CHAPTER 5 Functions Topics Introduction to Functions Defining

CHAPTER 5 Functions

Topics • Introduction to Functions • Defining and Calling a Void Function • Designing a Program to Use Functions • Local Variables • Passing Arguments to Functions • Global Variables and Global Constants

Benefits of Modularizing a Program with Functions • The benefits of using functions include: ◦ Simpler code ◦ Code reuse • write the code once and call it multiple times ◦ Better testing and debugging • Can test and debug each function individually ◦ Faster development ◦ Easier facilitation of teamwork • Different team members can write different functions

Introduction to Functions • Examples of built-in functions: input, float, int, etc • Group of statements within a program that perform a specific custom task ◦ A task of a large program with multiple tasks • Functions can be executed in order to perform overall program task ◦ Known as divide and conquer approach • Modularized program: A large task or program is broken up into smaller tasks or functions

Types of Functions • A function can just execute statements or return a value to the function that called it • A void function ◦ Simply executes the statements it contains and then terminates • A value-returning function ◦ Executes the statements it contains then it returns a value back to the statement that called it • Ex: input, int, and float functions

Naming a Function ◦ It follows the same rules as variable naming ◦ Function name should be descriptive of the task carried out by the function ◦ Often includes a verb ◦ Cannot use key words as a function name ◦ Cannot contain spaces ◦ First character must be a letter or underscore ◦ All other characters must be a letter, number or underscore ◦ Uppercase and lowercase characters are distinct

Defining a Function ◦ Function must be defined before they are used ◦ Could be in beginning of program before main program starts ◦ Function header: first line of function – Includes keyword def and function name, followed by parentheses and colon ◦ Block: set of statements that belong together as a group – Example: the statements included in a function ◦ Function definition template def function_name(): statement

Definition and Calling Function Example # This program demonstrates a void function. # First, we define a function named message. # Notice statements in function are indented since that shows # where the function statements begin and end # Statements in main are not indented def message(): print('I am Arthur') print('King of the Britons') # Main program # Call the message function. print (“Hello”) message() Output Hello I am Arthur King of the Britons

Example Function ◦ Notice that the function is defined before it is used ◦ The first statement to be executed is not in the function even though it is listed first ◦ The first statement to be executed is in the main program ◦ Then the function is executed which executes the statements in the function

Calling a Function within Function • You can call one function within another function • You cannot call the main program • This allows you to break up the code to be more readable and repeat things with custom values

Designing a Program to Use Functions • In a flowchart, a function call is shown as a rectangle with vertical bars on each side ◦ Function name written in the symbol ◦ Typically draw separate flow chart for each function in the program • End terminal symbol usually reads Return • Top-down design: technique for breaking algorithm into functions

Function Flowchart

Variable Types ◦ Global – value can be seen in all functions ◦ Local – value can only be seen in function that created it ◦ Parameter – used when passing a value to a function

Local Variables • Assigned a value inside a function ◦ Belongs to the function in which it was created • Only statements inside that function can access it • Error will be generated if accessed from outside the function • Scope: the part of a program that recognizes a variable ◦ For local variable – only in function that created it

Variables Definition and Usage • Define local variables first • Cannot be accessed by statements inside its function before it is created just like before • Different functions may have local variables with the same name ◦ Each function does not see the other function’s local variables ◦ Otherwise there would be confusion between functions

Problem With Local Variable # Definition of the name function. def name(): get_name() print('Hello', name) # This causes an error! # Definition of the get_name function. def get_name(): name = input('Enter your name: ') # Call the name function. name()

Arguments • Argument: value (data) that is sent into a function ◦ ◦ Function can use argument in calculations Argument is placed in parentheses after function name Received by parameter in function If variable is used, its value cannot be changed ◦ Only value is sent to function

Passing Arguments to Functions Example

Parameter Variable • Assigned the value of an argument when the function is called ◦ The parameter and the argument have same value ◦ General format: ◦ def function_name(parameter): ◦ Scope of a parameter: only the function in which the parameter is used

Parameter In Use

Passing Multiple Arguments • Python allows writing a function that accepts multiple arguments • Parameter list items are separated by comma • Arguments are passed by position to corresponding parameters ◦ First parameter receives value of first argument, second parameter receives value of second argument, etc.

Passing Multiple Arguments Example

Multiple Parameters In Action # multiple_args. py # This program demonstrates a function that accepts two arguments. def main(): print('The sum of 12 and 45 is') show_sum(12, 45) # The show_sum function accepts two arguments and displays their sum. def show_sum(num 1, num 2): result = num 1 + num 2 print(result) # Call the main function. main()

Making Changes to Parameters • Changes made to a parameter value within the function do not affect the argument ◦ Known as pass by value ◦ Provides a way for unidirectional communication between one function and another function • Calling function can communicate with called function

Making Changes to Parameters In Use

Making Changes to Parameters In Use • Figure 5 -18 ◦ The value variable passed to the change_me function cannot be changed by it