How use correctly the pointers in a multi dimensional array?

Question

I have to create a matrix and manipulate it with only one pointer

my code:

#include <stdio.h>
#include <time.h>
#include <stdlib.h>

void Matrix();
int Fun(int *A, int n, int key);

int main()
{
 Matrix();
 return 0;
}

void Matrix()
{
  int Nel, Larray, i, j, k, a, c, b, Nf;
  printf("Num of elements: ");
  scanf("%d", &Nel);

  int A[Nel][Nel];

    srand(time(NULL));
    for(b=0;b<Nel;b++)
    {
      for(c=0;c<Nel;c++)
      {
          A[b][c] = rand() % 100 + 1;
      }
    }

    printf("\n");

    for(b=0;b<Nel;b++)
    {
      for(c=0;c<Nel;c++)
      {
          printf("%d\t",A[b][c]);
      }
      printf("\n");
    }
    printf("\n\n");

    printf("Num to find? ");
    scanf("%d", &Nf);

     Fun(*A, Nel, Nf);
 }//end


 int Fun(int *A, int n, int key)
 {
   //just to see if it works
   int i,j;

   for(i=0;i<n;i++)
   {
     for(j=0;j<n;j++)
     {
      printf("%d\t",A);
      }
    printf("\n");
   }
 }

In the function Fun, gives me an error

printf("%d\t",A); subscripted value is neither array nor pointer nor vector

I have to use only one pointer to call the matrix. Can you expalin the pointer arithmetic in multi dimensional array? Thx

Zach P · Accepted Answer · 2016-10-23 08:46:03Z

1

Assuming you have int* arr; pointing to an array of size [ROWS_NUM][COLS_NUM], the calculation done when trying to access arr[X][Y] is arr + (X * COLS_NUM + Y).

A general rule would be that an absolute index of an element in a multi-dimensional array would be absolute_index = ((index1 * size2 + index2) * size3 + index3) * size4 + index4 ..... (to which you just add the offset of the array).

You can look at this sketch to understand the way a multi-dimensional array is formatted in memory:

answered Oct 23, 2016 at 8:46

Zach P

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2 Comments

HugoB Over a year ago

This is pretty clear, but how can I fix my problem? With 2 pointers I have no problem, but I have to use only one pointer!

Mr Tarsa Over a year ago

Replace printf("%d\t",A) with printf ("%d\t",*(A+(i*n+j))) or with printf ("%d\t",A[i*n+j]) and mark answer as resolved if that helps.

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