Functions in Python

Big \(O\) Notation

Big \(O\) notation is used to describe the performance or complexity of an algorithm. It specifically describes the worst-case scenario, and can be used to describe the execution time required or the space used by an algorithm.

Comparison computational complexity

Functions

• Functions are reusable blocks of code that perform a specific task
• They can take input parameters and return output values
• Functions are essential in modular programming, as they help organize code and make it more readable

Defining a Function

• To define a function in Python, use the keyword def followed by the function name and input parameters in parentheses
• The function body is indented below the header line
• Use the keyword return to specify the output value(s) of the function

def add_numbers(x, y):
    result = x + y
    return result

type(add_numbers)

function

Calling a Function

To call a function, use its name followed by input values in parentheses The function returns the output value(s), which can be stored in a variable or used directly

sum = add_numbers(2, 3)
print(sum)

5

Default Parameter Values

• Functions can have default values for input parameters, which are used when no value is provided
• Default values are specified in the function header

def greet(name, greeting = "Hello"):
    print(greeting + ", " + name)

greet("Alice")

Hello, Alice

greet("Bob", "Hi")

Hi, Bob

Variable-Length Arguments

• Variable-length arguments allow a function to accept any number of input arguments
• They are useful when the number of input arguments is unknown or can vary

def add_numbers(*args):
    result = 0
    for num in args:
        result += num
    return result

add_numbers

<function __main__.add_numbers(*args)>

add_numbers(1, 2)

3

add_numbers(1, 2, 3)

6

Function Annotation

def add_numbers(*args):
    
    """
    Computes the sum of n numbers
    
    Parameters:
    args: A tuple of numbers
    
    Returns:
    int: The sum
    """        
    
    result = 0
    for num in args:
        result += num
    return result

help(add_numbers)

Help on function add_numbers in module __main__:

add_numbers(*args)
    Computes the sum of n numbers
    
    Parameters:
    args: A tuple of numbers
    
    Returns:
    int: The sum

Lambda Functions

• Lambda functions are anonymous functions that can be defined inline and used immediately
• They are useful for simple tasks that don’t require a named function
• Lambda functions can only have one expression

double = lambda x: x * 2
print(double(3))

6

Recursion

• Recursion is a technique where a function calls itself
• It is useful for solving problems that can be broken down into smaller subproblems

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n-1)

print(factorial(5))

120

Global vs Local Variables

• Global variables are defined outside of any function and can be accessed from anywhere in the program
• Local variables are defined inside a function and can only be accessed within that function

global_var = 10

def my_func():
    local_var = 20
    print(global_var)
    print(local_var)

my_func()

10
20

print(global_var)

10

# print(local_var)

Exercises

Exercise 1

Python function that takes a list of numbers and returns their product

multiply_numbers([2, 3, 4])

24

def multiply_numbers(numbers):
  product = 1
  for num in numbers:
    product *= num
  return product

Exercise 2

Write a Python function to find the maximum and minimum numbers in a list

find_min_max ([2, 3, 4])

(2, 4)

def find_min_max(numbers):
  min_num = numbers[0] # Set the min to the first number in the list
  max_num = numbers[0] # Set the max to the first number in the list
  for num in numbers:
    if num > max_num:
      max_num = num
    elif num < min_num:
      min_num = num
  return (min_num, max_num)

find_min_max ([2, 3, 4])

(2, 4)

Exercise 3

Write a Python function that takes a list of strings and returns the longest string.

find_longest_string(["Vicky", "Cristina", "Barcelona"])

'Barcelona'

def find_longest_string(strings):
  # Set the longest string to first string in the list
  longest_string = strings[0]
  for string in strings:
    # Compare the length of the current string to the longest string so far
    if len(string) > len(longest_string):
      longest_string = string
  return longest_string

find_longest_string(["Vicky", "Cristina", "Barcelona"])

'Barcelona'