Problem with matrix multiplication

[ChloroPhyte]

I'm trying to create the multiplication of matrix A and matrix B. I almost done, i think, but the result is a little bit off. Do you guys have any idea?

#include <bits/stdc++.h>
using namespace std;

int main () {
	int n, m, p;
	cin >> n >> m >> p;
	int a[n][m], b[m][p], c[n][p];
	
	//scan matrix A
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < m; j++) {
			cin >> a[i][j];
		}
	}
	
	// scan matrix B
	for (int i = 0; i < m; i++) {
		for (int j = 0; j < p; j++) {
			cin >> b[i][j];
		}
	}
	
	// getting matrix C
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < p; j++) {
			c[i][j] = 0;
			for (int k; k < m; k++) {
				c[i][j] += a[i][k] * b[k][j];
			}
		}
	}
	
	// output Matrix C
	for (int i = 0; i < n; i++) {
		for (int j = 0; j < p; j++) {
			cout << c[i][j];
		}
		cout << endl;
	}
	
	return 0;
}

[TheIdeasMan]

Hi,

The compiler is your friend:

In function 'int main()':
7:12: warning: array of array of runtime bound [-Wvla]
7:21: warning: array of array of runtime bound [-Wvla]
7:30: warning: array of array of runtime bound [-Wvla]
27:13: warning: 'k' may be used uninitialized in this function [-Wmaybe-uninitialized]

Line 1 : don't do this, include the headers you need.
Line 2: Try to get out of the habit of doing this. Fine for now, but it will bite you one day. Lots written about this on the web.

Line 7: Illegal. Array size must be known at compile time.

Line 27: what is the value of k ? Results a little bit off, by a lot I would say.

Learn how to use a debugger. Set up a watch list of variables; step through the code 1 line at a time; see how values change; deduce what went wrong. Hopefully there is a debugger in your IDE.

Good Luck !!

[ChloroPhyte]

Thank you so much for the info. i'll try to fix the problem by adding the value of k(duh!) and removing some unnecessary things. again, thank you so much!

[jonnin]

maybe, initialize c to zero?