# 3.8. Parameters and Arguments¶

Some of the built-in functions we have used have parameters, which are values that you provide to let the function do its job. For example, if you want to find the sine of a number, you have to indicate what the number is. Thus, sin takes a double value as a parameter.

Some functions take more than one parameter, like pow, which takes two double s, the base and the exponent.

Notice that in each of these cases we have to specify not only how many parameters there are, but also what type they are. So it shouldn’t surprise you that when you write a class definition, the parameter list indicates the type of each parameter. For example:

void printTwice (char phil) {
cout << phil << phil << endl;
}


This function takes a single parameter, named phil, that has type char. Whatever that parameter is (and at this point we have no idea what it is), it gets printed twice, followed by a new line. I chose the name phil to suggest that the name you give a parameter is up to you, but in general you want to choose something more illustrative than phil.

In order to call this function, we have to provide a char. For example, we might have a main function like this:

int main () {
printTwice ('a');
return 0;
}


The char value you provide is called an argument, and we say that the argument is passed to the function. In this case the value ’a’ is passed as an argument to printTwice where it will get printed twice.

Alternatively, if we had a char variable, we could use it as an argument instead:

int main () {
char argument = 'b';
printTwice (argument);
return 0;
}


Notice something very important here: the name of the variable we pass as an argument (argument) has nothing to do with the name of the parameter (phil). Let me say that again:

Note

The name of the variable we pass as an argument has nothing to do with the name of the parameter.

They can be the same or they can be different, but it is important to realize that they are not the same thing, except that they happen to have the same value (in this case, the character ’b’).

Warning

The value you provide as an argument must have the same type as the parameter of the function you call.

This rule is important, but it is sometimes confusing because C++ sometimes converts arguments from one type to another automatically. For now you should learn the general rule, and we will deal with exceptions later.

The following code will show the output of the printTwice function. Try modifying the value of argument to change the output.