Calculating histogram of a 2 dimensional array in Parallel - parallel-processing

Question

I have a 9x9 2D array. I fill the array with random numbers and then send the 1st 3 rows of the array to process with rank=1, 2nd 3 rows of the array to process with rank=2 and at last, the 3rd 3 rows of the array to process with rank=3. I mean each process will get 3 rows of the array.
After the respective rows of the main array are received by the process, it will calculate the histogram of the 3 rows which it received from the root process. Then return the histogram back to the root process. All the 3 processes will do this task.
The problem is that the process with rank=3 can not receive the rows which are sent by the root process. I could not figure it out why?
I would be very grateful if someone have a look at my code and find a solution for my problem.
Thanks in advance.
I am using MPI with c. here is my code
#include "mpi.h"
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
#include <time.h>
int main(int argc, char* argv[])
{
int my_rank;
int p;
int i;
int k;
int m;
int tag=0;
int array[9][9];
int sub_array[3][9]; // 3 rows and 9 columns for each process
int c[20];
MPI_Status status;
// MPI_Datatype MPI_INT;
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&my_rank);
MPI_Comm_size(MPI_COMM_WORLD,&p);
// MPI_Type_contiguous(9,MPI_INT,&MPI_INT);
// MPI_Type_commit(&MPI_INT);
srand(time(NULL));
for(i=0; i<20 ; i++)
c[i] = 0;
if(my_rank == 0)
{
for(i=0; i<9 ; i++)
for(k=0; k<9 ; k++)
{
array[i][k] = rand()%20+1; // fill the array with random numbers from 1 to 20;
}
for(i=0; i<9 ; i++)
{
printf("\n");
for(k=0; k<9 ; k++)
{
printf("%d ",array[i][k]); // prints the array here
}
}
// here each process will be sent 3 rows of the array;
for(i=0; i<3 ; i++)
MPI_Send(&(array[i][0]),9,MPI_INT,1,tag,MPI_COMM_WORLD); // 1st 3 rows of the array are sent to process 1
for(i=3; i<6 ; i++)
MPI_Send(&(array[i][0]),9,MPI_INT,2,tag,MPI_COMM_WORLD); // 2nd 3 rows of the array are sent to process 2
for(i=6; i<9 ; i++)
MPI_Send(&(array[i][0]),9,MPI_INT,3,tag,MPI_COMM_WORLD); // 3rd 3 rows of the array are sent to process 3
for (i=1;i<=3;i++)
MPI_Recv(&sub_array, 9, MPI_INT,i, 0, MPI_COMM_WORLD, &status); // root receives the subarrays from each node;
}
if(my_rank != 0)
{
// for the process with rank=1;
for(i=0; i<3 ; i++)
{
MPI_Recv(&(sub_array[i][0]),9,MPI_INT,0,tag,MPI_COMM_WORLD,&status);
for(k=0; k<9 ; k++)
{
for(m=0 ; m<20 ; m++) // here the process with rank=1 calculates the histogram
{
if(sub_array[i][k] == m)
c[m]++;
}
}
}
// for the process with rank=2
for(i=3; i<6 ; i++)
{
MPI_Recv(&(sub_array[i][0]),9,MPI_INT,0,tag,MPI_COMM_WORLD,&status);
for(k=0; k<9 ; k++)
{
for(m=0 ; m<20 ; m++) // here the process with rank=2 calculates the histogram
{
if(sub_array[i][k] == m)
c[m]++;
}
}
}
// for the process with rank=3
for(i=6; i<9 ; i++)
{
MPI_Recv(&(sub_array[i][0]),9,MPI_INT,0,tag,MPI_COMM_WORLD,&status);
for(k=0; k<9 ; k++)
{
for(m=0 ; m<20 ; m++) // here the process with rank=3 calculates the histogram
{
if(sub_array[i][k] == m)
c[m]++;
}
}
}
}
// here the histogram must be printed.
for(k=0; k<20 ; k++)
{
printf("\n");
printf("%2d : ",k);
for(i=0 ; i<c[k] ; i++)
{
printf("=");
}
}
MPI_Finalize();
return 0;
}

Answer 1

#include <stdio.h>
#include <conio.h>
#include <time.h>
int main(int argc, char* argv)
{
int my_rank;
int p;
int Array[9];
int localArray[3];
int i;
int k;
int m;
int a[9][9];
int b[10][50];
int c[20];
MPI_Status status;
MPI_Datatype my_type;
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&my_rank);
MPI_Comm_size(MPI_COMM_WORLD,&p);
MPI_Type_contiguous(9,MPI_INT,&my_type);
MPI_Type_commit(&my_type);
srand(time(NULL));
for(i=0; i<20 ; i++)
c[i] = 0;
if(my_rank == 0)
{
for(i=0; i<9 ; i++)
for(k=0; k<9 ; k++)
{
a[i][k] = rand()%20;
}
for(i=0; i<9 ; i++)
{
printf("\n");
for(k=0; k<9 ; k++)
{
printf("%d ",a[i][k]);
}
}
for(i=0; i<3 ; i++)
MPI_Send(&(a[i][0]),1,my_type,1,0,MPI_COMM_WORLD);
for(i=3; i<6 ; i++)
MPI_Send(&(a[i][0]),1,my_type,2,0,MPI_COMM_WORLD);
for(i=6; i<9 ; i++)
MPI_Send(&(a[i][0]),1,my_type,3,0,MPI_COMM_WORLD);
}
if(my_rank != 0)
{
for(i=0; i<3 ; i++)
{
MPI_Recv(&(b[i][0]),1,my_type,0,0,MPI_COMM_WORLD,&status);
}
for(i=0; i<3 ; i++)
for(k=0; k<9 ; k++)
{
for(m=0 ; m<20 ; m++)
{
if(b[i][k] == m)
c[m]++;
}
}
printf("\n \n ");
for(m=0 ; m<20 ; m++)
printf("%d ",c[m]);
for(i=3; i<6 ; i++)
{
MPI_Recv(&(b[i][0]),1,my_type,0,0,MPI_COMM_WORLD,&status);
}
for(i=0; i<3 ; i++)
for(k=0; k<9 ; k++)
{
for(m=0 ; m<20 ; m++)
{
if(b[i][k] == m)
c[m]++;
}
}
printf("\n \n ");
for(m=0 ; m<20 ; m++)
printf("%d ",c[m]);
for(i=0; i<3 ; i++)
{
MPI_Recv(&(b[i][0]),1,my_type,0,0,MPI_COMM_WORLD,&status);
}
for(i=0; i<3 ; i++)
for(k=0; k<9 ; k++)
{
for(m=0 ; m<20 ; m++)
{
if(b[i][k] == m)
c[m]++;
}
}
printf("\n \n ");
for(m=0 ; m<20 ; m++)
printf("%d ",c[m]);
}
for(k=0; k<20 ; k++)
{
printf("\n");
printf("%2d : ",k);
for(i=0 ; i<c[k] ; i++)
{
printf("=");
}
}
MPI_Finalize();
return 0;
}
#Colin D I edited the code above and it's working now BUT the problem is that 3.processor doesn't receive or 0.processor couldn't send the rows.I didn't achieve that.

Answer 2

In the following loop:
// for the process with rank=3
for(i=6; i<9 ; i++)
{
// snip
for(i=0; i<3 ; i++) // notice the reuse of the var i here!
//snip
}
In you loop, you are reassigning the value for i. this might be your problem. for the loops for procs with rank 1 and 2, you use a different variable k for this loop.