Laboratory Exercise on Function Parameters, Testing,
and More About Passing Values and Addresses as Parameters

This lab covers three main topics:

• Function parameters
• Testing
• More passing values and addresses as parameters

Preparation before Class

• Reading from Text
  • King: Sections 9.3, 11.1-11.2, pages 191-200, 241-245
  • K&R: 1.7, 4.1-4.3, 5.1, 5.2
• Reading: Function pointers
• Reading: Correctness and Testing

Work Started in Class

Function Parameters

As in Scheme, a function can be a parameter for another function. In Scheme, of course, all parameters are un-typed — the parameter may have one type (e.g., a number) at one time, then another type (e.g, a string) for another call, and yet another type (e.g., a function) for a third call. In C, each variable and parameter has a designated type. However, as we shall see, the designated type can indicate a function. For example, a function might have the following signature:

  void printTable (double func (double, double))

1. Program func-parm.c uses a function printTable with the signature above to print a table of metric equivalents.

   1. Save, compile, and run the program, and describe what is printed. Also, explain the roles of the functions toLiters and toCenti, and how these functions are utilized to obtain the output.

   2. Function printTable prints a table, with rows always going from 0.0 to 10.0 and the columns going from 0.0 to 10.0, based on local variables rowStart, rowEnd, colStart, and colEnd. In this example, however, it would make more sense for the number of pints to go from 0.0 to 7.0 (number of pints less than a gallon), and the number of inches to go from 0.0 to 11.0 (number of inches less than a foot).

      Replace these local variables with parameters, so that function printTable has the new signature:

        void printTable (double rowStart, double rowEnd,
                         double colStart, double colEnd,
                         double func (double, double))
      

      Then change the calls to printTable in the main program to more appropriate limits for each type of table.

2. Within a program to control a Scribbler 2 robot, write a function with the following signature:

   void boxMove (void movement (double, double))
   

   The function should move the Scribbler 2 robot 5 times using the following sequence:

   • move forward for 1 second
   • call the movement function with values 0.6, 1.0.

   The main part of the program should call boxMove using the MyroC rTurnLeft function as parameter and then call boxMove again using rTurnRight.

   Why do you think the name boxMove was chosen? Is there a better name?

Arrays of Functions

As noted earlier in this lab, functions as parameters provide one mechanism to take advantage of common elements within an algorithm. A second approach involves utilizing an array of functions.

Program func-parm-arrays.c produces exactly the same output as program func-parm.c from Step 1.

3. Copy this program to your account, compile it, and run it to check that the output matches the output from Step 1.

   1. Check that the functions toLiters, toCenti, and printTable are unchanged from Step 1.

   2. In the main program, note how variable titles is declared and initialized as an array of two strings. (In this context, char * indicates titles will refer to the start of strings, and [2] indicates the array will refer to the start of 2 strings.)

   3. In the main program, note how the variable f refers to an array, each containing the address of a function of two double parameters.

4. Write 3 functions that cause the Scribbler 2 robot to react in different ways (e.g., move/turn, beep, spin). Then, write a program that uses these three functions as follows.

   • Within main, make an array that references each of these functions.
   • Use a loop to execute these functions in order, and then use a second loop to execute the functions in reverse order.

Testing

Pre- and Post-Conditions

5. Review the MyroC.h header file.

   1. In the documentation, find at least two functions that have stated a "pre-condition" and at least two functions that have stated a "post-condition".

   2. Using these functions as examples, write a careful definition of what is meant by the terms ,"pre-condition" and "post-condition".

   3. In anticipation of later work in this lab, review the documentation for the function rMotors. The documentation does not explicitly state pre-conditions for this function, but one might infer such conditions. Write a careful statement of the implied pre-condition(s) for rMotors.

6. The program motors-test.c sets the motor speeds of the Scribbler to the given leftspeed and rightspeed.

   1. Initialize the variables leftspeed and rightspeed to 1.

   2. Initialize the variables leftspeed and rightspeed to 1 and -1 respectively.

   3. Initialize the variables leftspeed and rightspeed to 2 and -1. respectively.

   4. Now try 6 and 5 respectively.

   5. Try other numbers that you might need to figure out what works and what doesn't.

   6. How do these experimental results compare with the pre-conditions that you inferred in Step 5c?

The assert Function in C

7. Modify the same code, motors-test.c, to use assert  so that it will test the precondition(s) you wrote for rMotors in motors-test.c.

   Note: You can read about C's assert function in the reading for this lab and/or using the command man assert in a terminal window. Better yet, read about assert in both places!

Choosing Test Cases

8. Copy the program object-avoid.c to your working directory. This program attempts to control the robot in avoiding obstacles. Run the program a few times and observe what it does.

   1. Develop two test plans for object-avoid.c to figure out if the program works correctly. That is, apply both black-box and white box testing by identifying test cases that will cover a full range of situations that might be encountered in executing the program object-avoid.c . Remember that:

      • Black-box testing is when the problem is examined to determine the logical cases that might arise. Test cases are developed without reference to details of code.

      • White-box testing is when the code is examined to determine each of the possible conditions that may arise, and tests are developed to exercise each part of the code.

   2. You should have found that the program is not working properly. Now fix the program so that it performs in the intended way and the robot avoids obstacles.

Homework

Debugging

9. Program follower-test.c is supposed to control the robot in following a sheet of paper which is held in front of the robot.

   1. Develop two test plans for this program, using both black-box and white-box testing strategies. Apply both black-box and white box testing to identify tests cases that will cover a full range of situations that might be encountered.

   2. Run follower-test.c  with all the cases from your test plan to determine if the program works correctly.

   3. Fix the errors you found in the program.

   4. Run the program again with all the cases from your test plan to be sure that it now works correctly.

More Values and Addresses as Parameters

ints by Reference

10. Consider the program rand-beep.c.

    1. Copy this program to your account, review the code, and run it a few times to see what it does.

    2. Write a few sentences explaining what the program is doing, making sure include the following points:

       • Identify every time the address of an int is passed into a function.

       • When addresses are passing into a function, explain how the values at these addresses are referenced within the function.

       • What durations are possible for the robot to beep for?

       • What frequencies are possible for the robot to beep for?

       • When will the loop end?

Indicating Function Success

11. Write a simple function which finds the square root of a number. It will have the following signature: int safe_sqrt (double * num).

    • It will use the double sqrt(double num) function from the math.h library to find the square root of num. Make sure you compile with the -lm flag when using sqrt().

    • If num is zero or positive, your function will modify num to be its square root and return 1 to indicate success.

    • If num is negative, your function will not modify num and return 0 to indicate failure.

    • You will have to use the & operator when calling your function so that num is passed in by reference.

Addresses (If you have time — Optional)

12. Copy get-ir.c into your directory for this lab and write a few sentences explaining what the program is doing.

    Complete the parts of the program which say "code here!". It should only require one line of code at each spot, and there are three locations where you must add code.

    Hint: If you are getting the warning "assignment makes pointer from integer without a cast", try putting * (an asterisk) in front of the integer you are assigning to.

Feedback Welcome

Development of laboratory exercises is an iterative process. Prof. Walker welcomes your feedback! Feel free to talk to him during class or stop by his office.