Where to initialize array then to scatter it. MPI_Scatter - mpi

Question

I need to send array pieces to all processes using MPI_Scatter then to get sum of all elements. Where should I initialize array then to scatter it? In root rank?
If I initialize array on root rank then other ranks dont get their data. Otherway I can initialize array for everyone (out of if(rank == root)...else), but it means, that I create array several times.
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
#include <iostream>
#include <time.h>
using namespace std;
int main(int argc, char* argv[])
{
int size;
int rank;
srand(time(NULL));
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
int arr_size = size * 2;
int block = arr_size / (size);
int* B = new int[block];
if (rank == 0)
{
int* A = new int[arr_size];
cout << "generated array: " << endl;
for (int i = 0; i < arr_size; i++)
{
A[i] = rand() % 100;
cout << A[i] << " ";
}
cout << endl;
MPI_Scatter(A, block, MPI_INT, B, block, MPI_INT, 0, MPI_COMM_WORLD);
}
cout << "process " << rank << " received: " << endl;
for (int i = 0; i < block; i++)
{
cout << B[i] << " ";
}
cout << endl;
int local_sum = 0;
for (int i = 0; i < block; i++)
{
local_sum += B[i];
}
cout << "sum in process " << rank << " = " << local_sum << endl;
cout << endl;
int global_sum;
MPI_Reduce(&local_sum, &global_sum, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);
if (rank == 0)
{
cout << "sum = " << global_sum << endl;
}
MPI_Finalize();
return 0;
}
I get something like this (only root rank got its data):
process 1 received:
process 3 received:
-842150451 -842150451
-842150451 -842150451
sum in process 1 = -1684300902
sum in process 3 = -1684300902
process 2 received:
-842150451 -842150451
sum in process 2 = -1684300902
process 0 received:
4 9
sum in process 0 = 13
sum = -757935397

Answer 1

MPI_Scatter() is a collective operation and must hence be invoked by all the ranks.
Declare int *A = NULL; on all ranks and only allocate and populate on rank zero.
int* A = NULL;
int* B = new int[block];
if (rank == 0)
{
A = new int[arr_size];
cout << "generated array: " << endl;
for (int i = 0; i < arr_size; i++)
{
A[i] = rand() % 100;
cout << A[i] << " ";
}
cout << endl;
}
MPI_Scatter(A, block, MPI_INT, B, block, MPI_INT, 0, MPI_COMM_WORLD);