — Alnitak
– Alnitak, Commented Mar 30, 2019 at 16:37
I would also suggest swapping. Swap the i th element with length-i element. — arc
– arc, Commented Mar 30, 2019 at 16:56
You have selected an answer that will cause you to fail your assignment. — Alnitak
– Alnitak, Commented Mar 31, 2019 at 7:50

brk · Accepted Answer · 2019-03-30 16:52:12Z

0

Not sure why you need the unshift you can just iterate and return the array where you are pushing the value

let myArray = [1, 2, 3, 4];

function reverse(myArray) {
  let newArray = [];
  for (i = myArray.length - 1; i >= 0; i--) {
    newArray.push(myArray[i])
  }
  return newArray;
}
console.log(reverse(myArray))

answered Mar 30, 2019 at 16:52

brk

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1 Comment

Alnitak Over a year ago

This is not an in-place reversal!

Ori Drori · Accepted Answer · 2019-03-30 16:47:08Z

You can iterate the array 'till the middle, and switch between the current (i) and the opposite (length - i - 1):

const myArray = [1, 2, 3, 4];

function reverse(myArray) {
  const length = myArray.length;
  const middle = Math.floor(length / 2);
  
  for(let i = 0; i < middle; i++) {
    let tmp = myArray[i];
    myArray[i] = myArray[length - i - 1];
    myArray[length - i - 1] = tmp;
  }
}


reverse(myArray);
console.log(myArray) // expected output is [4, 3, 2, 1]

karthick · Accepted Answer · 2019-03-30 16:49:28Z

0

You can swap first and last element in an array and iteratively swap the next and prev respectively.

You don't have to visit the complete set in the loop, get the middle element and rotate the index around that

function reverseInArray(arr){
  let len = arr.length;
  let temp;
  for(let i=0; i < len/2; i++){
  	temp = arr[i];
    arr[i] = arr[len - i - 1];
    arr[len - i - 1] = temp;
  }  
  return arr;
}

console.log(reverseInArray([1,2,3,4,5]));

edited Mar 30, 2019 at 16:49

answered Mar 30, 2019 at 16:44

karthick

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Comments

Nina Scholz · Accepted Answer · 2019-03-30 16:51:52Z

0

You could swap the first and the last element and start from the most inner item.

function reverse(array) {
    var i = array.length >> 1, // take half of the length as integer
        l = array.length - 1;  // last index value to calculate the other side

    while (i--) [array[i], array[l - i]] = [array[l - i], array[i]];
}

var a = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

reverse(a);
console.log(...a);

answered Mar 30, 2019 at 16:51

Nina Scholz

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1 Comment

Alnitak Over a year ago

interesting use of structured assignment to achieve the element swap :)

2 revs · Accepted Answer · 2019-03-31 07:49:21Z

Just swap pairs starting at either end of the array, until there's none left:

function reverse(a) {
    for (let i = 0, j = a.length - 1; i < j; ++i, --j) {
        let tmp = a[i];
        a[i] = a[j];
        a[j] = tmp;
    }
    return a;  // not required, but allows use in an expression
}

In ES2016 you can use destructuring assignments to perform the swap in one operation without the use of a temporary variable:

function reverse(a) {
    for (let i = 0, j = a.length - 1; i < j; ++i, --j) {
        [ a[j], a[i] ] = [ a[i], a[j] ];
    }
    return a;
}

mbojko · Accepted Answer · 2019-03-30 16:36:25Z

-1

Here:

  while (newArray.length){

    myArray.unshift(newArray)
  }

You're adding to myArray, but not taking from newArray, hence infinite loop. Methinks it should be myArray.unshift(newArray.pop()).

answered Mar 30, 2019 at 16:36

mbojko

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