Functions

• Functions in Python are ways to “encapsulate” parts of code in order to be organized and reused

• A programmer who uses a function does not need to know how a function works. He just informs the inputs and outputs!

• Example: math.sqrt(3)

  • Input: what is inside the parentheses: 3
  • Output: what is the expression evaluated to: 1.7320508075688772

Why use functions?

• Programs could be written without functions and all code written in one block, but:
  • the program it would become large
  • a lot of code would be duplicated
• Breaking big programs into smaller functions:
  • allows work to be divided among several programmers
  • enables functions to be re-used in other programs
  • makes testing and maintenance easier
  • improves readability

Defining a function

• Python has many built-in functions like print(), but you can also create your own functions
  • these functions are called user-defined functions

def function_name():
    statement1
    statement2

• The code to be executed when the function is called needs to be indented - “inside” the function definition
  • The code inside the definition will not run unless it is called later!
• The name for a function should follow the same general rules as for variables - as they also are variables pointing to objects.
  • Functions are also objects: of the “callable” type. Check, for example, type(print).

Examples

# define the function
def greeter():
    print('Hello world!')

# call the function
greeter()

Hello world!

# define function
def print_square():
    print('****')
    print('****')
    print('****')
    print('****')

# call the function
print_square()

****
****
****
****

• What is the flow of execution of the programs above? (state the sequence using the line numbers)