1. Neal Stublen nstublen@jccc.edu

3. Breaking Down Tasks  Reduce large tasks into smaller units of code  Smaller units of code might be called any of the following: Procedure Function Subroutine Method

4. Why use methods?  Provides high level perspective  Play MP3 Open file Read file contents Decode file contents Send decoded content to sound card  We say methods encapsulate details

5. Why use methods?  Smaller tasks are generally easier to understand  Smaller tasks can be reused  Examples: Verify a date Authenticate a user Spell check a word Calculate the area of a circle Play an alarm Take a picture

6. Naming Methods  Methods encapsulate behavior on specific data  Usually named with a verb-noun combination validateDate() authenticateUser() spellCheckWord() calculateArea() playAlarm() takePicture()

7. Program Execution  When calling a method, program execution jumps to the statements within the method  When the method returns, program execution continues after the method call

8. Example Method main() displayAddress() return displayAddress() output "S. Holmes" output "221 B Baker Street" output "London, England" return

9. Declaring Variables  We've looked at declaring variables: type identifier = value num counter = 0 string name = "John Smith"  When we declare variables within a method, we say they are local to the method or local variables  They can only be accessed from within the method  When the method is executing, they are in scope; when the method returns they go out of scope

10. Local Variables class Program main() displayAddress() return displayAddress() string name = "S. Holmes" string address1 = "221 B Baker Street" string address2 = "London, England" output name output address1 output address2 return endClass

11. Class Variables class Program string name = "S. Holmes" string address1 = "221 B Baker Street" string address2 = "London, England" main() displayAddress() return displayAddress() output name output address1 output address2 return endClass

12. Global Variables string name = "S. Holmes" string address1 = "221 B Baker Street" string address2 = "London, England" class Program main() displayAddress() return displayAddress() output name output address1 output address2 return endClass

13. Class vs. Global  The book uses the term "global" to refer to variables that are declared within a class and available to all methods within the class  It will be much better to refer to these as "class" variables  The term "global" should be used to refer to variables that are accessible from any code in an application  Global variables are often considered unwise  If you refer to class variables as global variables, you will very likely be misunderstood

14. Local Variable Scope class Program main() displayPerson() displayPartner() return displayPerson() string name = "S. Holmes" output name return displayPartner() string name = "J. Watson" output name return endClass

15. Method Parameters  When we need to provide information to a method, we use method parameters validateDate(string date) authenticateUser(string user, string password) spellCheckWord(string word) playAlarm(num seconds)  We send an argument to the method  The method receives values in parameters  Most likely you'll hear these terms used interchangeably

16. Parameters by Value main() num count = 6 doubleValue(count) output count // displays "6" return doubleValue(num value) // value contains a copy of count // changing the copy does not change count value = value * 2 output value // displays "12" return

17. Parameters by Reference main() num count = 6 doubleValue(count) output count // displays "12" return doubleValue(ref num value) // value refers to the variable count // value is just another name for count value = value * 2 output value // displays "12" return

18. Array Parameters  The memory address of the first element is passed  If a method changes an element, it is actually changing the original array contents  Array parameters are passed by value, but may appear to be passed by reference

19. Returning a Value  Methods can return a value to the caller by declaring a return type num calculateArea(num radius) num area = PI * radius * radius return area string requestName() string name output "Please enter your name." input name return name

20. Not Returning a Value  We specify a void return type if the method does not return a value void main() return void playAlarm() play beep return

21. Using a Return Value void main() // We can store the return type and use it later num volume = calculateVolume(3, 6) output volume return num calculateArea(num radius) num area = PI * radius * radius return area num calculateVolume(num radius, num height) // We can use the return type without storing it num volume = calculateArea(radius) * height return volume

22. Guess the Pattern  What logic is necessary to have a user guess a random ordering of the numbers 1, 2, 3, and 4?  After each guess, the user will be told how many numbers are in the correct location.

23. Overloading Methods  Some languages allow multiple methods with the same name, but different parameters  The method is overloaded num calculateSeconds(num minutes) num seconds = minutes * 60 return seconds num calculateSeconds(num hours, num minutes) num seconds = (hours * 60 + minutes) * 60 returnseconds

24. "Predefined" Methods  Most languages and platforms provide a library of commonly used methods  Accessing a file system  Receiving input from a keyboard or mouse  Writing information to a display  Complex mathematical operations

25. Recursive Methods  How would you implement a factorial function? 5! = 5 * 4 * 3 * 2 * 1 5! = 5 * 4! 4! = 4 * 3!, etc.  Some methods can be designed to call themselves  These methods implement recursion

26. Factorial num factorial(num value) num f = 1 for index = 2; index < value; index += 1 f = f * index endfor return f num factorial(num value) num f = 1 if value <> 0 f = f * factorial(value – 1) endif return f

27. Summing an Array num sumArray(num[] values, num length) num sum = 0 for index = 0; index < length; index += 1 sum = sum + values[index] endfor return sum num sumArray(num[] values, num length) num sum = 0 if length <> 0 sum = sum + sumArray(values, length – 1) sum = sum + values[length – 1] endif return sum

28. Summary  Creating methods  Local variables and scope  Method parameters  Return values  Overloading methods  Predefined methods  Recursive methods