Multiply 2 matrices in Javascript
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));
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(` ${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));
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 */