Thread: Matrix Multiplication in C

Hi,

I am trying to create a program in C that will multiply a matrix with a vector. Both the size of the vector and dimensions of the matrix are given by the user. I am having the most trouble trying to declare and use the 2D array, which is my matrix. I just don't get it. Here is what I have so far. there are probably a lot of errors because I don't quite understand how to use pointers correctly.

Code:

#include <stdio.h>
#include <stdlib.h>
int main(){
	
	int s, z;	 /*s is number of columns, z number of rows and length of vector*/
	int i, x, y, ns, nz;
	double *X, *E, *a;		/*X is input vector, E is result vektor, a is matrix*/
	FILE *fd;
	
	
	/*Vektor*/
	fd = fopen("vektor3_2.dat", "r");
	fscanf(fd, "%d", &z);
	nz=z;
	
	if (z < 2) return 2;          
	
	X = malloc(z * sizeof(*X));    
	E = malloc(z * sizeof(*E));
	a = malloc(z * s * sizeof(*a));
	
	for (i = 0; i < z; i++)
		fscanf(fd, "%lf", &X[z]);  
	
	
	
	/*Matrix*/
	fscanf(fd, "%d", &s);
	ns=s;
	nz=z;
	for (z = 0; z<nz; z++)
		for (s=0; s<ns; s++)
			fscanf(fd, "%lf", &a[z][s]);
		
	
	fclose(fd);
	
	/*Multiplication*/
	
	for (z=0; z<nz; z++)
		for (s=0; s<ns; s++)
			E[z] = E[z] + a[z][s] * X[z];
	
	/*result out*/
	fd = fopen("Ergebnis3_2.dat", "w");
		for(i=0; i<z; i++)
			printf("%lf\n", E[z]);
	
	
}