1. Lecture 11 B Methods and Data Passing
Richard Gesick

2. Overview Terminology Why we have Methods Data Passing The Mantra
Calling Methods
Scope of Variables

3. Terminology A method is a logical grouping of statements
Reusable chunks of code
Write once
Call as many times as you like
Benefits
Reusable
Easy to work at higher level of abstraction
Reduces complexity
Reduces size of code

4. AKA (also known as)
Methods can be called several things, depending on the book or context
Examples:
Procedure
Module
Method (OOP)
Behavior (OOP)
Member Function (OOP)

5. You’ve Already Seen Methods
Denotes a Method of the class
Denotes a property of the class

6. MidiPlayer.Play(new NoteOn(0, GeneralMidiPercussion.BassDrum, 127));
Invoking Methods
MidiPlayer.Play(new NoteOn(0, GeneralMidiPercussion.BassDrum, 127));
Two things going on here
Creating a new NoteOn object
Passing it three parameters
“Constructing” (more later)
Invoking the Play method of the MidiPlayer class
Passing it one parameter (the NoteOn object)
This is a “static” method (more later)

7. Why have Methods? (see anything similar?)
double average; int userNum1, userNum2; Console.WriteLine(“Please enter the 2 numbers”); userNum1 = Int32.Parse(Console.ReadLine()); userNum2 = Int32.Parse(Console.ReadLine()); average = (userNum1 + userNum2) / 2; … // a lot of other code

8. Why have Methods? (see anything similar?)
double average; int userNum1, userNum2; Console.WriteLine(“Please enter the 2 numbers”); userNum1 = Int32.Parse(Console.ReadLine()); userNum2 = Int32.Parse(Console.ReadLine()); average = (userNum1 + userNum2) / 2; … // a lot of other code
All of this code
is the same!

9. Basic Idea
double average; int userNum1, userNum2; Console.WriteLine(“Please enter the 2 numbers”); userNum1 = Int32.Parse(Console.ReadLine()); userNum2 = Int32.Parse(Console.ReadLine()); average = (userNum1 + userNum2) / 2; … // a lot of other code

10. Basic Idea Create method instead
double average; int userNum1, userNum2; Console.WriteLine(“Please enter the 2 numbers”); userNum1 = Int32.Parse(Console.ReadLine()); userNum2 = Int32.Parse(Console.ReadLine()); average = (userNum1 + userNum2) / 2; … // a lot of other code
Create method instead
userNum1 = Int32.Parse(Console.ReadLine());
userNum2 = Int32.Parse(Console.ReadLine());
average = (userNum1 + userNum2) / 2;

11. Give the Method a Name myMethod
double average; int userNum1, userNum2; Console.WriteLine(“Please enter the 2 numbers”); userNum1 = Int32.Parse(Console.ReadLine()); userNum2 = Int32.Parse(Console.ReadLine()); average = (userNum1 + userNum2) / 2; … // a lot of other code
myMethod
userNum1 = Int32.Parse(Console.ReadLine());
userNum2 = Int32.Parse(Console.ReadLine());
average = (userNum1 + userNum2) / 2;

12. Call the Method (instead of writing all that code)
double average; int userNum1, userNum2; Console.WriteLine(“Please enter the 2 numbers”); myMethod … // a lot of other code
myMethod
userNum1 = Int32.Parse(Console.ReadLine());
userNum2 = Int32.Parse(Console.ReadLine());
average = (userNum1 + userNum2) / 2;

13. What have we done? Written the code once, but called it many times
Reduced the size of our code
Easier to comprehend
This is called procedural abstraction
Tracing of code skips all around (no longer linear)

14. Scope of Variables Scope – “who can see what”
Variables that are defined within a method can only be seen by that method!
We need a way to send information to the method
We need a way for the method to send back information
Example: method1 can’t see myInt
method1
method2
char myChar;
int myInt;

