Multiply 2 matrices in Javascript
arrays, javascript, matrix, matrix-multiplication
Solution
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(' '+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));
Problem
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 ```