Transpose a Matrix – JavaScript

Transpose or swap the rows and columns of the given matrix – 2D array in JavaScript. Matrix swapping takes place diagonally.

🟢 Easy 🧩 Pattern – Arrays

Example

Original Matrix:
[ 
  [ 1, 1, 1 ], 
  [ 2, 2, 2 ], 
  [ 3, 3, 3 ] 
]

Transposed Matrix:
[ 
  [ 1, 2, 3 ], 
  [ 1, 2, 3 ], 
  [ 1, 2, 3 ] 
]Code language: JavaScript (javascript)

JS code to transpose a square matrix

Since a square matrix has the same number of rows and columns, we only need to swap half of them. We exchange the values below the diagonal with values above the diagonal – think of it like swapping two triangles.

function transposeSquareMatrix(matrix) {
  const length = matrix.length;
  // Get the dimension of the square matrix,
  // iterate through the lower triangle,
  // swap elements below with elements above the diagonal
  for (let i = 0; i < length; i++) {
    // First loop to iterate through each row
    for (let j = 0; j < i; j++) {
      // Second loop to iterate each column until the diagonal
      // And swap the elements
      const temp = matrix[i][j];
      matrix[i][j] = matrix[j][i];
      matrix[j][i] = temp;
    }
  }
  return matrix;
}

const matrix = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9],
];

console.log(transposeSquareMatrix(matrix));
// [
//   [1, 4, 7],
//   [2, 5, 8],
//   [3, 6, 9],
// ];Code language: JavaScript (javascript)

JS code to transpose a rectangular matrix

function transpose(matrix) {
  const output = [];
  const rows = matrix.length;
  const columns = matrix[0].length;
  // We start with the columns first as we need to transpose
  for (let j = 0; j < columns; j++) {
    // Initialise a new row first
    output[j] = [];
    for (let i = 0; i < rows; i++) {
      // Swap values row to column and column to row
      output[j][i] = matrix[i][j];
    }
  }
  return output;
}

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

console.log(transpose(matrix));
// [
//   [ 1, 2, 3 ],
//   [ 1, 2, 3 ],
//   [ 1, 2, 3 ],
//   [ 1, 2, 3 ]
// ]Code language: JavaScript (javascript)

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