Functions

Goals

This laboratory exercise provides initial practice with functions and passing values as parameters within C programs.

Consider the program quadratic.c that solves the quadratic formula ax² + bx + c = 0, while illustrating several common variations for simple functions.

void and non-void Functions

1. Copy quadratic.c to your account, compile and run it, and review the code to determine how it works.

   1. Procedure printEqn has a void return type. Parameters are passed into printEqn, but nothing is passed back. What happens if the statement return a+b+c; is inserted at the end of this procedure? (Try compiling the code and explain what happens.)
   2. Restore the program to its original state.
   3. Procedure printEqn is called in both procedures eqn1 and eqn2 with just a simple statement, such as:

      printEqn(1.0, -3.0, 2.0);

      What happens if you try to assign the result of printEqn to a variable, such as

      double value = printEqn(1.0, -3.0, 2.0);

      (Try this and explain what happens.)
   4. Restore the program again to its original state.

   5. Procedure disc returns a double. Sometimes beginning coders separate the call to a function with the assignment of the return value. Here are three examples:

      Attempt 1:

      double discriminant;
      disc(a, b, c);

      Attempt 2:

      disc(a, b, c);
      double discriminant;

      Try each of these attempts within procedure printRoots.

      • Does the code compile?
      • Does the code give the desired result?
      When you are finished, restore the program to its original state.
   6. Change the body of disc to the following

      printf("r = %lf, s = %lf, t = %lf\n", r, s, t);
      return sqrt(s*s - 4*r*t);

      Compile and run the program.
      • Describe what is printed by the overall program. What might you conclude about what happens when printing is done in the middle of a computation? For example, how does this impact the overall program's output?
      • Can you identify any guidelines regarding the advisability of including print statements in functions designed for computation (e.g., disc)?
      • In addition to the printf statement in function disc, insert the code from Step 1c (into printRoots). What happens when procedure with a return value (e.g., disc) is called on a line by itself?
      Restore the program to its original state.

Writing Numeric Functions

2. Within a C program, define and use the following functions:
   • Function circum that takes a circle's radius as parameter and returns the circumferences of the circle.
   • Function area that takes a radius as parameter and returns the area of the circle with that radius.

Functions, Values as Parameters, and Robot Motion

Program yoyo.c uses the function yoyo to control a Scribbler 2 robot.

3. Copy yoyo.c to your account, compile and run it, and review to code to determine how it works.
   1. Explain what the program does.
      • What movements does the robot make? Why?
      • What output is printed? Why?
   2. In the main program, duplicate the line

      result = yoyo (repetitions);

      (I.e., this line should appear twice in succession.)

      Does making this call twice change how many times the robot yoyos in each call? Why or why not?

   3. In the main program, replace the line

      result = yoyo (repetitions);

      by

      repetitions = yoyo (repetitions);
      repetitions = yoyo (repetitions);

      Does making this call twice change how many times the robot yoyos in each call? Why or why not?

   4. In C (as in Scheme), variables declared within a function are separate from variables declared elsewhere (e.g., in main). In the original program, change the name of the repetitions variable to reps throughout the main procedure. Compile and run the program.

      Does changing yoyo's variable reps have any impact on the variable reps in the main procedure?

   5. Again, return to the original program. This time nest the call to yoyo in the main procedure. That is, the call to yoyo will become:

      yoyo (yoyo (repetitions));

      • Make a prediction of what will happen with the above nested call.
      • Test out your nested call and explain what happened.
4. Recall the reading's discussion of memory diagrams. Draw a sequence of schematic memory diagrams for program quadratic.c from Step 1.

   Important: To benefit from this exercise, you must actually draw these diagrams. It is not a mental exercise.

5. Consider the program value-params.c.
   1. Copy the program to your account, compile, and run it.
   2. Draw schematic memory diagrams for the state of memory when each print statement is executed. Be sure you can explain the running of the program at each step, based upon this series of schematic memory diagrams.