multiply 2 matrixes

this code should multiply 2 matrixes but it seems it doesn't work how it should.

output:

Code:

Matrix A : 
0  4  7  
1  5  5  

Matrix B : 
0  5  3  6  
0  7  1  2  
0  3  2  1  

Matrix C : 
3  103  50  53  
0  77  32  35

the first number of C[1][1] should be 0!!!

C[1][1] = 0*0+4*0+7*0 = 0 not 3 !! can soneone help me?
C[1][2] = 0*5+4*7+5*3 = 43 not 103..
and so on..

it works sometimes, other times it does not

source:

Code:

#include "stdio.h"
#include "stdlib.h"
#include "string.h"

#define RIGAA 2    
#define COLA  3
#define RIGAB 3    
#define COLB  4

int A [RIGAA][COLA];
int B [RIGAB][COLB];
int C [RIGAA][COLB];


void GenerateMatrixes();
void GenerateMatrix(int* matrix,int riga,int col);
void MultiplyaMatrix();
void PrintMatrix(char* name,int* matrix,int row,int col);
void CleanScreen();


int main(int argc, char* argv[])
{

	int ch=0;  //scelta dal menu iterattivo

	
	printf("************************************\n");
	printf("**     Matrix Multiplication      **\n");
	printf("************************************\n\n");


	GenerateMatrixes();
	printf("\n\n");
	

	while(ch!=5) //loop cycle, repeat untill user choose to exit with value 5
	{   //application menu 
		ch=0;
		printf("Regenerate matrixes    : 1 \n");
		printf("Multiplicate           : 2 \n");
		printf("Re-print matrixes      : 3 \n");
		printf("Clean Screen           : 4 \n");
		printf("Exit                   : 5 \n");
		printf("Select command: ");
		scanf("%d",&ch);

		switch(ch)
		{
		case 1:
			GenerateMatrixes();
			break;
		case 2:
			{
			MultiplyaMatrix();
			break;
			}
		case 3:
			{
			PrintMatrix("A",(int*)A,RIGAA,COLA);
			PrintMatrix("B",(int*)B,RIGAB,COLB);
			break;
			}
		case 4:
			{
			//clear the screen
			CleanScreen();
			break;
			}
		default:
			CleanScreen();
			break;
			
		};
	
	}

	return 0;
}



//random generation of matrix values
void GenerateMatrixes()
{
	
	//generate matrixes A B
	GenerateMatrix((int*)A,RIGAA,COLA);
	GenerateMatrix((int*)B,RIGAB,COLB);

	//automatically print of generated matrix
	printf("\n\n\n");
	PrintMatrix("A",(int*)A,RIGAA,COLA);
	PrintMatrix("B",(int*)B,RIGAB,COLB);

	printf("\n\n");

}


//Print output of the specified matrix
void PrintMatrix(char* name,int* matrix,int row,int col)
{
	int i,j;
	char str[20];
	printf("\n\n");

	sprintf(str,"Matrix %s : ",name);
	printf(str);
	printf("\n\n");

	for(i=0;i<row;i++)//ciclo e genero casualmente i valori 
	{
		for(j=0;j<col;j++)
		{
			sprintf(str,"%d  ",matrix[(i*col)+j]);
			printf(str);
		}
	   printf("\n");
	}

	printf("\n");


}

//Matrix multiplication
void MultiplyaMatrix()
{
	int i; int j; 
	int m;
    for (i=0; i<RIGAA; i++)
      for (j=0; j<COLB; j++)
		{
			for(m=0;m<COLA;m++)
				C[i][j]+= (A[i][m] * B[m][j]); 
		}

		PrintMatrix("C",(int*)C,RIGAA,COLB);

}


//Randomly generate velues of a matrix given by pointer
void GenerateMatrix(int* matrix,int riga,int col)
{
	int i,j=0;

	for(i=0;i<riga;i++)//random generation of values
		for(j=0;j<col;j++)
			matrix[(i*col)+j]=rand()%8;
}


void CleanScreen()
{
	system("clear"); // unix system
}

In your MultiplyaMatrix() function you are not initialising C before summing the individual elements:

Code:

void MultiplyaMatrix()
{
  int i; int j; 
  int m;

  for (i=0; i<RIGAA; i++) {
    for (j=0; j<COLB; j++) {
      C[i][j] = 0;   /* <-- initialise before using a loop to sum a number of elements */
      for(m=0;m<COLA;m++) {
        C[i][j]+= (A[i][m] * B[m][j]);
      }
    }
  }

  PrintMatrix("C",(int*)C,RIGAA,COLB);
}

you are soooo right
thanx

Jacopo