1. Functions Jay Summet

2. Aug 29 2007 2 Functions A function is a piece of code you can use over and over again Treat it like a black box You pass it (optional) values, it does some work, and it (optionally) returns values You “call it”,”invoke it”, or “use it” by using its name and parentheses The things you pass it go inside the parentheses output = function( input )‏ function input output

3. Aug 29 2007 3 Using Simple Functions print( “Some Text” ) ‏ You call a function by using it's identifier (name) followed by parenthesis. If the function takes parameters, you insert them inside the parenthesis.

4. Aug 29 2007 4 Writing Simple Functions def sayHi(): print(“Hi there!”)‏ print(“Nice to meet you!”) sayHi()‏ Indent Defining functions Creates function Does not execute/run them Indenting indicates a “block” of code Call functions from top-level or other functions No Indention “Top Level”

5. Aug 29 2007 5 Format of a function definition def function-name(): statement …

6. Aug 29 2007 6 Writing Functions with Parameters def sayHi(name): print( “Hi there”, name) print( “Nice to meet you!”) sayHi(“Jay”)‏ sayHi(“Bob”)‏

7. Aug 29 2007 7 Parameters are Variables When you pass values into functions as parameters, they get assigned to the variable names declared in the definition line of the function. For example, when you call sayHi(“Jay”) The name variable is assigned (points to) the value “Jay” When the code in the function refers to the name variable, it evaluates to the string “Jay” So, when you call sayHi(“Jay”) and the sayHi function calls print(“Hi there”, name), it's the same as if it called print(“Hi there”, “Jay”)

8. Aug 29 2007 8 Format of a Function Definition with Parameters def function-name(param0, param1,...): statement … function-name(param0, param1,...)‏

9. Aug 29 2007 9 Using Functions that Return Values name = getName()‏ print( “Hello”, name)

10. Aug 29 2007 10 Composing Functions You can use the output (return value) of one function as the input (parameter) to another function. sayHi( getName() ) In this example, the getName() function executes first (things inside parenthesis execute before things outside parenthesis)‏ The getName() function returns a name, which is then given to the sayHi() function as a parameter.

11. Aug 29 2007 11 Writing Functions that Return Values def area(radius): return 3.14 * radius**2 def circumference(diameter): return 3.14 * diameter print( “Area of a 3 ft circle”, area(3)‏ ) print( “Circumference”, circumference(2*3)‏ )

12. Aug 29 2007 12 Return Statements The return statement is used to return a value from a function The return statement also affects the flow of execution Whenever the flow of execution hits a return statement it jumps back to the place where the function was called All functions have an implicit return statement at the end of the block of indented code, even if you do not specifically place one at the end of your function

13. Aug 29 2007 13 Functions with Local Variables def area(radius): a = 3.14 * radius**2 return a def circumference(diameter): c = 3.14 * diameter return c print( “Area of a 3 ft circle”, area(3)‏ ) print( “Circumference”, circumference(2*3)‏ )

14. Aug 29 2007 14 Variables in a Function are Local Variables in a function are private – Including the parameters Each function has its own variables – Even when the names are the same Allows you to write functions independently without worrying about using the same name

15. Aug 29 2007 15 Different Variables - Same Name def area(radius): a = 3.14 * radius**2 return a def circumference(radius): a = 3.14 * 2 * radius return a a = 20 print( “Area of a 3 ft circle”, area(3) )‏ print( “Circumference”, circumference(3)‏ ) print( a )

16. Aug 29 2007 16 Writing Functions with Return Values def function-name(list-of-params): statement … return value output = function-name(list-of-params)‏

17. Aug 29 2007 17 Passing variables to a functions If you pass a variable to a function, the function gets the value that the variable is pointing at userInput = input(“Enter your Name”)‏ sayHi(userInput)‏

18. Aug 29 2007 18 Functions in general # description of this function # what it expects as input # what is provides as output def function (p 0, p 2, …, p n ): statement … return value z = function(a 0, a 2, …, a n )‏

19. Aug 29 2007 19 Where’s the Error? def area: a = 3.14 * radius**2 return a print( “Area of a 3 ft circle”, area( 3‏ )

20. Aug 29 2007 20 Not enough Parentheses! def area: a = 3.14 * radius**2 return a print( “Area of a 3 ft circle”, area(3)‏ )

21. Aug 29 2007 21 Where’s the Error? def area( radius ): a = 3.14 * radius**2 return a print( “Area of a 3 ft circle”, area(3)‏ )

22. Aug 29 2007 22 No Indentation! def area(radius): a = 3.14 * radius**2 return a print( “Area of a 3 ft circle”, area(3)‏ )

23. Aug 29 2007 23 Where’s the Error? def area(radius): a = 3.14 * radius**2 return a print( “Area of a 3 ft circle”, area()‏ )

24. Aug 29 2007 24 No argument to the function! def area(radius): a = 3.14 * radius**2 return a print( “Area of a 3 ft circle”, area(3)‏ )

25. Aug 29 2007 25 Where's the Error? def area(radius): a = 3.14 * radius**2 print( “Area of a 3 ft circle”, area(3)‏ )

26. Aug 29 2007 26 No Return Value! def area(radius): a = 3.14 * radius**2 print( “Area of a 3 ft circle”, area(3)‏ )

27. Aug 29 2007 27 Where’s the Error? def area(radius): a = 3.14 * radius**2 return a area(3)‏ print( “Area of a 3 ft circle”, a )

28. Aug 29 2007 28 Don't save the return value into a! def area(radius): a = 3.14 * radius**2 return a area(3)‏ print( “Area of a 3 ft circle”, a )

29. Aug 29 2007 29 What’s the result? def area(radius): a = 3.14 * radius**2 return a v = area(3)‏ a = 16 print( “Area of a 3 ft circle”, v ) print( “value of a”, a)