Modifying an array in a nested loop in a function

Question

I think I'm misunderstanding something about variable scope in JavaScript. The goal is, given an array of numbers, to generate a 2D array that contains all the rotations of that array. So, the array [1,2,3,4] should yield a 2D array of:

[ [1,2,3,4],[2,3,4,1],[3,4,1,2],[2,3,4,1] ]

I'm coming from Ruby, where the following would work just fine:

row = [3,1,6,4];

function rotations(arr) {
  var rotations = [];
  var i = 0;
  var k = arr.length;
  while(i < k) {
    arr.unshift(arr.pop());
    rotations.push(arr);
    i++;
  };
  return rotations;
};

console.log(rotations(row));

However, what this logs is a 2D array containing 4 iterations of the original array:

[ [ 3, 1, 6, 4 ], [ 3, 1, 6, 4 ], [ 3, 1, 6, 4 ], [ 3, 1, 6, 4 ] ]

So it appears that the original array row is not being modified in the scope of the function - only in the scope of the nested while loop.

Pavlo Zhukov · Accepted Answer · 2020-05-14 13:23:02Z

2

You just need to copy your array before modifying it with unshift, because it modifies origin array

var row = [3,1,6,4];

function rotations(arr) {
  var rotations = [arr];
  var i = 1;
  var k = arr.length;
  var copiedArr = arr
  while(i < k) {
    copiedArr = [...copiedArr]
    copiedArr.unshift(copiedArr.pop());
    rotations.push(copiedArr);
    i++;
  };
  return rotations;
};

console.log(rotations(row));
console.log(rotations([1,2,3,4]))

More about unshift is here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/unshift

Or if you need the first element also being rotated

var row = [3,1,6,4];

function rotations(arr) {
  var rotations = [];
  var i = 0;
  var k = arr.length;
  var copiedArr = arr
  while(i < k) {
    copiedArr = [...copiedArr]
    copiedArr.unshift(copiedArr.pop());
    rotations.push(copiedArr);
    i++;
  };
  return rotations;
};

console.log(rotations(row));
console.log(rotations([1,2,3,4]))

edited May 14, 2020 at 13:23

answered May 14, 2020 at 13:17

Pavlo Zhukov

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1 Comment

Duncan Boatright Over a year ago

Excellent, this makes a ton of sense! Thank you very much.

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