How to use malloc to assign large two-dimensional arrays in C

Question

I know a very large two-dimensional arrays in C needs a long identifier for the variable type when assigning memory to the array to avoid stack overflow errors and memory issues. Suppose I intend to assign memory to my matrix A which should hold a very large number of numbers using malloc, how am I supposed to go about it?

int main() {

    int rows = 20000;
    int cols = 20000;

    // How do I define the two-dimensional
    // array using 'long int'?
    long long int** A = new long long int(rows);

    // I got that from this site, but it's
    // wrong. How do I go about it?
}

Whatever you think to know, it is not what you think it is. Large arrays require a large (enough) type for addressing elements. That does not mean you must use large elements. That is why `size_t` is used to query size of an element and to address an element. You can also create a huge array of single chars. — Gerhardh, Feb 10 '22 at 12:15
In C++, `new long long int(rows)` allocates a *single* `long long int`, and initialize that single `long long int` to the value `rows`. — Some programmer dude, Feb 10 '22 at 12:17
@mch, no am not using c , how to use malloc is the problem am updating the post though — Son of Man, Feb 10 '22 at 12:24
[**Correctly allocating multi-dimensional arrays**](https://stackoverflow.com/questions/42094465/correctly-allocating-multi-dimensional-arrays). I'll let others decide if this is a dupe. `long long int** A` will never refer to a 2-dimensional array. — Andrew Henle, Feb 10 '22 at 12:27
@PubliusFlaviusTiberius If you're not programming in C, then why do your question say that you do? Or do you mean that you're not programming in C++? If you program in C++, then **[edit]** your question to reflect that (title, body, tags). But if you do program in C then you really need to get a book or take classes, as that should teach you all you need to know, and much more. — Some programmer dude, Feb 10 '22 at 12:31
@Some programmer dude: Because it is [paid homework](https://meta.stackoverflow.com/questions/417632/request-to-let-banned-users-ask-one-question-every-week#comment906279_417632). It is posted on behalf of somebody else. It is one of the ways to use Stack Overflow. And it works (if not discovered) due to the gamification system (e.g., the willingness to close as a duplicate is dwindling due to the impact of fewer reputation points). — Peter Mortensen, Jun 05 '22 at 11:16

score 3 · Accepted Answer · answered Feb 10 '22 at 12:32

Use pointers to array:

int main()
{
    size_t rows=20000;
    size_t cols=20000;

    long long int (*A)[cols] = malloc(rows * sizeof(*A));

    //You can use it as normal array.
    A[6][323] = rand();
}

or (but access has different syntax)

int main()
{
    size_t rows=20000;
    size_t cols=20000;

    long long int (*A)[rows][cols] = malloc(sizeof(*A));

    //You can use it almost as normal array.
    (*A)[6][323] = rand();
}

How to use malloc to assign large two-dimensional arrays in C

1 Answers1