Question

Can someone explain to me how C retrieves the correct memory address for a row when you only use one subscript to access a 2d array?

Example –

int array2D[2][2] = {1,2,3,4};
printf ( "starting address of row2 = %p" , array2D[1]);

I understand that when subscripting in C that what is actually going on is pointer addition so for a 1d array the array name points to element 0. In this case if I had wanted element 1 the compiler would take the starting address (say 4000) and add 4 to it (assuming a 4 bit int) so that what is returned is the item at memory address 4004.

My understanding is that when you populate a 2d array, as in my example, they are allocated sequentially so I would have

1 2
3 4
at addresses

4000 4004

4008 4012

So how does C work out that in this case array2D[1] should point to 4008 and not 4004? Does it run a sizeof() operator or have I misunderstood a fundamental here?

Thanks in advance

Answer 1

C knows how long each row is, so it does the multiplication to find the row.

int x[][3] = {{1,2,3},{4,5,6}};

then &x[1][0] is &x[0][0] plus 3 * sizeof(int).

That’s why in a multidimensional C array declaration, all but the first dimension must be specified.