writeAssignment.c

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

int main(int argc, char **argv) {
  int rank, nprocs;
  MPI_File file;
  MPI_Init(&argc, &argv);
  MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  
  MPI_File_open(MPI_COMM_WORLD, "test.txt", MPI_MODE_CREATE|MPI_MODE_WRONLY,
      MPI_INFO_NULL, &file);
  
  char str[32]; 
  int idx[2][2] = {1,3,0,2};
  int val[2][2] = {10,2,1,4};
  for (int i=0; i< 2; i++) {
    sprintf(str, "%-15d\n", val[rank][i]);
    MPI_Offset offset = idx[rank][i]*16;
    MPI_File_seek(file, offset, MPI_SEEK_SET);
    MPI_File_get_position(file, &offset);
    MPI_Status status;
    printf("%d len %lu val %d offset %lld idx %d str \"%s\"\n", rank, strlen(str), val[rank][i], offset, idx[rank][i], str);
    MPI_File_write(file, str, strlen(str), MPI_CHAR, &status);
  }
  MPI_File_close(&file);

  MPI_Finalize();

  return 0;
}
~           

that should work, depending on how fast it is for all processes to seek once per element.
I vaguely remember an MPI File set size command, which might make things faster.

http://www.mpich.org/static/docs/latest/www3/MPI_File_set_size.html

note, this is setup for scattered indices, we will be collecting a range of contiguous indices on each part so only a single write is needed