Functions

Note: This lesson covers defining functions in JavaScript and Python.

Definition

A function is a block of reusable code designed to perform a particular task. The function is executed when it is "called".

Syntax

functions

JavaScript
Python

function function_name(parameter1, parameter2, ... parameterN) {
	block;
}

JavaScript Note:

A JavaScript function is defined using the function keyword, followed by the function name, followed by parentheses (). The parentheses () may contain a list of parameters.
The code to be executed by the function is placed inside curly brackets.

def function_name(parameter1, parameter2, ... parameterN):
	block

Python Note:

A Python function is defined using the def keyword, followed by the function name, followed by parentheses () ending with a colon. The parentheses () may contain a list of parameters.
The code to be executed by the function is indented.

Note: The function will only perform an operation when the function is 'called'.

Examples

Example 1: Without Parameters

A function message() is defined. The parentheses () are empty as this function does not use parameters. This means the function is not passed data to operate on.

When the function is called, i.e., message(), it displays a simple message 'Hello'.

This is a simple example. When you create functions you can optionally define:

if the function should be passed data to operate on (Example 2: With Parameters);
if the function should return a value (Example 3: Returning Results).

JavaScript
Python

Defining a function
```
function message() {
	alert("Hello");
}
```
Calling a function
```
message();		// Hello
```

JavaScript Note: Use the function keyword, followed by the function name message, followed by parentheses (). The function code block is placed inside curly brackets. This function uses an alert to display a simple message.

Defining a function
```
def message():
	print("Hello")
```
Calling a function
```
message()			# Hello
```

Python Note: Use the def keyword, followed by the function name message, followed by parentheses () ending with a colon. The function code block is indented.

Example 2: With Parameters

Here the function will be passed data to operate on. We define a function greet with a parameter named message.

When the function is called, the function assigns the data passed to it, i.e., morning, afternoon or evening to the parameter message. Note the difference between an argument and parameter.

Argument: A value passed to a function when the function is called. E.g., greet("morning");.
Parameter: the variable listed inside the parentheses in the function definition - used to refer to the value passed to it. E.g., message.

Functions can be defined with a list of parameters by separating each parameter name with a comma.

JavaScript
Python

Defining a function

function greet(message){
	alert("Good " + message);
}

Calling a function

greet("morning"); 		// Good morning
greet("afternoon");		// Good afternoon
greet("evening");		// Good evening

Defining a function

def greet(message):
	print("Good " + message)

Calling a function

greet("morning")		# Good morning
greet("afternoon")		# Good afternoon
greet("evening")		# Good evening

Example 3: Returning Results

To allow a function to return a value, use the return statement.
The return statement immediately terminates a function execution and sends the return value back to the code that called the function. Therefore, line 3 will not run.
We save the returned value in a variable result. Then we simply display the value returned and saved to result.

JavaScript
Python

function square(x){
	return x * x;
	alert("this line will never be printed");
}

var result = square(10);
alert(result);

def square(x):
	return x * x
	print("this line will never be printed")

result = square(10)
print(result)

Example 4: Returning Ticket Price

Here is another example that returns results. We have taken the code used in the if else lesson (Example 1: Ticket Price) and rewritten it to use a function.
The function accepts a value for age and calculates and returns the appropriate entry message.
The returned value is displayed.
This code could be modified to accept the age value from keyboard input.

JavaScript
Python

function ticket(age){
	var entry = "";
 	if ( age <= 12 ) {
		entry = "Free";
	} else if ( age <= 16 ) {
		entry = "£10";
	} else {
		entry = "£20";
	}
	return entry;
}

alert(ticket(10));		// Free
alert(ticket(14));		// £10
alert(ticket(21));		// £20

def ticket(age):
	entry = ""
	if age <= 12:
		entry = "Free"
	elif age <= 16:
		entry = "£10"
    else:
		entry = "£20"
	return entry

print(ticket(10))       # Free
print(ticket(14))       # £10
print(ticket(21))       # £20