Creating Functions

Function definition

Specifies the name of a new function
Specifies the sequence of statements that execute when the function is called
The function can be reused throughout the program

Flow of Execution

Programs generally run from top to bottom
Function definitions are not executed
Functions run only when called

Example

def print_twice(value):
    print(value)
    print(value)

print_twice("Hello there")

Void functions

Void functions do not return a value
They may still do useful work
print is an example of a void function

Fruitful functions

Fruitful functions return a value
This value can be used later in your programs
input is an example of a fruitful function

Example

def square(n):
    return n*n

print(square(2))
print(square(7))

Parameters and arguments

Arguments are the values passed to a functions
Parameters are the variable names used inside the function

Return

Return allows us to terminate a function and return control to the caller
If used with a value, the function returns the value

We can reimplement min ourselves

def min(a, b):
    if a < b:
        return a
    else:
        return b

Functions may call other functions

Example

def square(n):
    return n*n

def cube(n):
    return n * square(n)

print(cube(2))

Recursive functions

Functions may call themselves
This must be done carefully to avoid infinite recursion

def repeat(task, num_times):
    if num_times > 0:
        task()
        repeat(task, num_times - 1)

def greet():
    print("Hello!")

repeat(greet, 3)

def fib(n):
    if n <= 1:
        return n
    return fib(n-2) + fib(n - 1)

print(fib(20))