C - Bidimensional Array without constants

Question

I've got a school project where I have to create a game with a bi dimensional array where the rows and columns will change. So what I want to do is asking to user what's the number of rows and the number of columns and then create the game board. So if the user says 6 rows and 10 columns I'll save it in two variables and then I'll create the bi dimensional array. The problem here is that Visual Studio said that I've got to have constant values, there's a way to make a bi-dimensional array without constants? Thanks for the help.

Answer 1

With your compiler, you have to use malloc to allocate your array. Although C99 supports the int[x][y] Syntax when x or y are not compile-time constant, Visual Studio does not.

Allocate your array like this:

int* ar = malloc(sizeof(int) * x * y);

Then you can use it like this:

int getIndex(int* array, int col, int row){
    return ar[row*y+col];
}

Of course, you have to store the y somewhere so that you can use it for index calculations (as shown in the getInt example function. For example, you could wrap the int* pointer to the array and the y into a struct and always pass around this struct.

Answer 2

If you want to use normal array indexing for a dynamic 2d array:

int* array_mem = malloc( sizeof(*array_mem) * x * y ) ;
int** array = malloc( sizeof(*array) * x ) ;
int i = 0;
for( i = 0; i < x; i++ )
{
    array[i] = &array_mem[i * y] ;
}

The array and be indexed as a two dimensional array such as array[0][5], and shares the same contiguous memory properties of a normal array. However sizeof(array) will not return the size of the allocated memory as it would for a normal array.

Note that in this case you are responsible for returning the memory to the heap when done - it does not do that automatically except on termination of the process.

If you have not covered dynamic memory or pointers in class, you may be better off using a simpler alternative solution, such as simply creating an array large enough for the largest dimensions a user will request (and include some range checking on the input).

Answer 3

You get this answer because you are using a crappy, non-standard compiler. The solution to your problem is to compile your C code on a good compiler instead.

Code such as int arr[x][y];, where x and y are plain int variables, is perfectly fine.

This is known as a variable-length array (VLA). Visual Studio doesn't know this, because it doesn't follow the C standard.