You write the same block of code in three places. You find a bug. You fix it in one place. But the other two still have the bug.

Functions solve this. A function gives a block of code a name so you can call it from anywhere.

Think of a function as a machine. Boxes go in; those are the parameters. The machine does its work. A box comes back out; that is the return value. The machine does not care who feeds it. Give it the same inputs and it always produces the same output.

Defining a function

A function has four parts.

int add(int a, int b) {
  return a + b;
}

void

Some functions do not send anything back. They just do something.

void greet(String name) {
  print('Hi, $name!');
}
 
void main() {
  greet('Ali'); // Hi, Ali!
}

void means nothing comes out. The machine does the work, but returns no box.

Return values

The return keyword sends a value back to whoever called the function.

int square(int n) {
  return n * n; // sends back the result
}
 
void main() {
  int result = square(4);
  print(result); // 16
}

The return type in the definition must match what you actually return. Write int and return an int. If they do not match, Dart will not compile.

What if you forget return? Dart will tell you: A non-null value must be returned from the function 'square'.

Calling a function

Write the function name followed by arguments in parentheses.

void main() {
  int result = add(3, 5); // assign the result to a variable
  print(result);          // 8
 
  print(add(10, 20));     // 30
}
 
int add(int a, int b) {
  return a + b;
}

Notice that add is defined after main here. Dart reads the whole file before running it. Order does not matter.

Parameters and arguments

These two words mean different things.

In practice, many developers use them interchangeably. But now you know the distinction.

Scope

A variable declared inside a function only exists inside that function. Try to use it outside and Dart will not find it.

void greet() {
  String message = 'Hi'; // lives inside greet only
}
 
void main() {
  print(message); // Error: Undefined name 'message'.
}

When greet finishes, message is gone. Nothing outside can reach it.

This is a feature. Each function gets its own private space. They do not interfere with each other.

main revisited

You have been writing void main() since Lesson 1. Now you know exactly what it means.

main is a function. void means it returns nothing. The empty () means it takes no parameters. Dart calls it automatically when your program starts. Every other function, you call yourself.

There is more to functions than this lesson covers. Named parameters, optional parameters, arrow functions, anonymous functions, and functions-as-variables are all in Dart in Depth.

Summary