Function

A function in programming is a separate block of code that performs a specific task. A function acts as a subprogram inside a larger program. Instead of writing all logic in one place, functions process a part of the main program’s work. By using functions, programs become more structured, cleaner, and easier to develop.

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Why Functions Are Important

Functions help to:

• break complex tasks into simpler ones
• reduce duplicated code
• reuse the same code in different programs
• divide large programs into smaller steps
• hide implementation details from users
• improve debugging and error tracking

In short, functions make programs easier to understand, maintain, and scale.

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Types of Functions

In general, there are two types of functions:

1. Built-in Functions

Built-in functions are functions provided directly by a programming language. They can be used immediately, without defining them yourself.

Example: Built-in Function

Python

print("Hello, World")

• print() is a built-in function
• It outputs text to the screen

Other examples of built-in functions:

• len()
• type()
• input()

2. User-Defined Functions (UDF)

User-defined functions are functions created by the programmer.

They are used when built-in functions are not enough to solve a problem.

Defining a User-Defined Function

def greet():
    print("Hello, World")

Calling the function:

greet()

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Functions With and Without Return Values

Function Without Return Value

This type of function performs an action but does not return a value.

def say_hello():
    print("Hello")

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Function With Return Value

A function can return a value using the return keyword. The returned value is often stored in a variable and used later.

def add(a, b):
    return a + b

Using the return value:

result = add(3, 5)
print(result)

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Function Naming Rules

Function names must follow identifier rules, just like variables:

• must not start with a number
• must not contain spaces
• must not use reserved keywords
• names are case-sensitive

Common naming styles:

• snake_case
• camelCase

Examples

# Valid
calculate_sum
getUserName

# Invalid
2calculate
my function
print

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Parameters and Arguments

Functions can accept input values.

Parameters

Parameters are variables listed in the function definition. They act as placeholders for input values.

def greet(name):
    print("Hello,", name)

Here, name is a parameter.

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Arguments

Arguments are the actual values passed to a function when it is called.

greet("Achly")

Here, "Achly" is an argument.

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Summary

• A function is a reusable block of code
• Functions help structure and simplify programs
• There are built-in and user-defined functions
• Functions may return a value or not
• Parameters define input, arguments provide input

Functions are a core building block of programming and prepare you for:

• modular programming
• clean code practices
• advanced topics like recursion and OOP