C++ Function Intro

We will be using code written by other programmers a lot in this course. Most of the time we will access this code by calling the code into action using a C++ function. We need to start early in understanding these things, otherwise a lot of your code is not going to make sense!

The easiest way to understand functions is to consider them blocks of code inside a box with a name painted on the outside. Hopefully, the code inside the box does something interesting. It would be especially nice if we needed the code inside the box in several places in our program, but that is not really necessary.

When we want the code inside the box to do its thing, we need a way to activate it. This is called calling the function. Perhaps we need to provide the code inside the box some information that it needs to work, and perhaps the result of activating the code will be to produce some answer that we will take and use in other parts of our program.

Defining a Function

We create a function by coming up with a name, which should describe what the function is going to do for us. Next, we identify any information the function will need to accomplish its task; we call this information parameters. Next, we describe what kind of answer the function will produce (if any); this is called the return value. Finally, we write the code that defines the action of the function.

Here is an example of how it all looks in C++:

float my_sin(float angle) {
    float answer;
    // code to calculate the sin of an angle

    return answer;

The first float keyword defines the kind of value the function will return, a floating point number in this case. The name of the function is my_sin. The function needs one piece of data to work with, that parameter will be named angle inside the function, and it is another floating point number.

The body of the function is the code between the curly braces. At some point in that code, we need to include a return statement that causes the function to quit work and hand back the value specified in the statement. The thing after the word return is an expression, usually just the name of the variable you used to calculate the answer, that you want to send back to whoever called this function.

Calling a Function

When we want the code inside the function to do its thing, we call the function by creating a statement that contains the name of the function. That reference will include parentheses inside of which we will provide actual parameters we want the function to use to do its work. The parameters are separated by commas if there are more than one of them.

Parameters can be literal values, or the names of variables defined in our calling code, or expressions the system will evaluate before calling the function. The system will pass the data we specify into the function when the function wakes up. For now, we will assume that the function has no idea where the parameter information came from, it just uses the values passed in and refers to those values using the parameter names we defined when we wrote the function.

Parameter names defined in the function code do not need to be the same as the names from the caller code we place between the parentheses when we call the function.

Phew - sounds complicated, but it is actually fairly simple. (There are more rules on these parameters, but this is enough for now!)

Here is code that demonstrates calling the function:

float mySin, myCos;
float myAngle = 37.0;
float PI = (float)acos(-1.0); // this is a pretty accurate PI

myAngle = myAngle * PI / 180.0;
mySin = my_sin(myAngle);
mySin = my_sin(180 * PI);           // the expression is evaluated and the result passed in

myCos = sqrt(1.0 - my_sin(myAngle)*my_sin(myAngle));        // sqrt is a new function

In each of these examples, the reference to the my_sin function is part of an expression that is being evaluated. Once the entire expression has been evaluated, the value is stored in the container named on the left side of the equal operator. The normal rules of evaluating expressions are still being followed, but when the system gets to the reference to the function, the expression evaluation pauses for a bit while the code for the function figures out the answer needed. That answer is returned to the expression evaluator which continues on with its work.