How to initialize an array using pointers?

Question

I need to initialize a 0 matrix using pointer to address its elements

Answer 1

See if this is what you want:

int main()
{
    int matriz[5][5], i, *ponteiro;
    ponteiro = matriz[0];
    for (i = 0; i < 25; i++)
    {
        *ponteiro = 0;
        ponteiro++;
    }

    return 0;
}

It’s pretty much the same thing that was answered in your question vector sum with pointers.

Answer 2

Four ways to zero a matrix come to mind, among them the easiest would be this:

int matriz[5][5] = {0};

The normal, basic level, that would be:

int matriz[5][5], i, j;
for(i=0; i<5; i++)
{
    for(j=0; j<5; j++)
    {
        matriz[i][j]=0;
    }
}

A middleman, I think that’s the one you want, would be to work with pointers. As the allocation is sequential, the next or previous values will always be in the next house, going from the beginning to the end of matriz. I only see use for something of the kind in an auxiliary function, because that way it is completely a waste of time, there is no need.

int matriz[5][5], i, j, *ptr=matriz;
for (i=0; i<25; i++, ptr++)
{
    *ptr=0;
}

And it has the advanced method, with memset(), here you mess with memory directly, the first argument is the vector, the second is the value to be setate and the third the amount of values of the vector to be setate in bytes. You first need to know what kind of data you are working with, its size in bytes, it is used function sizeof() that takes the data and returns the amount of bytes allocated to that data. So, to reset matriz you could do just that:

memset(matriz, 0, sizeof(matriz));

Answer 3

#include <string.h>

/* ... */

int matrix[3][4];
memset(matrix, 0, 3 * 4 * sizeof(int));

The function memset receives a pointer, a value and a quantity. It will fill that amount of bytes from the pointer with that value.

Since the size is in bytes, the quantity must be multiplied by the size of the matrix type.

Edit: As suggested in the comment of that reply, it is possible to do the following as well:

memset(matrix, 0, sizeof(matrix));

This is possible because the compiler knows the full size of the matrix, since its dimensions are defined in the variable. But if you instead had a pointer to the matrix, that wouldn’t be possible:

int *ptr = &matrix[0][0];
memset(ptr, 0, sizeof(ptr));         /* ERRADO:  vai zerar apenas o primeiro elemento */
memset(ptr, 0, 3 * 4 * sizeof(int)); /* CORRETO: vai zerar toda a matriz */