Pointer

A lot of beginning programers consider that pointers are difficult to use. But there is nothing conceptually challenging in understanding pointers. It is true that careless uses of pointers (or memory management) can cause head-aches in debugging. But pointers are useful and powerful tools.

• Simplified view of computer memory
  • Computers use RAM (random access memory) to store data.

    

  • It's like a bunch of storage containers linked up like chains.
    
  • Each conainer has a unique address.
  • When you declare a variable, a box become automatically dedicated to store the value of the variable.

    int x = 111;
    float y = 0.23;
    
    

• Pointer variable
  • A pointer is just a variable that points at, or refers to, another variable.

  • The pointer variable stores the address of another variable.
    

    Let's think about the departmental mail boxes, each of you have your own box. In addition to individual boxes, there may be a box for the ``organizer of biobites seminar series''. Inside of this box, there is a piece of paper, saying who is the current organizer. So the person who is distributing the mails will take a look at this paper, and put the thank-you letter for the organizer to the person's mailbox. The piece of paper in the organizer's box was a pointer to another mailbox. Pointer variables have this kind of functionality.

• Basic pointer operations
  • Declaration:

        int *xPtr;
    

    Notice ``*''. This is saying xPtr is a variable, which stores the address of int variable.
    You should associate * with int, and read this as xPtr is a type (int *).

    If you want to declare 2 pointer variables and 1 simple variable:

        double *ptr1, y, *ptr2;

  • Assignment:
    

        int x = 111;
        xPtr = &x;

    ``&'' means ``address-of''. So in this case address of x is created by ``&x'' (in our case ``3''), this value is stored in the space allocated for the pointer variable (xPtr)

  • Read the value of variable from the pointer variable (dereferncing)

       printf("%d\n", *xPtr);
          

    In this context, ``*'' means ``contents-of''. This statement will print ``111''.
    
    

  • Set the value with a pointer variable.

        *xPtr = 6;
    
    Now the value of x become 6.

  • Exercises:
    
    • Write a program which will do the following steps.
      1. Declare two integer variables (i, j), and two pointer variables to integers (ptr1 and ptr2).
      2. Initialize i and j with 1 and 2, respectively.
      3. Store the address of i in both ptr1 and ptr2. In other words, both ptr1 and ptr2 are pointing to i.
         
      4. Use ``dereferencing'' to print the value of the variable which ptr1 is pointing (i.e. the value of i.
      5. Now increment the *ptr2 by 3.
      6. printf("Direct: %d, throughPntr: %d \n", i, *ptr1);
      7. Assign the address of j to ptr1

      8. j = *ptr1 + *ptr2;
      9. Print i, j, *ptr1, and *ptr2.
    • What is wrong with the following program? Does it compile? Does the compiled program run correctly? If not, fix the problem.

      #include <stdio.h>
      
      int main (void) {
        int *ptr;
      
        *ptr =  11;
        printf ("%d\n", *ptr);
      }