Returning an array of 2D arrays in C

Question

First, I'll admit this is homework but it has been around six years since I last programmed in C and ever since I have been only programming in Python and Java.

I want to generate successor 2D arrays to a 2D array for example:

[1][2][3] 
[4][5][6] 
[7][8][ ]

For the 2D array above, the successor 2D arrays would be:

[1][2][3]
[4][5][6]
[7][ ][8]

and

[1][2][3]
[4][5][ ]
[7][8][6]

This wouldn't be a problem if I just placed the code for this in a main() method.

However I want to separate the code for this part and encapsulate it in a function and just call it when I need it. In other words, I want to generate both arrays from inside a function and return both of them.

In C this is isn't as straightforward because I can't make a function that can pass an array of 2D arrays.

I have some ideas like

return a struct with a 2d array and next variable that is a pointer to another successor 2D array (I want to process all the successor arrays in a loop).
create a global pointer where I will point the head to the first struct, which in turn points to the next succesor 2d array and so on.

But I am not really confident which one to try. Looking for other helpful leads.

It's not clear at all what "successor" means. The examples are not doing it for me. — Chris Eberle
– Chris Eberle, Commented Jul 20, 2011 at 16:49
Terminology nitpick: C doesn't have methods, it has procedures and functions. — cdhowie
– cdhowie, Commented Jul 20, 2011 at 16:49
What makes you think that you can not pass an array of 2D arrays ? It is perfectly straightforward. What have you tried ? — Paul R
– Paul R, Commented Jul 20, 2011 at 16:54
Looks to me like a slid puzzle. Maybe you should add the tag homework to your question. — Nobody moving away from SE
– Nobody moving away from SE, Commented Jul 20, 2011 at 16:56

Chris Eberle · Accepted Answer · 2011-07-20 17:12:14Z

3

To return an array of 2D arrays:

int*** getArrayOf2DArrays(int num_arrays, int rows_per_array, int cols_per_array)
{
    int*** arr = malloc(num_arrays * sizeof(int**));
    // check that arr isn't null

    for(int i = 0; i < num_arrays; i++)
    {
        arr[i] = malloc(rows_per_array * sizeof(int*));
        // again, check result

        for(int j = 0; j < rows_per_array; j++)
        {
            arr[i][j] = malloc(cols_per_array * sizeof(int));
            // yet again, check result

            // NOT necessary, but if you want to initialize the values
            // here, you could. Either use memset or:
            for(int k = 0; k < cols_per_array; k++)
                arr[i][j][k] = 0;
        }
    }

    return arr;
}

And then you can access it with arr[array_number][row][col]. Make sure to free it when you're done (similar process, only in reverse):

void freeArrayOf2DArrays(int*** arr, int num_arrays, int rows_per_array)
{
    // sanity checks here
    for(int i = 0; i < num_arrays; i++)
    {
        // and here
        for(int j = 0; j < rows_per_array)
            // and here
            free(arr[i][j]);
        free(arr[i]);
    }

    free(arr);
}

And of course you can just pass this pointer around to any of your functions using a int*** data type.

edited Jul 20, 2011 at 17:12

answered Jul 20, 2011 at 16:52

Chris Eberle

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3 Comments

Jose Over a year ago

Nope not just ONE 2D array, I want to return an array of 2D arrays :)

Chris Eberle Over a year ago

Ah ok, well then that's just one more level. Let me adjust this.

Chris Eberle Over a year ago

By the way, in most circles an array of 2D arrays is also known as a 3D array (unless each array is a different size, in which case it's a 3D jagged array).

Vitor · Accepted Answer · 2011-07-20 17:03:15Z

1

An array of 2D arrays is just a 3D array, so you can pass them the same way as parameters/return values. Since you have a fixed size in 2 dimensions, it may be a lot easier to just use a int* as type, and treat it as a 3D array

answered Jul 20, 2011 at 17:03

Vitor

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