Memory map for a 2D array in C

Solution 1:

See my question here.

That is not the way you access information about 2-d arrays. In fact, you can just think of them as 1-d, where you multiply and add the indices in a special way.

e.g.

int x[10] = {0,1,2,3,4,5,6,7,8,9};
int y[2][5] = {{0,1,2,3,4},{5,6,7,8,9}};

These are formatted exactly the same in memory, and they look like this:

|0|1|2|3|4|5|6|7|8|9|

So to get the 8 element, you can either ask for x[8], or y[1][3].

For the second way, you can think of it as (1 * 5) + 3.

This is why your first 4 were the same. You have:

• arr: this is the address of the start of the array
• arr[0]: this is address of the start of the first sub-array, which is the same as the start of the overall array
• &arr[0][0]: this is the address of first element of the first sub-array, also the start of the overall array
• arr[0][0]: this is the value stored in the first element of the first sub-array.

Solution 2:

Your code just uses a plain multidimensional array, but the image describes an array of pointers, like the kind you usually make when malloc-ing things.

A multidimensional array is basically just a normal, flattened, array (in memory) with some extra syntatic sugar for accessing. So while it is possible to get a pointer from arr[i], there isn't an extra "variable" just to store this, as happens in your image.

To correct the image, remove the parts with the arr[0], arr[1]... and change the value of arr to 1245039 (the same as &arr[0][0]).