Directions for Lab #4

( Latest Revision: Oct 17, 2007 )

Directions for Lab #4

In this lab we will work on if-(then)-else logic, techniques of top-down program design, and input checking.

We will work on a program whose purpose is first to read two quiz scores entered from the keyboard and then to print their maximum.

However, if the user enters an invalid (out-of-range) quiz score, we want the program to print an error message and then just stop (without asking for more input, or trying to calculate a maximum, or trying to print out a maximum). We need to put if-(then)-else logic into the program to make it work this way.

It would be possible to design the program to read both quiz scores and then check to see if they are both valid. However, in this lab you must write the program code so that if the first score that the user enters is not valid the program will print an error message and stop without reading or prompting for a second score.

There is a structure diagram of the assigned program here. It shows that the main program calls four sub-functions: PrintHowTo, GetScore, RangeError, and maximum.

Individual steps:

Make a file called lab04.cpp from the program skeleton you see when you click here.
Read the program. Pay special attention to the (header) comments above each of the function definitions. The header comments explain what each of the functions is supposed to do.
In the main function, add the needed C++ instructions under each of these comments:
(Be careful to group the statements in the first else clause with braces to form a compound statement. Remember that you have to write this part of the program so that if the user enters an invalid score the program will immediately print an error message and then stop immediately.)

Use your understanding of what each function is supposed to do to guide you as you write the function calls in this part of the program.

After you complete this step, raise your hand and ask me to check your work.
In the next several steps we will put stub code (placeholder code) into the sub-functions (the functions that are called by the main function). After doing this the program will still be unfinished. Nevertheless we will be able to compile and execute the program. We will be able to check to see if the main function is performing correctly.

Through the use of stubs, a programmer can build a program one function at a time and test each function individually. This is a very valuable technique. It keeps things pretty simple for the programmer, even when the program being written is very large and complex.
Replace the comment in the body of function GetScore with the following stub code:
```
     cout << "Entering function GetScore" << endl ;
     return 300 ; 
```
Replace the comment in the body of function RangeError with the following stub code:
```
     cout << "Entering function RangeError" << endl ;
     return false ; 
```
Replace the comment in the body of function maximum with the following stub code:
```
     cout << "Entering function maximum" << endl ;
     return x ; 
```
Compile and execute the program. Debug the program before continuing. The output should have a structure like this example:
```
This program interactively reads two quiz
scores and prints the maximum.

Entering function GetScore
Entering function RangeError
Entering function GetScore
Entering function RangeError
The maximum score is: Entering function maximum
300.
```
(In other words you should see two sets of outputs from the GetScore and RangeError stubs, and then the output from your main program that reports the value returned by function maximum, with the stub screen output of function maximum inserted in between.)

After you have verified that the program produces output like this, send me a copy of the source code by e-mail. Use the subject line: CS1500_Lab4_Vers1
Change the stub code in function GetScore so that it returns 750 instead of 300. Change the stub code in function RangeError so that it returns true instead of false. Then compile and execute the program. Now the output should have a structure like this example:
```
This program interactively reads two quiz
scores and prints the maximum.

Entering function GetScore
Entering function RangeError
You entered an invalid score.
Scores must be in the range 0-600 (inclusive).
Please try again.
```
(In other words, you should see exactly one output from GetScore, one from RangeError, and then the error message about invalid data that you put in your main program earlier. If you see more than one output from GetScore and RangeError then you probably forgot to enclose between braces the statements in the main program after the first else. If so, you need to fix that.)

When you have the program producing output like the example above, send me a copy of the source code by e-mail. Use the subject line: CS1500_Lab4_Vers2
Change the stub code in function RangeError so that it returns false (as before). Then delete the stub code from inside GetScore and put code inside it that makes it do what it is supposed to do.

The (header) comment above function GetScore says: "Function to prompt for a score, read it, and return the score to the caller." Therefore you write the code to make GetScore do exactly that - no more or less.

The code in GetScore should declare a (local) int variable, write a prompt for the user, input a number into the variable, and then return the variable. If you want to see an example of the type of function I mean, look at the function GetDim in this program.

Compile and execute the program (lab04.cpp). Debug the program before continuing. If you run the program and enter scores of 250 and 340, the output should have a structure like this example:
```
This program interactively reads two quiz
scores and prints the maximum.

Enter a quiz score: 250
Entering function RangeError
Enter a quiz score: 340
Entering function RangeError
The maximum score is: Entering function maximum
250.
```
Replace the stub body of RangeError with code that makes it work correctly (i.e. put code in the function that makes it do what the header comment above RangeError says it should do - no more and no less). You can do this with one line of code: a return followed by a Boolean expression. You can also use if-(then)-else logic.
Similarly, replace the stub body of function maximum with code that makes it do just what its header comment says it should do. Try writing the code this way: declare a temporary variable. Use if-(then)-else logic to test which variable is the max and store the max in the temporary variable. Then return the value of the temporary variable. Make sure that your function returns a correct result in all cases: x less than y, x greater than y, or x equal to y.
Compile and test the program by running it five times on different kinds of inputs:
- The first score entered is out of range.
- The first score entered is in range, but the second is not.
- Both scores are in range and the first score is larger than the second.
- Both scores are in range and the second score is larger than the first.
- Both scores are in range and the scores are equal.
Debug and re-test as necessary. When the program is working correctly on all five types of input, make a single script that shows all five tests. E-mail me a copy of the final version of the program source code with subject line: CS1500_Lab4_Fin and e-mail me a copy of the (filtered) script with subject line: CS1500_Lab4_Scr

As usual, you can refer to the Hello World! example for details on how to make and filter a script file, and on how to e-mail files to me.