HugoB HugoB - 1 month ago 7
C Question

How use correctly the pointers in a multi dimensional array?

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

Answer

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: enter image description here

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