Exam 1 Lab

You should be reasonably comfortable with simple function parameters by now. The parameters you have been using are called value parameters, because the actual parameters provided by the caller are evaluated, and the final value is passed to the function. Inside the function, the parameter name is initialized with that value as the function starts processing.

There is another kind of parameter, the reference parameter. You cannot place an expression on the call if you use one of these parameters. Instead, you provide the name of a suitable variable in the function call. What happens is different. The addres of the caller’s variable is passed to the function. Instead of setting up a new variable inside the function, the function uses the caller’s variable directly! That means it can mess with the data in the caller’s world. This is both powerful, and a bit scary!

In this lab project, we are going to run a simple experiment to see how reference parameters work.

Creating Reference Parameters

The only new thing you need to know is that you must add a “&” symbol after the parameter type specification to set up a reference parameter:

int myfunc(int& vara) {
    vara = 42;
    return -42;
}

When you call this function, provide the name of a variable from your world:

int testdata = 0;
int dummy;

dummy = myfunc(testdata);

With this, we can get a normal return value from our function. But a hidden side effect is possible as well. If myfunc modifies the parameter variable (vara in this case), the caller’s testdata will be modified. It is important that you realize this!

Your Assignment

You are to build a project that has three files:

  • main.cpp
  • compare.cpp
  • compare.h

The Compare Function

The “compare.cpp” file contains one function, also named “compare”. This function takes two integer reference parameters with names “param1” and “param2” and compares them. It returns a single character based on what it sees. Here is what you are to return:

  • “=” if the two values are equal
  • “<” if param1 is less than param2
  • “>” if param1 is greater than param2

Just to prove that the “compare” function really does have access to your caller’s variables, have the function flip the sign of each parameter before you process the return statement you need in this function:

param1 = -param1;
param2 = -param2;

(Yes, this will mess up our final output, but that is fine for what we are trying to see here!)

The Main Function

Set up the main program that asks the user for two integer numbers, which are stored in variables named “var1” and “var2”. The main program then calls the “compare” function described above, passing in the parameters “var1” and “var2”. Once you have the return character, you are to test that character and print out a more complete English statement about the result. For example, if you get back an “equal sign” character, you will print out this:

  • 1234 is equal to 1234

(Since you flipped the sign in the compare function, you will actually see negative values here, not the positive values the user entered).

Modify the output so it makes sense for the other two possible return characters.

What to turn in

Place your code in your Homework repository under a folder named Exam1Lab. Push your code to GitHub and make sure you can see it in your web browser before the deadline. You can (and probably should) use a Makefile for this project, but I will not grade anything but the three files required for this project.

Your program must compile and run with no errors to be considered for grading. If it does not run properly, you will at least get partial credit.

This is due by midnight on Thursday, Oct 11. I will check the time of your submission as it is recorded on GitHub. Any work submitted after midnight will not be graded, so make sure you have something submitted before that time.