Passing multidimensional arrays [solved]

Question

How do you pass a variable length multidimensional array to a function in C?

Take the code snippet below as an example.

Void foo(int **);

main()
{
int a;
scanf("%d",&a);
int ar[a][a];
foo(ar);
printf("%d",ar[0][0]);
}

Void foo(int **a)
{
a[0][0]=12;
}

in that snippet, I wanted to pass a VLA to the function foo which will then set element [0][0]. Then I wanted to print that change from main. But my function prototype is not correct. It will be easier if the second index was known before hand like ar[][5], but in this case, neither of the index is known at compile time as it depends on user input.
I tried declaring the array as global but that was worse as it was declared before it's index. A quick solution would be to define the length of the array, but I want the user to enter the length of the array instead of tampering with the source code to change the defined value.

So help!!

Answer 1

You can pass the variable length array to a function by first passing its dimensions:
void foo(int m, int n, int a[m][n]);

if you need, the first dimension can be omitted:

void foo(int n, int a[][n]);

Note that this approach only works in C, not in C++.

https://code.sololearn.com/c3s1saJvYi13/?ref=app

Answer 2

There are many ways to represent a 2-dimensional array but the following example with a user-selected 1 dimensional array should get you started:

#include <stdio.h>
#include <stdlib.h>

void foo(int *, int);

int main()
{
int a;
scanf("%d",&a);
int *ar = (int*)malloc(a * sizeof(int));
foo(ar, a);
printf("%d",ar[0]);

// if this wasn't main, it would be important to call free(ar);
// since this is main, free isn't needed as the memory will be freed automatically as the program closes.
}

void foo(int *a, int len)
{
a[0]=12;
}

I included a len parameter because if you were really looping through or processing all elements in the array, you'd need to know how many elements are in it.

If you just want a square 2-dimensional array, I would actually just use a single block of memory like this example and you could translate any [x][y] indexes by doing something like x * size + y or x + y * size consistently. size would be the number of elements along a single dimension of the square array so 16 if it was supposed to be a 16 by 16 array.

Another way to represent a 2-dimensional array would be an array of pointers to your allocated blocks of memory. This way, each block could be a different size to represent different array lengths. An array of pointers would leave you accessing each element with notations like [x][y].

Answer 3

Martin Taylor Thanks man. I appreciate your varied examples and the comments. Thanks

Answer 4

Josh Greig Your example is pretty new to me. I didn't know you could traverse multidimensional arrays that way. I will read up on it. Thank you.

Answer 5

Volodymyr Chelnokov Thanks man. That was to the point.