Question

I am having some trouble dealing with some C code. Can someone explain this syntax:

void some_function(Int16 omegaFlag[2][8])
{
    for(i = 0; i < 2; i++)
    {
        Int16 *Flag = omegaFlag[i] + 1;
        for(j = 0; j < k; j++)
        {
            // do some stuff
            *Flag++ = some_integer_value;
        }
    }
}

1. Why the parameter Int16 omegaFlag[2][8] passed in some_function() declares index values? How are they helping the code(in general, not specific to this code)?
2. *Flag++ = some_integer_value;: What does this line mean?

Answer 1

The first index to omegaFlag[2][8], is not required and is ignored by the compiler. The second, however, is important, because it tells the compiler this a two-dimensional array where each row contains 8 elements, so advancing the pointer will advance by 8 elements.

omegaFlag[2][8] is equivalent to omegaFlag[][8] or (*omegaFlag)[8]. However it is different than **omegaFlag because of the memory layout. **omegaFlag is an array of pointers, while omegaFlag[2][8] is an array of arrays — with space for exactly 8 elements, or a total space for 16 elements.

*Flag++ does two things, it de-references Flag, and then also increments its value by 1. Flag is a pointer to and Int16 representing the second value in the ith row of the matrix omegaFlag. The assignment assigns a value to that element, and then advances Flag to point to the next element, which will be assigned in the next iteration of the loop.