// CS1621 Fall 2005
// Array access in C++ using indexing and pointers
#include <iostream>
using namespace std;

int main()
{
    int A[100];

    // With "normal" index access, we must evaluate the index expression
    // at each iteration through the loop.  This can add some overhead to
    // access, especially if it is sequential through the array.
    for (int i = 0; i < 100; i++)
    {
         A[i] = i;
         cout << A[i] << " ";
    }
    cout << endl << endl;

    // With pointer access the pointer moves down the locations so that
    // at each iteration all we must do is derereference the pointer to
    // obtain the data
    for (int * p = A; p < A + 100; p++)
    {
         *p = 100 - (p - A + 1);
         cout << *p << " ";
    }
    cout << endl << endl;

    // Traditional 2-D array access using two index values
    int B[10][5];
    for (int i = 0; i < 10; i++)
    {
        for (int j = 0; j < 5; j++)
        {
             B[i][j] = i+j;
             cout << B[i][j] << " ";
        }
        cout << endl;
    }
    cout << endl;

    // Single pointer 2-D array access.  Note the deferencing of the
    // array when used with the pointer.  This is because a 2-d array
    // in C++ is really a pointer to an array (as opposed to a pointer
    // to an int.  Dereferencing the array gives us a pointer to an int
    // that is compatible with our normal pointer.

    // Note that if we are just iterating through the array, our pointer
    // access is again much more efficient than access by index.  However,
    // if we need to calculate the i, j indexes anyway, we probably do not
    // save wih pointer access.
    for (int *p = *B; p < *(B + 10); p++)
    {
        int diff = p - *B;
        int i = (diff / 5);
        int j = (diff % 5);
        cout << "B[" << i << "][" << j << "] = " << *p << " ";
        if (j == 4)
            cout << endl;
    }
}