Lab 12: Pointers

Objectives

The objectives of this lab are to become familiar with using pointers in C. Once again we will be using a makefile to build these programs; however, this time you must write the makefile yourself from scratch (1 Point).

(Hint: the easiest way to do this is to copy a makefile from a previous lab and modify if). Your makefile should properly compile and link the source files needed to make the three programs described below. The commands we will use to compile your programs are:

          make swap

          make swapbig

          make reorder

You will also need data to test the second and third programs below. You should create your own data files that verify that your functions work correctly under all conditions (1 point). We will run your programs with the following commands:

          swapbig < swapbig.dat

          reorder < reorder.dat

Programs

• (2 Points). Exploring Pointers
  • Copy the files swap.c and swaptest.c to your directory. Create the appropriate header file, swap.h, for the function defined in swap.c. Compile this program using your makefile with the command:
    • make swap
  • Run the program and observe the output. Make sure you understand what this program is doing. On a piece of paper, draw the picture showing the trace of the code and state what the memory addresses are for the three variables, a, b, and c. You will turn in this paper to the TA before leaving the lab.

• (3 Points). Using Pointers
  • Write a function, swap_big(), which is given two pointers to floats and swaps the values in the cells pointed to such that the value pointed to by the first is less than (or equal to) the value pointed to by the second. The prototype for your function is:
    • void swap_big(float *a, float *b);
  • Your data file, swapbig.dat, should show that your function works under all conditions. Write a driver to allow you to test this function for different pairs of floats when redirected from your data file. There is more than one way to do this program. You should use the swap() function from the previous problem to implement your function.

• (3 Points). More Pointers and Functions
  • Write a function, reorder(), which is given three pointers to floats and reorders the values pointed to into ascending order. The prototype for this function is:
    • void reorder(float *a, float *b, float *c);
  • i.e. when your function is done, the following should hold:
    • *a <= *b <= *c
  • Use your swap_big() function from the previous problem to implement reorder(). Your data file, reorder.dat, should show that your function works under all conditions. Write a driver to allow you to test this function for different triplets of floats when redirected from your data file.

What You Turn In

Use the "grade" command to turn in the all of the source and data files (.c,.h, and .dat files) and the makefile in your Lab12 directory. You will turn in your trace of the first problem on paper to the TA BEFORE leaving the lab. Make sure you have your name and login id on the paper. Your command will look like the following.

If you are in Section 001 use:

            grade -lab12s1,ee160  *.c *.h *.dat  makefile

If you are in Section 002 use:

            grade -lab12s2,ee160  *.c *.h *.dat  makefile

If you are in Section 003 use:

            grade -lab12s3,ee160  *.c *.h *.dat  makefile

If you are in Section 004 use:

            grade -lab12s4,ee160  *.c *.h *.dat  makefile

You should verify that you turned in things successfully, which you can do with the command (which simply leaves the file names off from the previous command).

            grade -lab12s1,ee160

            OR

            grade -lab12s2,ee160

            OR

            grade -lab12s3,ee160