Thread: Pointers, Arrays, Assigning values

  1. #1
    Registered User
    Join Date
    Feb 2008

    Pointers, Arrays, Assigning values

    I have zero idea why this is occuring, I have looked at so many examples of pointers and arrays and I just cannot figure this out.

    All I am trying to do right now is create an n by m matrix and populate it with random numbers...but all I get are null pointers (ie 0)

    #include <stdio.h>
    #include <stdlib.h>
    #include "mpi.h"
    int main(int argc, char* argv[]){
            int rank;
            int p;
            int i, j;
            double *A;              //Matrix A
            double *b;              //Vector b
            int m = atoi(argv[1]);
            int n = atoi(argv[2]);
            //printf("M=%d, N=%d\n", m,n);  //Strictly used for testing purposes
            A = malloc((n*m)*sizeof(double));
            for (i=0; i<n; i++){
                    for(j=0; j<n; j++){
                            A[i*n + j] =  rand() % 100;//assigns *A random numbers between 0 - 99
                            printf("%d\n",A[i*n +j]);
            //genMatrix(n, m, A);
            MPI_Init(&argc, &argv);
            MPI_Comm_rank(MPI_COMM_WORLD, &rank);
            MPI_Comm_size(MPI_COMM_WORLD, &p);
            return 0;
    I even tried writing
    double *A;
    A = malloc((n*m)*sizeof(double);
    A[0] = 5;
    printf("%d", A[0]);
    And this is returning 0, rather, a null pointer as well. Any help would be greatly appreciated

  2. #2
    Join Date
    Jul 2009
    srand() before calling rand() and:
    for(j=0; j<n; j++);  // change to: for (j = 0; j < m; j++);
    Disclaimer: This post shows my ignorance at the time of its making. I claim ownership of but not responsibility for all errors in it. Reference at your own peril.

  3. #3
    Registered User
    Join Date
    Feb 2008
    I found the solution to my error...

    I am supposed to use %.1f....not %d


  4. #4
    and the hat of int overfl Salem's Avatar
    Join Date
    Aug 2001
    The edge of the known universe
    If you're using gcc as your compiler, then seriously consider adding at least -Wall as a command line option.

    One of the many things it can diagnose is mis-matched printf/scanf formats.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

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