If I declare an array in C:
Answer by an example:
int A; int i = 2; int j = 4; int* p = A[i]+j;
Of course, since any array is a consecutive piece of memory (consisting of 18
int elements in the example above), you can also cast it into a simple pointer, and then refer to whichever element you want.
int* p = (int*)A+i*6+j;
You can compute the exact number of columns (6 in the example above), using any of the following:
sizeof(A[x])/sizeof(A[y][z]) // x,y,z are any integer constants or variables
But you can do so safely only within the array's scope of declaration (function if it's a local variable, or file if it's a global variable).