Matrix Multiplication in C

This is a discussion on Matrix Multiplication in C within the C Programming forums, part of the General Programming Boards category; Hi, I am trying to create a program in C that will multiply a matrix with a vector. Both the ...

  1. #1
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    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]);
    	
    	
    }

  2. #2
    ATH0 quzah's Avatar
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    Code:
    a = malloc(z * s * sizeof(*a));
    Shouldn't you have some size for s at this point?
    Code:
    for (i = 0; i < z; i++)
    		fscanf(fd, "%lf", &X[z]);
    That should be i not z.


    Quzah.
    Hope is the first step on the road to disappointment.

  3. #3
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    Ya, that was a dumb error. But as far as the matrix a goes, the compiler doesn't recognize it. This is the error:
    pr3_2.c: In function ‘main’:
    pr3_2.c:33: error: subscripted value is neither array nor pointer
    pr3_2.c:42: error: subscripted value is neither array nor pointer

    Code:
    fscanf(fd, "%lf", &a[z][s]);
    and
    Code:
    E[z] = E[z] + a[z][s] * X[z];
    how declare the matrix and then use it?

  4. #4
    and the hat of wrongness Salem's Avatar
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    > a = malloc(z * s * sizeof(*a));
    Well you've allocated a flattened array (1D), not something you can subscript with [][]

    If you want to sub-script by hand, then can do if you want.
    fscanf(fd, "%lf", &a[z][s]);
    would be
    fscanf(fd, "%lf", &a[z*ns+s]);



    Alternatively, you need
    Code:
    double **a;
    a = malloc( z * sizeof(*a) );
    for ( i = 0 ; i < z ; i++ ) a[i] = malloc( s * sizeof (*a[i]) );
    This results in something you can use [][] notation on.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  5. #5
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    Ok, so in this new code, the first line creates a pointer to a. The second reserves locations for z (aka rows) and the third line reserves locations for columns within each row
    Is that correct?

    And so now I should be able to use [][] for each location in the matrix?

  6. #6
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    That fixed the compiling errors, but now I just get an empty matrix out (i.e 0 0 0 0). I can't tell if my error lies in my multiplication part, or the entry of data.

  7. #7
    and the hat of wrongness Salem's Avatar
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    Dunno without seeing the latest code.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  8. #8
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    Code:
    #include <stdio.h>
    #include <stdlib.h>
    int main(){
    	
    	int s, z;			/*nummer von Spalten*/
    	int i, x, y, ns, nz;
    	double *X, *E, **a;		/*nummer von Zeilen*/
    	FILE *fd;
    	
    	
    	/*Vektor*/
    	fd = fopen("vektor3_2.dat", "r");
    	fscanf(fd, "%d", &z);
    	nz=z;
    	
    	if (z < 2) return 2;          /* Programm beenden, wenn n < 2 */
    	
    	X = malloc(z * sizeof(*X));           /*Speicherplaetze fuer Vektorcomponenten reservieren*/
    	E = malloc(z * sizeof(*E));
    	a = malloc(z * sizeof(*a));
    	for (i=0; i<z; i++)
    		a[i] = malloc(s*sizeof(*a[i]));
    	
    	for (i = 0; i < z; i++)
    		fscanf(fd, "%lf", &X[i]);  /*Vektor Inhalt bekommen*/
    	
    	
    	
    	/*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];
    	
    	
    	fd = fopen("Ergebnis3_2.dat", "w");
    		for(i=0; i<z; i++)
    			printf("%lf\n", E[z]);
    	
    	
    }

  9. #9
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    Sorry about the comments in German.
    s is number of columns
    z is number of rows
    E is result vector
    X is input vector
    a is input matrix

  10. #10
    and the Hat of Guessing tabstop's Avatar
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    You still can't be using s in your malloc before you know what s is.

  11. #11
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    Ok, I moved my input of s above malloc.
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    int main(){
    	
    	int s, z;			/*nummer von Spalten*/
    	int i, x, y, ns, nz;
    	double *X, *E, **a;		/*nummer von Zeilen*/
    	FILE *fd;
    	
    	
    	/*Vektor*/
    	fd = fopen("vektor3_2.dat", "r");
    	fscanf(fd, "%d", &z);
    	fscanf(fd, "%d", &s);
    	ns=s;
    	nz=z;
    	
    	if (z < 2) return 2;          /* Programm beenden, wenn n < 2 */
    	
    	X = malloc(z * sizeof(*X));           /*Speicherplaetze fuer Vektorcomponenten reservieren*/
    	E = malloc(z * sizeof(*E));
    	a = malloc(z * sizeof(*a));
    	for (i=0; i<z; i++)
    		a[i] = malloc(s*sizeof(*a[i]));
    	
    	for (i = 0; i < z; i++)
    		fscanf(fd, "%lf", &X[i]);  /*Vektor Inhalt bekommen*/
    	
    	
    	
    	/*Matrix*/
    	
    	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];
    	
    	
    	fd = fopen("Ergebnis3_2.dat", "w");
    		for(i=0; i<z; i++)
    			printf("%lf\n", E[z]);
    	
    	
    }

  12. #12
    and the Hat of Guessing tabstop's Avatar
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    I suppose if you want the right answer it might not hurt to set E[z] to 0 before starting to add things up.

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