CODEWITHSUNDEEP
parallel-processingmpi
Pavel Shchegolevatykh
Pavel Shchegolevatykh

Reputation: 2634

Find min and max value in matrix MPI

I am really noob at MPI stuff. I just need to complete my college task. Could someone help me?

The task is to find Min and Max value in N*M matrix. Process count must be equal to N. I try to find min and max using MPI_Reduce but I get error results. Here is my code.

#include  <mpi.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

const int n = 5;
const int m = 4;

void fill_matrix_randomly(int matrix[n][m], int max_value);
void write_matrix(int matrix[n][m]);
int find_max(int* vector, int vector_size);
int find_min(int* vector, int vector_size);
void write_vector(int* vector, int vector_size);

int main(int argc, char* argv[])
{
    int my_rank = 0;
    int comm_size = 0;

    int a[n][m];
    int receive_buffer[m];
    int partial_max[m];
    int partial_min[m];

    MPI_Init(&argc, &argv);

    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
    MPI_Comm_size(MPI_COMM_WORLD, &comm_size);

    if (comm_size != n)
    {
        printf("Please set process count = %d and run again.", n);
        MPI_Finalize();
        return 0;
    }

    if (my_rank == 0)
    {
        fill_matrix_randomly(a, 10);
        write_matrix(a);    
    }
    /* MPI Scatter(address of send buffer, number of elements sent to each process, data type of send buffer, address of receive buffer, number of elements in receive buffer, data type of receive buffer, rank of sending process, communicators space) */
    MPI_Scatter(a, n, MPI_INT, receive_buffer, n, MPI_INT, 0, MPI_COMM_WORLD);
    /* MPI_Reduce(address of send buffer, address of receive buffer, number of elements in send buffer, data type of elements in send buffer, reduce operation, rank of root process, communicators space) */
    MPI_Reduce(receive_buffer, partial_max, n, MPI_INT, MPI_MAX, 0, MPI_COMM_WORLD);
    MPI_Reduce(receive_buffer, partial_min, n, MPI_INT, MPI_MIN, 0, MPI_COMM_WORLD);
    if (my_rank == 0)
    {
        printf("Vector of partial max values.\n");
        write_vector(partial_max, n);
        printf("Vector of partial min values.\n");
        write_vector(partial_min, n);

        int max = find_max(partial_max, n);
        int min = find_min(partial_min, n);
        printf("Matrix boundaries = [%d..%d]\n", min, max);
    }
    MPI_Barrier(MPI_COMM_WORLD);
    MPI_Finalize();

    return 0;
}

int find_max(int* vector, int vector_size)
{
    int max = vector[0];
    int i = 0;  
    for (i = 0; i < vector_size; i++)
    {
        if (vector[i] > max)
        {
            max = vector[i];
        }       
    }
    return max;
}

int find_min(int* vector, int vector_size)
{
    int min = vector[0];
    int i = 0;
    for (i = 0; i < vector_size; i++)
    {
        if (vector[i] < min)
        {
            min = vector[i];
        }
    }
    return min;
}

void fill_matrix_randomly(int matrix[n][m], int max_value)
{
    int i = 0;
    int j = 0; 
    srand(time(NULL));
    for (i = 0; i < n; i++)
    {
        for (j = 0; j < m; j++)
        {
            matrix[i][j] = rand() % max_value;
        }
    }   
}

void write_matrix(int matrix[n][m])
{
    int i = 0;
    int j = 0;
    for (i = 0; i < n; i++)
    {
        for (j = 0; j < m; j++)
        {
            printf("%4d", matrix[i][j]);            
        }
        printf("\n");
    }
}

void write_vector(int* vector, int vector_size)
{
    int i = 0;
    for (i = 0; i < vector_size; i++)
    {
        printf("vector[%d] = %d\n", i, vector[i]);
    }
}

And here is the output.

$ mpiexec -n 5 ./main
   8   1   3   9
   1   0   3   1
   4   9   5   5
   1   7   4   9
   8   5   7   4
Vector of partial max values.
vector[0] = 9
vector[1] = 8
vector[2] = 5
vector[3] = 9
vector[4] = 16
Vector of partial min values.
vector[0] = -1
vector[1] = -1
vector[2] = -1
vector[3] = -1
vector[4] = 1
Matrix boundaries = [-1..16]

Where am I wrong? What I should change? I spend the whole day trying to find solution.

Upvotes: 0

Views: 6587

Answers (1)

Hristo Iliev
Hristo Iliev

Reputation: 74455

Your matrix is n rows by m columns. When you distribute this matrix to n processes, each process has to process m elements, but instead you use a count of n in all your vector calls. You should pass a length of m in:

  • the MPI_Scatter call;
  • both MPI_Reduce calls;
  • both write_vector calls;
  • the find_max and find_min calls.

Note that you have correctly declared receive_buffer, partial_max, partial_min to be of m elements.

Upvotes: 2

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