CODEWITHSUNDEEP
carrayspointersmultidimensional-array
minimalistic
minimalistic

Reputation: 37

Pointer to a 3d array

I would like to declare a pointer to a 3d array in a manner that I could index it with one dimension less than original array, example: ptr[i][j]. I'm using some structure in which I would like this pointer to be stored so I could access it later.

I've done this with the 2d arrays but I declared a pointer to an array of pointers to 2d arrays:

typedef const unsigned char* ptrType;
const ptrType ptrArray[] = {...};

And this is what I'm trying for the 3d array:

typedef const unsigned char** ptrType;

typedef struct structP
{
    ptrType arrayPtr;
};

and in the main I'm doing something like this:

struct structP test;
test.arrayPtr = *myThreeDArray;

when trying to access elements via pointer this is the only thing that compiler let me do:

&(test.arrayPtr[i][j]);

Also myThreeDArray is defined like this:

const unsigned char myThreeDArray[2][23][25] = { ... };

Doing it in a way that is described gives me unspected results at the output. Some garbage values.

Any ideas how to do this in a proper way?

Upvotes: 0

Views: 8291

Answers (1)

4386427
4386427

Reputation: 44246

What you seem to want is a pointer to a 2D array

For integers this could be like:

#include <stdio.h>

void print_3d(int (*p3d)[3][4])
{
    for (int i=0; i<2; ++i)
        for (int j=0; j<3; ++j)
            for (int k=0; k<4; ++k)
                printf("%d ", p3d[i][j][k]);
}

int main(int argc, char *argv[])
{
    int arr3d[2][3][4] = {
                          {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 10, 11, 12}},
                          {{13, 14, 15, 16}, {17, 18, 19, 20}, {21, 22, 23, 24}}
                         };

    int (*p_to_arr3d)[3][4] = arr3d;  // Get a pointer to int[3][4]
    print_3d(p_to_arr3d);             // Use the pointer
}

Output:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

In case you are dealing with strings, it could be like:

#include <stdio.h>

void print_3d(char (*p3d)[3][20])
{
    for (int i=0; i<2; ++i)
        for (int j=0; j<3; ++j)
            printf("%s ", p3d[i][j]);
}

int main(int argc, char *argv[])
{
    char arr3d[2][3][20] = {
                           {"this", "is", "just"},
                           {"a", "little", "test"}
                         };

    char (*p_to_arr3d)[3][20] = arr3d;    // Get a pointer to char[3][20]
    print_3d(p_to_arr3d);
}

Output:

this is just a little test

Using the same syntax as above you can store the pointer in a struct:

struct myData
{
    char (*p_to_char_arr)[3][20];
    int (*p_to_int_arr)[3][4];
};

Upvotes: 2

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