multiply 2 matrixes

This is a discussion on multiply 2 matrixes within the C Programming forums, part of the General Programming Boards category; this code should multiply 2 matrixes but it seems it doesn't work how it should. output: Code: Matrix A : ...

  1. #1
    Registered User
    Join Date
    Mar 2004
    Posts
    161

    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
    }
    This forum is the best one I've ever seen. Great ppl, great coders

  2. #2
    Registered User
    Join Date
    Mar 2003
    Posts
    143
    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);
    }
    DavT
    -----------------------------------------------

  3. #3
    Registered User
    Join Date
    Mar 2004
    Posts
    161
    you are soooo right
    thanx

    Jacopo
    This forum is the best one I've ever seen. Great ppl, great coders

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