Question

The C++ program below contains a function that adds a specific row of two matrices and store the result in the corresponding row of a third matrix. This is done by a loop to invokes a function that adds a single row of the two matrices. By the end of the loop all rows should be added and stored in the result matrix. The rows are added sequentially one after the other. You are required to modify the program below to use C++11 threads to add the rows of the two matrices using multithreading, such that all the rows are added concurrently. You will need to join on the running threads in the correct location of the code to be able to wait for all the threads to finish before you start printing the results.

THE PROGRAM:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>


void add_matrix_row (int * mat1,int * mat2, int * result, int cols)
{
for ( int c = 0 ; c < cols ; c++)
result[c] = mat1[c]+mat2[c];
}

int main ()
{
int rows = 3;
int cols = 4;
int array1[3][4]= {{100,20,3,50}, {40,33,56,2}, {150,23,17,22}};
int array2[3][4]= {{56,13,9,100},{22,15,55,60}, {19,200,27,14}};
int result[3][4];
for ( int r = 0 ; r < rows ; r++)
add_matrix_row(array1[r],array2[r],result[r],cols);

for ( int r = 0 ; r < rows ; r++) {
for ( int c = 0 ; c < cols ; c++) printf ("%4d ",array1[r][c]);
printf ("\n");
}
printf (" + \n");
for ( int r = 0 ; r < rows ; r++){
for ( int c = 0 ; c < cols ; c++) printf ("%4d ",array2[r][c]);
printf ("\n");
}
printf (" = \n");
for ( int r = 0 ; r < rows ; r++) {
for ( int c = 0 ; c < cols ; c++) printf ("%4d ",result[r][c]);
printf ("\n");
}
return 0;
}

Answer 1

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <thread>
using namespace std;

// One thread per row
#define THREADS_NUMBER 3

void add_matrix_row (int * mat1, int * mat2, int * result, int cols)
{
    for ( int c = 0 ; c < cols ; c++){
            result[c] = mat1[c] + mat2[c];
        }
}

int main ()
{
    int rows = 3;
    int cols = 4;
    int array1[3][4]= {{100,20,3,50}, {40,33,56,2}, {150,23,17,22}};
    int array2[3][4]= {{56,13,9,100},{22,15,55,60}, {19,200,27,14}};
    int result[3][4];

//    for ( int r = 0 ; r < rows ; r++)
//       add_matrix_row(array1[r],array2[r],result[r],cols);

    for ( int r = 0 ; r < rows ; r++)
    {
        for ( int c = 0 ; c < cols ; c++) printf ("%4d ",array1[r][c]);
        printf ("\n");
    }
    printf (" + \n");
    for ( int r = 0 ; r < rows ; r++)
    {
        for ( int c = 0 ; c < cols ; c++) printf ("%4d ",array2[r][c]);
        printf ("\n");
    }
    printf (" = \n");

    // Create an array of threads
    std::thread threads[THREADS_NUMBER];
    for (int i = 0; i < THREADS_NUMBER; i++) {
       // Initialize each thread with the function responsible of adding row of the matrices
        threads[i] = std::thread(add_matrix_row, array1[i], array2[i], result[i], cols);
    }

    // Waiting for join threads after compute
    for (int i = 0; i < THREADS_NUMBER; i++) {
            threads[i].join();
    }

    for ( int r = 0 ; r < rows ; r++)
    {
        for ( int c = 0 ; c < cols ; c++) printf ("%4d ",result[r][c]);
        printf ("\n");
    }
    return 0;
}