Nested Loop not defined (Javascript)

Question

My resultant matrix seems to be undefined. At line 25, this is the error my chrome console gives: "Cannot set property "0" of undefined."

On looking at similar problems, most of the matrix multiplication solutions I've seen use 3 nested loops, as opposed to my 4. Those are probably the better way, but four loops is the only way that makes sense to me, since the iterations are over two separate rows and two separate columns. If this is the cause of my bug issue, could someone please explain why?

const A = [ [-4,0,5], [-3,-1,2], [6,7,-2], [1, 1, 2]],B = [ [1, 0, 3, 0], [4,5,-1, 2], [2, 4, 3, 1]],C = [];
for (var i = 0; i < A.length; i++) {
	//C[i] = 0;
	for (var j = 0; j < A[j].length; j++) {
		//console.log(A[i][j]);
		for (var y = 0; y < B[0].length; y++) {
			C[i][y] = 0;
			for (var x = 0; x < B.length; x++) {
				//console.log(B[x][y]+ "["+x+","+y+"]");
				console.log(C[i][y] + "[" + i + "," + y);
				C[i][y] += A[i][j] * B[x][y];
			}
			console.log(C[i][y] + "[" + i + "," + y + "] is the resultant matrix");
		}
	}
}

@mbojko Oh, right, of course! Line 25: C[i][y] += A[i][j] * B[x][y]; — S K
– S K, Commented Jul 2, 2019 at 13:18

ponury-kostek · Accepted Answer · 2019-07-02 15:02:51Z

1

Change //C[i] = 0; to C[i] = [];. You need to initialize array under C[i] to access it later C[i][y] = 0;

const A = [ [-4,0,5], [-3,-1,2], [6,7,-2], [1, 1, 2]],B = [ [1, 0, 3, 0], [4,5,-1, 2], [2, 4, 3, 1]],C = [];
for (var i = 0; i < A.length; i++) {
	C[i] = [];
	for (var j = 0; j < A[j].length; j++) {

		for (var y = 0; y < B[0].length; y++) {
			C[i][y] = 0;
			for (var x = 0; x < B.length; x++) {
				C[i][y] += A[i][j] * B[x][y];
			}
		}
	}
}
console.log(C);

edited Jul 2, 2019 at 15:02

answered Jul 2, 2019 at 12:59

ponury-kostek

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2 Comments

S K Over a year ago

Yes! This worked! Thank you so much! -- Can I please further ask how to make my resultant matrix output in a matrix/array form?

ponury-kostek Over a year ago

@SK your output matrix is in C, so just do console.log(C)

S K · Accepted Answer · 2019-07-02 18:40:18Z

0

const A = [ [-4,0,5], [-3,-1,2], [6,7,-2], [1, 1, 2]],B = [ [1, 0, 3, 0], [4,5,-1, 2], [2, 4, 3, 1]],C = [];
for (var i = 0; i < A.length; i++) {
	C[i] = [];
	for (var j = 0; j < A[j].length; j++) {

		for (var y = 0; y < B[0].length; y++) {
			C[i][y] = 0;
			for (var x = 0; x < B.length; x++) {
				C[i][y] += A[i][j] * B[x][y];
			}
		}
	}
}
console.log(C);

answered Jul 2, 2019 at 18:40

S K

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1 Comment

S K Over a year ago

Answered by @ponury-kostek. (Hopefully I did this right; new to Stack.)

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