MPI_Reduce on different communicators not working as expected - c++

Question

#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
using namespace std;
int ceil(int x, int y) {
return x / y + (x % y > 0);
}
void create_group_and_comm(MPI_Group *world_group, MPI_Group *group, MPI_Comm *comm, int size, bool is_even) {
int *ranks;
int count = is_even ? ceil(size, 2) : size / 2;
ranks = (int *)malloc(count * sizeof(int));
int i = is_even ? 0 : 1, j=0;
while(i < size) {
ranks[j] = i;
j++;
i+=2;
}
MPI_Group_incl(*world_group, j, ranks, group);
MPI_Comm_create(MPI_COMM_WORLD, *group, comm);
free(ranks);
}
int main(int argc, char *argv[])
{
int size, rank, *result_odd, *result_even;
int rank_gr;
char processor_name[MPI_MAX_PROCESSOR_NAME];
MPI_Status status;
MPI_Comm even_comm, odd_comm;
MPI_Group even_group, odd_group, world_group;
int *A, *Rows;
int namelen;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Get_processor_name(processor_name, &namelen);
if (rank == 0)
{
A = (int *)malloc(size * size * sizeof(int));
for (int i = 0; i < size * size; i++) {
A[i] = rand() / 1000000;
}
printf("Initial data:\n");
for (int i = 0; i < size; i++)
{
putchar('|');
for (int j = 0; j < size; j++)
printf("%.4d ", A[i*size+j]);
printf("|\n");
}
MPI_Barrier(MPI_COMM_WORLD);
}
else
MPI_Barrier(MPI_COMM_WORLD);
MPI_Comm_group(MPI_COMM_WORLD, &world_group);
create_group_and_comm(&world_group, &even_group, &even_comm, size, true);
create_group_and_comm(&world_group, &odd_group, &odd_comm, size, false);
Rows = new int[size];
MPI_Scatter(A, size, MPI_INT, Rows, size, MPI_INT, 0, MPI_COMM_WORLD);
result_odd = new int[size];
result_even = new int[size];
if(rank % 2 == 0) {
MPI_Reduce(Rows,result_even,size,MPI_INT,MPI_MAX,0,even_comm);
} else {
MPI_Reduce(Rows,result_odd,size,MPI_INT,MPI_MIN,0,odd_comm);
}
if(rank == 0) {
printf("Max values for columns on even:\n");
for(int idx = 0; idx < size;idx++) {
printf("Column %d: %d\n", idx+1, result_even[idx]);
}
printf("Max values for columns on odd:\n");
for(int idx = 0; idx < size;idx++) {
printf("Column %d: %d\n", idx+1, result_odd[idx]);
}
}
//MPI_Comm_free(&even_comm);
//MPI_Comm_free(&odd_comm);
MPI_Group_free(&even_group);
MPI_Group_free(&odd_group);
MPI_Finalize();
return 0;
}
Hello i'm writing an application using MPI library, i'm trying to create 2 groups with each of them with their own communicator. Basically one group which holds processors with rank even calculates the maximum value per column using MPI_Reduce between them(processors in group), and the second one calculates the minimum for each column in matrice. For even rank MPI_Reduce works as expected but for processors with odd rank is not working as it should, can someone help me what i'm doing wrong? Below is a picture with the problem i described:
image here