Matrix multiplication using multiple threads?

Question

I am supposed to multiply 2 matrices using threads. Two things: I keep getting 0's when I run the program. I also get message errors(for each, it says "warning: passing argument 1 of 'printMatrix' from incompatible pointer type" on the bolded lines(where I try to print the output). Also to note, the first block that is bolded, I that was my attempt at solving the problem. I think I am close, but I may not be. Can anyone help? Thanks :) Output looks like this: A= 1 4 2 5 3 6 B= 8 7 6 5 4 3 A*B= 0 0 0 0 0 0 0 0 0

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define M 3
#define K 2
#define N 3

struct v
{
    int i; //row
    int j; //column
};

int A[M][K] = {{1,4},{2,5},{3,6}};
int B[K][N] = {{8,7,6},{5,4,3}};
int C[M][N];

void *workerThread(void *data)
{
    int i=((struct v*)data)->i;
    int j=((struct v*)data)->j;
    int accumulator = 0;

/*this is where you should calculate the assigned Cell. You will need to use the row(i) of
            A and column[j] of B. Accumulate the result in accumulator */
    **int k;
    for(k=0; k<k; k++)
    {
            accumulator = A[i][k]*B[k][j];
    }
    C[i][j]=accumulator;
    pthread_exit(NULL);**
 }

 void printMatrix(int *matrixIn, int rows, int columns)
{
    int *matrix = matrixIn;
    int i,j;
    for (i=0;i<rows;i++)
    {
}

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

    pthread_t threads[M*N];
    int i,j;
    int counter = 0;
    int numThreadsCreated = 0;

    /*the following 5 lines demonstrates how to create 1 thread to calculate C[0][0], you
            will need to create a loop for all of C's cells*/
    struct v *data = (struct v *)malloc(sizeof(struct v));
    data->i = 0; //assign the row of C for thread to calculate
    data->j = 0; //assign the column of C for thread to calculate
    pthread_create(&threads[0], NULL, workerThread, data);
    numThreadsCreated++;

    /*wait for all the threads to finish before printing out the matrices*/
    for(j=0; j < numThreadsCreated; j++)
    {
            pthread_join( threads[j], NULL);
    }

    printf("A=\n");
    **printMatrix(A,3,2);**
    printf("B=\n");
    **printMatrix(B,2,3);**
    printf("A*B=\n");
    **printMatrix(C,M,N);**
    pthread_exit(NULL);
}

Solution

Your program seems to have implemented a wrong coding algorithm for matrix multiplication.

The following piece of code seems absurd :-

for(k=0; k<k; k++)
{
        accumulator = A[i][k]*B[k][j];
}
C[i][j]=accumulator;    //   Not a code for matrix multiplication...

You should implement something like :-

for(i=0;i<M;i++){
 for(j=0;j<N;j++){
  accumulator=0;
   for(int something=0;something<K;something++){
    accumulator=accumulator+A[i][something]*B[something][j];
   }
  C[i][j]=accumulator;
  accumulator=0;
 }
}

Comments

[Am_I_Helpful]

This line is flawed--->for(k=0; k<k; k++)!

[Chuck Walbourn]

Unless your M and N are huge, this is not going to be much of a performance win. You should search for "parallel matrix multiplication" to see various alternative approaches to the problem when dealing with large matrices.

[Jay]

I just tried this. I replaced my previous code with the one you recommended. It is giving me these error messages ("error: 'k' undeclared (first use in this function)" & ("error: (each undeclared identifier is reported only once for each function it appears in") @shekharsuman

[Am_I_Helpful]

You need to use the capital OR block alphabet K instead of lower-case k!!! YOu've #define K=3. How can it give error? Please check your cases!

[Jay]

The error messages refer to this line: for (int something = 0; something<k; something++)

[Jay]

Well, it is still giving me the same 3 error messages for printing the matrices, and also another error message for the last line(end bracket) that says: error: expected declaration or statement at end of input

[Jay]

It worked! ... I think, it still gives me that error message but it multiplied.