Pointer Comparison with Arrays

• Pointers and Arrays

  • As we have already learned, arrays are collections of objects of the same type

    • E.g., the statement int myInts[100]; reserves memory space for 100 integer objects and refers (or names) this space myInts.

      • To get at any individual element (or object), we indicate which one we want by providing an index, e.g., myInts[7] allows us to access the *eighth* element of the array (remember we start counting at zero).

      • When we refer to myInts in our program without any index, the compiler sees the name as a reference (or pointer) to the entire array.

      • For example, in the following program fragment, the pointer variable ptr is set to point to the beginning of the array with the assignment ptr = myInts;  In this case, since the array is an array of integers, the variable ptr must be a pointer to integers or the compiler will complain of mis-matched types.

      • When pointers refer to arrays, they can be indexed just like the original array (see the last statement in the program fragment below).

        int myInts[100];  // declare an array of 100 integers

        int *ptr;               // declare a pointer variable (a pointer to int)

        ptr = myInts;      // causes ptr to point to the beginning of the array, just as myInts does

        ptr[3] = 10;       // sets the fourth element of the array myInts to 10.

         

  • So then, an array name is a pointer.  The only difference array names and general pointers is that array names are constant.  That is, they cannot be reassigned to point to other things (see example below):

    // suppose after the program fragment above, you had the following code

    // the first assignment is acceptable, because ptr can be assigned to point to any

    // valid integer (i.e,. either those in myInts or those in yourInts); however the

    // second assignment is unacceptable, because the myInts pointer cannot be

    // reassigned (since it is an array name)

    int yourInts[100];

    ptr = yourInts;

    myInts = yourInts;

     

  • Understanding the above, it should now make sense that all arrays are automatically passed to functions as reference parameters.  Because when we use the array name as a parameter, we are referring to the actual array (because the array name is a pointer) rather than a copy of the array.