Procedures

1
Procedures Functions

• Chapter 4
• CS0004
• http//www.cs.pitt.edu/bmills/cs4/

2
Administration

• Part 1 of Project 1 is due
• Will grade and return Wednesday
• Review on Wednesday and present good designs
• Instructions for turning in project
• Try to have this to you by Wednesday
• Questions?

3
Review

• Math operators
• Strings and their functions
• Declaring Variables and Data Types
• Decisions
• If blocks
• Select Case Blocks

4
Recall

• Already seen procedure and functions?
• Event Procedure
• Called when an event occurs, weve been doing
  this for a while
• Built-in Functions
• Math.Sqrt(number)
• SubString(index, length)

5
Sub-Procedures

• Event Procedures
• Private Sub btnAdd_Click() Handles btnAdd.Click
• --- Do Stuff In Here ---
• End Sub
• General Procedures - User Defined
• Sub MySubProcedure()
• --- I do MY stuff here ---
• End Sub

6
Why Use Procedures?

• Break difficult tasks into smaller pieces
• Example
• Need to convert meters into yards
• Display instructions to the user
• Open a file
• Save a file to a location

7
Parameters

• These are variables that you pass into the
  procedure or function.
• Recall all that junk in the Event Procedure
• Private Sub btn_Click(ByVal sender As
  System.Object,
• ByVal e As System.EventArgs)
• Handles btn.Click
• sender and e are parameters to btn_Click

8
Parameters

• Lets create a sub procedure to display the
  average given the sum and count
• Sub DisplayAvg(ByVal sum As Double, ByVal count
  As Double)
• txtOutput.Text sum / count
• End Sub

9
Parameters

• Lets create a sub procedure to display the
  average given the sum and count
• Sub DisplayAvg(ByVal sum As Double, ByVal count
  As Double)
• txtOutput.Text sum / count
• End Sub

count is also a parameter name
sum is the parameter name
10
Parameters

• Lets create a sub procedure to display the
  average given the sum and count
• Sub DisplayAvg(ByVal sum As Double, ByVal count
  As Double)
• txtOutput.Text sum / count
• End Sub

count is also of data type Double
sum is of data type Double
11
Parameters

• Lets create a sub procedure to display the
  average given the sum and count
• Sub DisplayAvg(ByVal sum As Double, ByVal count
  As Double)
• txtOutput.Text sum / count
• End Sub

Notice that inside the sub-procedure we can use
these variables.
Parameters are treated like variable
declarations inside the sub-procedure
12
Using a Procedure

• You Call these procedures
• Sub DisplayAvg(ByVal sum As Double, ByVal count
  As Double)
• txtOutput.Text sum / count
• End Sub
• Sub btnAvg_Click() Handles btnAvg.Click
• Dim the_sum, the_count As Double
• the_sum CDbl(txtSum.Text)
• the_count CDbl(txtCount.Text)
• DisplayAvg(the_sum, the_count)
• End Sub

This is called a procedure call
13
Variable Scope

• When can you access what variables?
• Sub MyProcedure(ByVal num As Integer)
• Dim x as Double
• x num 20.5
• txtOutput x
• End Sub
• Sub MyOtherProc(ByVal num As Integer)
• Dim x as Integer
• x num 1
• MyProcedure(x)
• End Sub

14
Variable Scope

• When can you access what variables?
• Sub MyProcedure(ByVal num As Integer)
• Dim x as Double
• x num 20.5
• txtOutput x
• End Sub
• Sub MyOtherProc(ByVal num As Integer)
• Dim x as Integer
• x num 1
• MyProcedure(x)
• End Sub

These are the same name but in different scopes
15
Variable Scope

• When can you access what variables?
• Sub MyProcedure(ByVal num As Integer)
• Dim x as Double
• x num 20.5
• txtOutput x
• End Sub
• Sub MyOtherProc(ByVal num As Integer)
• Dim x as Integer
• x num 1
• MyProcedure(x)
• End Sub

These too
16
Variable Scope

• When you use a variable the computer looks for
  where it is declared
• Starts close to home and expands search
• Close to Home Local Scope
• Farther from Home Class-level Scope
• As soon as it finds the variable declaration it
  uses that one

17
Class Variables

• Visible to every procedure
• Declared after the Public Class Form1 but
  before any procedures
• Limit the number of class variables used try to
  stick to local variables
• Example Time!

18
ByVal and ByRef

• ByVal tells the computer to copy the Value of the
  passed in data into the local scope
• The scope of the parameter is said to be local
• Like variables declared inside this procedure,
  this parameter will cease to exist when the
  procedure ends
• The value of the argument passed to this
  procedure is copied into the parameter
• The value of the argument in the calling
  procedure is UNCHANGED when control returns to
  the caller

19
ByVal and ByRef

• ByRef tells the computer to NOT copy the data but
  LINK them
• Changes made to the parameter in the called
  procedure are reflected in the calling procedure
• THIS IS DANGEROUS
• Try to avoid this if at all possible
• If you want a value returned, use a Function
• If you want a variable visible to all procedures,
  use a Class Variable

20
Functions

• Sub Procedures do not return any values
• Many times, this return value is desired
• Calculating an average
• Convert meters into yards
• Convert Fahrenheit into Celsius
• This is the reason we have functions

21
Functions

• Much like Sub-procedures
• Function FuncName(parameters) As Double
• Dim x As Double
• x 2.5
• Return x
• End Function
• Note that here we need to declare what type of
  value the Function will return
• The value we want passed is done so using the
  Return statement

22
Functions

• Notice that functions return values so you can
  use them just like other expression
• Function foo() As String
• Return This is my output
• End Function
• Sub btnClick(..) Handles btn.Click
• txtOutput.Text foo()
• End Sub

23
Functions Sub-Procedures

• Functions used where wed find expressions
• Subroutines used where wed find chunks of code
• Both perform similar tasks
• Both can call each other from other Subroutines
  or Functions

24
Modular Design

• Save on typing
• Break big tasks into small procedures and
  functions
• Keep from making errors (easy to test)
• What are some examples in your project?

25
Lets try it out

• Design a program to convert Fahrenheit to Celsius
• Use a function to compute the conversion
• Use a sub procedure to display the result