Question

all. I have an interesting question, posed to me by my C++ professor. It is also used in some programming job interviews, apparently.

Here’s the situation. I have a two-dimensional array (matrix) of int’s:

int array1[][2] = {{11, 12}, {21, 22}};

I would like to create a second two-dimensional array, array2, that points to this first array. That is, they would occupy the same space in memory.

The obvious solution is to use pointers. So, I declared array2 like so:

int (*array2)[2][2];
array2 = &array1;

This seems to work. Now, here’s the catch. I am trying to subtract 11 from array1, but do it by referencing array2. Here is the code for what I’m trying to accomplish:

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 2; j++) {
        // Trying to do the equivalent of this:
        array1[i][j] -= 11;
    }
}

I have tried the following in the body of the internal for-loop, all without success:

// Second row not affected
*(array2[i][j]) -= 11;

// Second row not affected
*(*(array2[i]) + j) -= 11;

// incompatible types in assignment of `int' to `int[2]'
*( *(array2 + i) + j) -= 11;

Any ideas? I suspect there’s something simple that I’m just not getting here.

Thanks!

Answer 1

Since this is C++, you can create an alias array using a reference, that’s exactly what they do.

int array1[][2] = {{11, 12}, {21, 22}};
int (&array2)[2][2] = array1;

for(int i=0; i<2; ++i)
    for(int j=0; j<2; ++j)
         array2[i][j] -= 11;

Although the problem is more likely a C problem with the expectation that array2 would be a pointer to the first row of array1

int array1[][2] = {{11, 12}, {21, 22}};
int (*array2)[2] = array1;

for(int i=0; i<2; ++i)
    for(int j=0; j<2; ++j)
         array2[i][j] -= 11;