Transposing a 2D-array in JavaScript

Question

I\'ve got an array of arrays, something like:

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

I would like to transpose it to get the following

Answer 1

Neat and pure:

[[0, 1], [2, 3], [4, 5]].reduce((prev, next) => next.map((item, i) =>
    (prev[i] || []).concat(next[i])
), []); // [[0, 2, 4], [1, 3, 5]]

Previous solutions may lead to failure in case an empty array is provided.

Here it is as a function:

function transpose(array) {
    return array.reduce((prev, next) => next.map((item, i) =>
        (prev[i] || []).concat(next[i])
    ), []);
}

console.log(transpose([[0, 1], [2, 3], [4, 5]]));

Update. It can be written even better with spread operator:

const transpose = matrix => matrix.reduce(
    ($, row) => row.map((_, i) => [...($[i] || []), row[i]]), 
    []
)

Answer 2

You can do it in in-place by doing only one pass:

function transpose(arr,arrLen) {
  for (var i = 0; i < arrLen; i++) {
    for (var j = 0; j <i; j++) {
      //swap element[i,j] and element[j,i]
      var temp = arr[i][j];
      arr[i][j] = arr[j][i];
      arr[j][i] = temp;
    }
  }
}

Answer 3

A library-free implementation in TypeScript that works for any matrix shape that won't truncate your arrays:

const rotate2dArray = <T>(array2d: T[][]) => {
    const rotated2d: T[][] = []

    return array2d.reduce((acc, array1d, index2d) => {
        array1d.forEach((value, index1d) => {
            if (!acc[index1d]) acc[index1d] = []

            acc[index1d][index2d] = value
        })

        return acc
    }, rotated2d)
}

Answer 4

shortest way with lodash/underscore and es6:

_.zip(...matrix)

where matrix could be:

const matrix = [[1,2,3], [1,2,3], [1,2,3]];

Answer 5

I found the above answers either hard to read or too verbose, so I write one myself. And I think this is most intuitive way to implement transpose in linear algebra, you don't do value exchange, but just insert each element into the right place in the new matrix:

function transpose(matrix) {
  const rows = matrix.length
  const cols = matrix[0].length

  let grid = []
  for (let col = 0; col < cols; col++) {
    grid[col] = []
  }
  for (let row = 0; row < rows; row++) {
    for (let col = 0; col < cols; col++) {
      grid[col][row] = matrix[row][col]
    }
  }
  return grid
}

Answer 6

Another approach by iterating the array from outside to inside and reduce the matrix by mapping inner values.

const
    transpose = array => array.reduce((r, a) => a.map((v, i) => [...(r[i] || []), v]), []),
    matrix = [[1, 2, 3], [1, 2, 3], [1, 2, 3]];

console.log(transpose(matrix));