Functions and Data Passing

1
Functions and Data Passing
2
Overview

• Terminology
• Why we have functions
• Data Passing
• The Mantra
• Calling Methods
• Scope of Variables

3
Terminology

• A function 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)

• Functions can be called several things, depending
  on the book or context
• Examples
• Procedure
• Module
• Method (OOP)
• Behavior (OOP)
• Member function (OOP)

5
Youve Already Seen Functions
Denotes a function of the class
Denotes a property of the class
6
Invoking Functions

• MidiPlayer.Play(new NoteOn(0, GeneralMidiPercussi
  on.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 functions?(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
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2

8
Why have functions?(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
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2

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
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2

10
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
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2

Create function instead
userNum1 Int32.Parse(Console.ReadLine()) userNu
m2 Int32.Parse(Console.ReadLine()) average
(userNum1 userNum2) / 2
11
Give the Function a Name

• 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
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2
• userNum1 Int32.Parse(Console.ReadLine())
• userNum2 Int32.Parse(Console.ReadLine())
• average (userNum1 userNum2) / 2

myFunction
userNum1 Int32.Parse(Console.ReadLine()) userNu
m2 Int32.Parse(Console.ReadLine()) average
(userNum1 userNum2) / 2
12
Call the Function(instead of writing all that
code)

• double average
• int userNum1, userNum2
• Console.WriteLine(Please enter the 2 numbers)
• myFunction
• // a lot of other code
• myFunction
• myFunction

myFunction
userNum1 Int32.Parse(Console.ReadLine()) userNu
m2 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 function can
  only be seen by that function!
• We need a way to send information to the function
• We need a way for the function to send back
  information
• Example function1 cant see myInt

function1
function2
char myChar
int myInt
15
Example Function Declarations(Youve seen all of
these before)

• private void buttonAdd_Click(object sender,
  EventArgs e)
• private void FormMain_Load(object sender,
  EventArgs e)
• private void buttonAddDigit_Click(object sender,
  EventArgs e)

16
Other Examples

• private int sum(int x, int y)
• private string getUserChoice()
• private void isMatch(int guess)

17
The Mantra

• All functions follow a mantra
• The mantra is
• Return type, function name, parameters
• Return type, function name, parameters
• Return type, function name, parameters

18
The return type

• A function has the option to return us (the
  calling method) some information
• If the function doesnt return us any info, the
  return type is void
• Otherwise, the return type is the data type its
  going to return
• Example of return types
• int
• char
• boolean

19
Figure out the return type

• Function Name
• average double or float
• getLetterGrade char
• areYouAsleep bool
• getGPA double or float
• printMenu void
• // Dont confuse what a function does with its
• // return type!
• getStudentName string

20
The functions name

• Similar to naming of variables
• Can be almost anything except
• A reserved word (keywords)
• Cant begin with a number
• Cant contain strange symbols except _ and
• Function names should begin with a lower case
• Different standards/conventions are used
• If multiple words in function name, capitalize
  the first letter in each word (except the first)
• Example
• thisIsAnExample

21
Parameters

• Functions cannot see each others variables
  (scope)
• Special variables used to catch data being
  passed
• This is the only way the main algorithm and
  functions have to communicate!
• Located between parentheses ( )
• If no parameters are needed, leave the
  parentheses empty

22
Examples

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

23
Getting the function 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, dont pass it a double!
• Parameters must match exactly
• If it returns something, do something with it!

24
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 (
(xy) / 2)
// Note the average function 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 (
(xy) / 2)
0
0
0
// Note the average function is currently
inactive
26
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 (
(xy) / 2)
0
0
0
result1
result2
0.0
0.0
// Note the average function is currently
inactive
27
Example (set values)

• 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 (
(xy) / 2)
5
0
0
result1
result2
0.0
0.0
// Note the average function is currently
inactive
28
Example (set values)

• 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 (
(xy) / 2)
5
7
0
result1
result2
0.0
0.0
// Note the average function is currently
inactive
29
Example (set values)

• 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 (
(xy) / 2)
5
7
4
result1
result2
0.0
0.0
// Note the average function is currently
inactive
30
Example (invoke function)

• 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 (
(xy) / 2)
5
7
4
result1
result2
0.0
0.0
31
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
5
7
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
0.0
0.0
32
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
5
7
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
0.0
0.0
// The function is now ACTIVE
33
Example (function active)

• int num1, num2, num3
• double result1, result2
• num1 5
• num2 7
• num3 4
• result1 average (num1, num2)
• result2 average (num3, num1)

x
y
5
7
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
0.0
0.0
// 5 7 is 12 12 / 2 is 6
34
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
5
7
6
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
0.0
0.0
35
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 (
(xy) / 2)
5
7
4
result1
result2
6.0
0.0
36
Example (call function 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 (
(xy) / 2)
5
7
4
result1
result2
6.0
0.0
37
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
4
5
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
6.0
0.0
38
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
4
5
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
6.0
0.0
// The function is now ACTIVE
39
Example (function active)

• int num1, num2, num3
• double result1, result2
• num1 5
• num2 7
• num3 4
• result1 average (num1, num2)
• result2 average (num3, num1)

x
y
4
5
Memory
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
6.0
0.0
// 4 5 is 9 9 / 2 4.5
40
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
4
5
Memory
4.5
num1
num2
num3
double average (int x, int y) return (
(xy) / 2)
5
7
4
result1
result2
6.0
0.0
41
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 (
(xy) / 2)
5
7
4
result1
result2
6.0
4.5
42
Function Rules

• You cannot define a function inside of another
  function
• Functions always reside in a class in C(as
  youve seen numerous time already)
• Functions cannot see each others variables
• Youve seen private static in our examples
  more on this later

43
An Example

• Improving the High-Low Game
• Add functions to clarify
• Rule of thumb
• Keep methods/functions small

44
Console High-Low Functions

• static private int getUserChoice()
• static private bool checkGuess(int guess, int
  secretNumber)
• static private bool playAgain()
• static void Main(string args)