15. Examples
private int sum(int x, int y) private string getUserChoice() private void isMatch(int guess)

16. The Mantra All methods follow a mantra The mantra is:
Return type, Method name, parameters …

17. The return type
A method has the option to return us (the calling method) some information
If the method doesn’t return us any info, the return type is void
Otherwise, the return type is the data type it’s going to return
Example of return types:
int
char
boolean

18. Figure out the return type
Method Name
average double or float
getLetterGrade char
areYouAsleep bool
getGPA double or float
printMenu void
// Don’t confuse what a method does with it’s
// return type!
getStudentName string

19. The Method’s name Similar to naming of variables
Can be almost anything except
A reserved word (keywords)
Can’t begin with a number
Can’t contain strange symbols except _ and $
Method names should begin with a lower case
Different standards/conventions are used
If multiple words in method name, capitalize the first letter in each word (except the first)
Example:
thisIsAnExample

20. Parameters Methods cannot see each other’s variables (scope)
Special variables used to “catch” data being passed
This is the only way the main algorithm and Methods have to communicate!
Located between parentheses ( )
If no parameters are needed, leave the parentheses empty

21. Examples Remember the mantra What can you tell me about these methods?
void doSomething (int data)
double average (int num1, int num2)
boolean didHePass ( )
char whatWasHisGrade ( )
void scareStudent (char gradeOfStudent)

22. Getting the Method to Work for You
Call it by name
Pass it the right stuff
Pass the right number of parameters
If it expects two things, pass it two things!
Pass the right type of parameters
If it expects a char, don’t pass it a double!
Parameters must match exactly
If it returns something, do something with it!

23. Example
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory
double average (int x, int y) {
return ( (x+y) / 2.0); }
// Note: the average method
is currently inactive

24. Example (declare variables)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); }
// Note: the average method is currently inactive

25. Example (declare variables)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); }
result1
result2
0.0
0.0

26. Example (set values) Memory
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5
result1
result2
0.0
0.0

27. Example (set values) Memory
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7
result1
result2
0.0
0.0

28. Example (set values) Memory
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
0.0
0.0

29. Example (invoke method)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1);
WAKE UP! x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
0.0
0.0

30. Example (data passing)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 5 7
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
0.0
0.0

31. Example (caller sleeps)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 5 7
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
0.0
0.0
// The method is now ACTIVE

32. Example (method active)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 5 7
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
0.0
0.0
// is 12; 12 / 2 is 6

33. Example (value returned)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 6 5 7
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
0.0
0.0

34. Example (caller resumes)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1);
sleep x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
0.0

35. Example (call method again)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1);
WAKE UP! x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
0.0

36. Example (data passing)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 4 5
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
0.0

37. Example (caller sleeps)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 4 5
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
0.0
// The method is now ACTIVE

38. Example (method active)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 4 5
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
0.0
// is 9; 9 / 2 = 4.5

39. Example (value returned)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1); x y
Memory 4 5
4.5
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
0.0

40. Example (caller resumes)
int num1, num2, num3; double result1, result2; num1 = 5; num2 = 7; num3 = 4; result1 = average (num1, num2); result2 = average (num3, num1);
sleep x y
Memory
num1
num2
num3
double average (int x, int y) {
return ( (x+y) / 2.0); } 5 7 4
result1
result2
6.0
4.5

41. Method Rules You cannot define a method inside of another method
Methods always reside in a class in C# (as you’ve seen numerous time already)
Methods cannot see each other’s variables
You’ve seen “private static” in our examples… more on this later

42. Overloading Methods
Method overloading is the process of using the same method name for multiple methods
The signature of each overloaded method must be unique
The signature includes the number, type, and order of the parameters
The compiler determines which version of the method is being invoked by analyzing the parameters
The return type of the method is not part of the signature

43. Overloading Methods float tryMe (int x) { return x + .375; } Version 1
float tryMe (int x, float y) {
return x*y; }
Version 2
result = tryMe (25, 4.32f)
Invocation

44. Overloading Operators
In C#, not only can methods be overloaded, operators can be overloaded as well.