Question 394049 in Javascript Question posted in
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Javascript – Matrix indexing/flattening

I am given a matrix with dimensions dim = nxmx4. I would like to flatten this matrix to a new 1-dimensional matrix with length l = n*m*4 using 3 nested for-loops.

for (let x = 0; x < n; x++) {
    for (let y = 0; y < m; y++) {
        for (let i = 0; i < 4; i++) {
            let index = ?;
            newMatrix[index] = matrix[x][y][i];
        }
    }
}

Additionally the condition index % 4 == i should always hold.

To be more precise, the values of matrix[x][y][i] don’t matter that much for now (can be arbitrary), as long as the index is increased by one each iteration.
For i = 0 -> index = 0, 4, 8, … should be returned | i = 1 -> index = 1, 5, 9, … | i = 2 -> index = 2, 6, 10, … | and i = 3 -> index = 3, 7, 11, …

I can’t come up with the correct indexing right now. Thanks in advance.

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2

Answers


  1. 0

    You can use index = x * m * 4 + y * 4 + i;:

    let newMatrix = [];
for (let x = 0; x < n; x++) {
  for (let y = 0; y < m; y++) {
    for (let i = 0; i < 4; i++) {
      let index = x * m * 4 + y * 4 + i;
      newMatrix[index] = matrix[x][y][i];
    }
  }
}
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  2. 0

    You’ll need to sum the total offset at each loop iteration. Here’s a reproducible example — based on the code in your question — with parametrized variables and comments explaining the logic:

    /** @type {number[][][]} */
const matrix = [
  [
    [1,  2,  3,  4],
    [5,  6,  7,  8],
    [9, 10, 11, 12],
  ],
  [
    [13, 14, 15, 16],
    [17, 18, 19, 20],
    [21, 22, 23, 24],
  ],
];

/** @type {number[]} */
const flat = [];

const a = matrix.length; // 2
const b = matrix[0].length; // 3
const c = matrix[0][0].length; // 4

for (let ia = 0; ia < a; ia++) {
  for (let ib = 0; ib < b; ib++) {
    for (let ic = 0; ic < c; ic++) {
      const index =
          ic // offset from inner loop
        + (c * ib) // offset from middle loop
        + (c * b * ia); // offset from outer loop
      flat[index] = matrix[ia][ib][ic];
    }
  }
}

console.log(flat); // [1, 2, … 23, 24]
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