3.5. Adding New Functions¶

So far we have only been using the functions that are built into C++, but it is also possible to add new functions. Actually, we have already seen one function definition: main. The function named main is special because it indicates where the execution of the program begins, but the syntax for main is the same as for any other function definition:

RETURN_TYPE NAME ( LIST OF PARAMETERS ) {
  STATEMENTS
}

The first line of this definition is called the declaration. A declaration introduces a new name into the program. For functions, the function name and all of the parameter types must unique. In the previous example, the declaration also included the definition, however, the declaration can appear as a separate statement:

RETURN_TYPE NAME ( LIST OF PARAMETERS );

Note the semicolon ; at the end of the statement, just like any other legal statement in C++.

You can make up any name you want for your function, except that you can’t call it main or any other C++ keyword. It should also not begin with an underscore _ -- variables and function that start with an underscore are reserved for the C++ implementation. The list of parameters specifies what information -- if any -- you have to provide in order to use, or call, the new function.

main doesn't take any parameters, as indicated by the empty parentheses () in it’s definition. The first couple of functions we are going to write also have no parameters, so the syntax looks like this:

void new_line () {
  cout << '\n';
}

This function is named new_line; it contains only a single statement, which outputs a new line character, represented by the special character '\n'.

In main we can call this new function using syntax that is similar to the way we call the built-in C++ commands:

int main () {
  cout << "First Line.\n";
  new_line ();
  cout << "Second Line.\n";
  return 0;
}

The output of this program is

First line.

Second line.

Notice the extra space between the two lines. What if we wanted more space between the lines? We could call the same function repeatedly:

int main () {
  cout << "First Line.\n";
  new_line ();
  new_line ();
  new_line ();
  cout << "Second Line.\n";
  return 0;
}

Or we could write a new function, named three_line, that prints three new lines:

Here we define the three_line function, which calls the new_line function three times. The result is a function that prints three lines after it is called.

The three_line Function

 1#include <iostream>
 2
 3void new_line () {
 4    std::cout << '\n';
 5}
 6
 7void three_line () {
 8    new_line ();  new_line ();  new_line ();
 9}
10
11int main () {
12    std::cout << "First Line.\n";
13    three_line ();
14    std::cout << "Second Line.\n";
15    return 0;
16}

You should notice a few things about this program:

You can call the same procedure repeatedly. In fact, it is quite common and useful to do so.
You can have one function call another function. In this case, main calls three_line and three_line calls new_line. Again, this is common and useful.
The space between the function name and the function parentheses () is not required. These space is included here to make it easier to see, but in most book examples, you will see no space after the function name.
In three_line I wrote three statements all on the same line, which is syntactically legal (remember that spaces and new lines usually don’t change the meaning of a program). On the other hand, it is usually a better idea to put each statement on a line by itself, to make your program easy to read. I sometimes break that rule in this book to save space.

Note

In general, you'll want to write your code so that it is easy for others to follow. This is especially important if you choose computer science as a career!

So far, it may not be clear why it is worth the trouble to create all these new functions. Actually, there are a lot of reasons, but this example only demonstrates two:

Creating a new function gives you an opportunity to give a name to a group of statements. Functions can simplify a program by hiding a complex computation behind a single command, and by using English words in place of arcane code. Which is clearer, new_line or cout << '\n'?
Creating a new function can make a program smaller by eliminating repetitive code. For example, a short way to print nine consecutive new lines is to call three_line three times. How would you print 27 new lines?

Which of these statements is false about functions?

You can name a function anything you want.

You can't name a function the same name as a reserved keyword.

You can have a function with several parameters or a function with none.

This is true! However, you must always use parentheses.

You can call a function inside of another function.

This is true! It is common and useful.

You can write multiple statements on one line of a function.

This is true! As long as each statement ends with a semicolon.

Click on all function DECLARATIONS.

Click on all function CALLS.

Construct a function that correctly prints the perimeter of a rectangle.

```
cout << perimeter_value<<'\n';
```

int perimeter_value = twice_length + twice_width;

```
int twice_length = 2*length;
```
```
int twice_width = 2*width;
```
```
return parameter_value; #distractor
```
```
void perimeter (int length,int width) {
```
```
}
```

More to Explore

From cppreference.com
- Function definitions and declarations
- Functions