Question

I am Trying to work through some class examples and have gotten stuck on the following:

The array grid should
have length width with each entry representing a column of
cells. Columns that have some occupied cells should be a
malloc’ed character array of length height.

with the given header:

void grid(char **grid, int width, int height)

grid is defined in another file as:

char **grid;

As I have said I have gotten stuck on using malloc, I currently have:

int x;
*grid = malloc(width * sizeof(char));

        for(x = 0; x < width; x++){
            grid[x] = malloc(height * sizeof(char));
        }

Can any one take a look at give me some pointers on the correct way to accomplish “Columns that have some occupied cells should be a
malloc’ed character array of length height.”, As I dont understand how the line:

grid[x] = malloc(height *
sizeof(char));

is equivalent to an array of char’s

Thanks

Answer 1

In C, an array is a pointer to the first element of the array. So an array is just a memory block, with the array variable pointing onto the first element in this memory block.

malloc() reserves a new memory block of the specified size. To know the size of a type (i.e. number of bytes needed to store one variable of that type), one uses the sizeof operator. Hence a character needs sizeof(char) bytes, and hence height characters need height * sizeof(char).

So with the malloc() call you allocate a memory block to store all the elements of the array, and malloc() returns a pointer onto the first of them.

With C’s definition for an array variable (a pointer onto the first element), you can assign the results of malloc(...) to your array variable.