Parameters

1
Parameters
2
Overview

• A Reminder
• Why Parameters are Needed
• How Parameters Work
• Value Parameters
• Reference Parameters
• Out Parameters

3
Youve Seen Parameters

• Defined
• private void buttonAdd_Click(object sender,
  EventArgs e)
• private void FormMain_Load(object sender,
  EventArgs e)
• private void buttonAddDigit_Click(object sender,
  EventArgs e)
• Invoked
• listBoxNames.Items.Add(textBoxInput.Text)
• choice MessageBox.Show("Do you want to exit the
  program?", "Quit?", MessageBoxButtons.YesNo,
  MessageBoxIcon.Question,
  MessageBoxDefaultButton.Button2)

4
Parameters (A Reminder)

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

5
Why Do We Need Parameters

• Functions are good
• Reuse
• Modularity
• Good design
• Scope is limited
• A variable declared in a function is only visible
  inside that function
• Using global variables is a bad idea
• So we need a way of passing data between functions

6
Parameter Example

• Defined
• private static void DoSomething(int x, double y,
  string name)
• Invoked
• DoSomething(42, 3.14, Bob Smith)
• Notice above that there is a 1-1 matching, and
  the types must match too

7
Parameter Details

• By default, all primitive types are passed by
  value
• Remember primitive types are all non-class types
  (int, float, double, string, char, etc.)
• The function gets a copy of the variable
• So if you change the variable in the function,
  the original value in the caller will remain
  unchanged

8
By Value Example

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

9
The Output

• The value of i is 4
• The value of x is 6
• Now the value of i is 4
• The variable i is not changed because the
  function gets a copy of the value.
• Lets see this traced out

10
By Value Example
MEMORY
Main
0
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

11
By Value Example
MEMORY
Main
4
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

12
By Value Example
OUTPUT
The value of i is 4
MEMORY
Main
4
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

13
By Value Example
OUTPUT
The value of i is 4
MEMORY
IBT
4
x
Main
4
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

The value in i is copied into the variable x
14
By Value Example
OUTPUT
The value of i is 4
MEMORY
IBT
6
x
Main
4
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

Waiting here
15
By Value Example
OUTPUT
The value of i is 4 The value of x is 6
MEMORY
IBT
6
x
Main
4
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

Waiting here
16
By Value Example
OUTPUT
The value of i is 4 The value of x is 6 Now the
value of i is 4
MEMORY
IBT
6
x
Main
4
i

• public static void IncreaseByTwo(int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(i)
• Console.WriteLine("Now the value of i is "
  i)

17
Ref Parameters

• We can alter how parameters are handled using the
  ref keyword
• This makes the parameter aBy Reference
  parameter
• Now the parameter IS NOT a copy of the value it
  refers to the original

18
By Reference Example

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

19
The Output

• The value of i is 4
• The value of x is 6
• Now the value of i is 6
• The variable i is changed because the function
  refers to the original variable i.
• Lets see this traced out

20
By Ref Example
MEMORY
Main
0
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

21
By Ref Example
MEMORY
Main
4
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

22
By Ref Example
OUTPUT
MEMORY
The value of i is 4
Main
4
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

23
By Ref Example
OUTPUT
MEMORY
The value of i is 4
IBT
R
x
Main
4
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

x is a reference to i
24
By Ref Example
OUTPUT
MEMORY
The value of i is 4
IBT
R
x
Main
6
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

Waiting here
25
By Ref Example
OUTPUT
MEMORY
The value of i is 4 The value of x is 6
IBT
R
x
Main
6
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

Waiting here
26
By Ref Example
OUTPUT
MEMORY
The value of i is 4 The value of x is 6 Now the
value of i is 6
IBT
R
x
Main
6
i

• public static void IncreaseByTwo(ref int x)
• x 2
• Console.WriteLine("The value of x is " x)
• static void Main(string args)
• int i
• i 4
• Console.WriteLine("The value of i is " i)
• IncreaseByTwo(ref i)
• Console.WriteLine("Now the value of i is "
  i)

27
The Necessity of Ref Parameters

• What if a function needs to return a value when
  it is complete?
• Use the return statement and declare the
  function of a particular type
• Ex public static int sum(int x, int y) return
  x y
• But what if the function should return more than
  one value?
• Create a collection of values to return and
  return the collection
• Use ref (by reference) parameters to allow
  multiple items to be updated/modified

28
Another By-Ref Example(notice we need to return
three values, modifying their original values)

• static public void RGBToGrey (ref byte red, ref
  byte green, ref byte blue)
• byte average
• average (red green blue) / 3
• red average
• green average
• blue average

29
Out Parameters

• Weve seen parameters that pass information into
  a function (via copying the value)
• Weve seen parameters that pass information into
  a function and receive information out of the
  function (via changed values to a reference)
• What about having values come out of a function?
• No initial values passed in

30
Out Parameters

• Think of these similar to ref (by reference)
  parameters
• No initial value passed in
• Its the job of the function to fill in a value
  for this type of parameter
• When the function ends, the calling method will
  retain the value stored in the parameter

31
Out Parameter Example

• public static void getUserInfo(out int age, out
  string name, out double gpa)
• Console.Write(What is your name? ")
• name Console.Read()
• Console.Write(What is your age? ")
• age Int32.Parse(Console.Read())
• Console.Write(What is your GPA? ")
• gpa Double.Parse(Console.Read())
• static void Main(string args)
• int user_age
• string user_name
• double user_gpa
• getUserInfo(out user_age, out user_name, out
  user_gpa)
• // more code follows...

32
Out Parameter Example

• public static void getUserInfo(out int age, out
  string name, out double gpa)
• Console.Write(What is your name? ")
• name Console.Read()
• Console.Write(What is your age? ")
• age Int32.Parse(Console.Read())
• Console.Write(What is your GPA? ")
• age Double.Parse(Console.Read())
• static void Main(string args)
• int user_age
• string user_name
• double user_gpa
• getUserInfo(out user_age, out user_name, out
  user_gpa)
• // more code follows...

33
Out Parameters Output Information

• Notice in the preceding example, the age,
  name and gpa variables must be filled in with
  values within the funtion
• After this is done, the calling method (Main) can
  make use of these values via the user_age,
  user_name and user_gpa variables
• How is this different from ref (by-reference)
  parameters?
• Notice we couldnt make use of these parameters
  (read their values) until we placed some value
  into them
• So the statementConsole.WriteLine("The users
  age is" age)at the beginning of the function
  (first line of the function) would be illegal
  because this parameter does not have a value
  assigned to it yet

34
The Necessity of Out Parameters

• Like ref parameters, out parameters are
  useful for returning information out from the
  function
• Unlike ref parameters, out parameters dont
  receive values from the caller
• No values passed in the parameters are empty
• Use out parameters when the function is
  generating the values and returning them to the
  calling method
• Just use the function return type and the
  return statement if the function returns only
  one value
• Use out parameters when more than one value is
  to be generated and returned to the calling method

35
Conclusion

• You should have a better/stronger feel on how
  parameters work
• You should understand the difference between
  by-value, by-reference, and out parameters
• Given a section of code, you should be able to
  trace through what the computer would do (and
  what output would be generated)