Functions and Data
Passing
Overview
•
•
•
•
•
•
Terminology
Why we have functions
Data Passing
The Mantra
Calling Methods
Scope of Variables
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
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)
You’ve Already Seen Functions
Denotes a function of the class
Denotes a property of the class
Invoking Functions
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)
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;
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!
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;
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());
userNum2 = Int32.Parse(Console.ReadLine());
average = (userNum1 + userNum2) / 2;
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());
userNum2 = Int32.Parse(Console.ReadLine());
average = (userNum1 + userNum2) / 2;
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());
userNum2 = Int32.Parse(Console.ReadLine());
average = (userNum1 + userNum2) / 2;
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)
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 can’t see myInt
function1
char myChar;
function2
int myInt;
Example Function Declarations
(You’ve 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)
Other Examples
private int sum(int x, int y)
private string getUserChoice()
private void isMatch(int guess)
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
…
…
…
The return type
• A function has the option to return us (the
calling method) some information
• If the function 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
Figure out the return type
• Function Name
average
double or float
getLetterGrade
char
areYouAsleep
bool
getGPA
double or float
printMenu
void
// Don’t confuse what a function does with it’s
// return type!
getStudentName
string
The function’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 $
• 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
Parameters
• Functions cannot see each other’s
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
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)
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, don’t pass it a double!
Parameters must match exactly
• If it returns something, do something with
it!
Example
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
// Note: the average function is currently inactive
Example (declare variables)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
0
0
0
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
// Note: the average function is currently inactive
Example (declare variables)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
0
0
0
result1 result2
0.0
0.0
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
// Note: the average function is currently inactive
Example (set values)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
0
5
0
result1 result2
0.0
0.0
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
// Note: the average function is currently inactive
Example (set values)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
0
result1 result2
0.0
0.0
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
// Note: the average function is currently inactive
Example (set values)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
0.0
0.0
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
// Note: the average function is currently inactive
Example (invoke function)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
0.0
0.0
WAKE UP!
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
Example (data passing)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
0.0
0.0
x
5
y
7
double average (int x, int y) {
return ( (x+y) / 2);
}
Example (caller sleeps)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
0.0
0.0
x
5
y
7
double average (int x, int y) {
return ( (x+y) / 2);
}
// The function is now ACTIVE
Example (function active)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
0.0
0.0
x
5
y
7
double average (int x, int y) {
return ( (x+y) / 2);
}
// 5 + 7 is 12; 12 / 2 is 6
Example (value returned)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
6
num1 num2 num3
7
5
4
result1 result2
0.0
0.0
x
5
y
7
double average (int x, int y) {
return ( (x+y) / 2);
}
Example (caller resumes)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
6.0
0.0
sleep
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
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);
num1 num2 num3
7
5
4
result1 result2
6.0
0.0
WAKE UP!
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
Example (data passing)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
6.0
0.0
x
4
y
5
double average (int x, int y) {
return ( (x+y) / 2);
}
Example (caller sleeps)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
6.0
0.0
x
4
y
5
double average (int x, int y) {
return ( (x+y) / 2);
}
// The function is now ACTIVE
Example (function active)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
6.0
0.0
x
4
y
5
double average (int x, int y) {
return ( (x+y) / 2);
}
// 4 + 5 is 9; 9 / 2 = 4.5
Example (value returned)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
6.0
0.0
4.5
x
4
y
5
double average (int x, int y) {
return ( (x+y) / 2);
}
Example (caller resumes)
int num1, num2, num3;
double result1, result2;
num1 = 5;
num2 = 7;
num3 = 4;
result1 = average (num1, num2);
result2 = average (num3, num1);
num1 num2 num3
7
5
4
result1 result2
6.0
4.5
sleep
x
y
double average (int x, int y) {
return ( (x+y) / 2);
}
Function Rules
• You cannot define a function inside of
another function
• Functions always reside in a class in C#
(as you’ve seen numerous time already)
• Functions cannot see each other’s
variables
• You’ve seen “private static” in our
examples… more on this later
An Example
• Improving the High-Low Game
• Add functions to clarify
• Rule of thumb
– Keep methods/functions small
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)