Multiply 2 matrices in Javascript

Question

I'm doing a function that multiplies 2 matrices.

The matrices will always have the same number of rows and columns. (2x2, 5x5, 23x23, ...)

When I print it, it doesn't work. Why?

For example, if I create two 2x2 matrices:

matrixA:

[1][2]

[3][4]

matrixB:

[5][6]

[7][8]

The result should be:

[19][22]

[43][50]

(http://ncalculators.com/matrix/2x2-matrix-multiplication-calculator.htm)

But, I get:

[19][undefined]

[22][indefined]

function multiplyMatrix(matrixA, matrixB) {
  var result = new Array(); //declare an array   

  //var numColsRows=$("#matrixRC").val();
  numColsRows = 2;

  //iterating through first matrix rows
  for (var i = 0; i < numColsRows; i++) {
    //iterating through second matrix columns
    for (var j = 0; j < numColsRows; j++) {
      var matrixRow = new Array(); //declare an array
      var rrr = new Array();
      var resu = new Array();
      //calculating sum of pairwise products
      for (var k = 0; k < numColsRows; k++) {
        rrr.push(parseInt(matrixA[i][k]) * parseInt(matrixB[k][j]));
      } //for 3
      resu.push(parseInt(rrr[i]) + parseInt(rrr[i + 1]));

      result.push(resu);
      //result.push(matrixRow);
    } //for 2
  } //for 1
  return result;
} // function multiplyMatrix

Answer 1

You're getting confused with your various temporary arrays. The undefined values are caused by out-of-bounds access on the line below your innermost loop.

I recommend that you stick to making a single array for the result of the multiplication. As you're probably aware, the hitch is that JavaScript doesn't allow you to initialize a multi-dimensional array. To make a two-dimensional array, you have to initialize a one-dimensional array, then iterate over its elements and initialize each one to a one-dimensional array.

function multiply(a, b) {
  var aNumRows = a.length, aNumCols = a[0].length,
      bNumRows = b.length, bNumCols = b[0].length,
      m = new Array(aNumRows);  // initialize array of rows
  for (var r = 0; r < aNumRows; ++r) {
    m[r] = new Array(bNumCols); // initialize the current row
    for (var c = 0; c < bNumCols; ++c) {
      m[r][c] = 0;             // initialize the current cell
      for (var i = 0; i < aNumCols; ++i) {
        m[r][c] += a[r][i] * b[i][c];
      }
    }
  }
  return m;
}

function display(m) {
  for (var r = 0; r < m.length; ++r) {
    document.write('&nbsp;&nbsp;'+m[r].join(' ')+'<br />');
  }
}

var a = [[8, 3], [2, 4], [3, 6]],
    b = [[1, 2, 3], [4, 6, 8]];
document.write('matrix a:<br />');
display(a);
document.write('matrix b:<br />');
display(b);
document.write('a * b =<br />');
display(multiply(a, b));

Answer 2

You can use multiplyMatrices() function from: http://tech.pro/tutorial/1527/matrix-multiplication-in-functional-javascript it works like charm. Example (You can print a matrix with style in Chrome and Firefox console with console.table() ):

function multiplyMatrices(m1, m2) {
    var result = [];
    for (var i = 0; i < m1.length; i++) {
        result[i] = [];
        for (var j = 0; j < m2[0].length; j++) {
            var sum = 0;
            for (var k = 0; k < m1[0].length; k++) {
                sum += m1[i][k] * m2[k][j];
            }
            result[i][j] = sum;
        }
    }
    return result;
}

var m1 = [[1,2],[3,4]]
var m2 = [[5,6],[7,8]]

var mResult = multiplyMatrices(m1, m2)

/*In Google Chrome and Firefox you can do:*/

console.table(mResult) /* it shows the matrix in a table */

Answer 3

I know it's an old question but I recommend to switch to my answer.

My solution's got good performance because it uses Map Reduce functions

//The chosen one
function matrixDot (A, B) {
    var result = new Array(A.length).fill(0).map(row => new Array(B[0].length).fill(0));

    return result.map((row, i) => {
        return row.map((val, j) => {
            return A[i].reduce((sum, elm, k) => sum + (elm*B[k][j]) ,0)
        })
    })
}

var print = m => m.forEach(r => document.write(`&nbsp;&nbsp;${r.join(' ')}<br/>`)) 

var a = [[8, 3], [2, 4], [3, 6]]
var b = [[1, 2, 3], [4, 6, 8]]

document.write('matrix a:<br />');
print(a);
document.write('matrix b:<br />');
print(b);
document.write('a * b =<br />');
print(matrixDot(a,b));