Introduction to Computing Dr. Nadeem A Khan. Lecture 18.

Introduction to Introduction to ComputingComputing

Dr. Nadeem A KhanDr. Nadeem A Khan

Lecture 18Lecture 18

Subprograms: Passing Subprograms: Passing Variables Variables

►Example 1: SubroutineExample 1: Subroutine

Sub Triple( num As Single)Sub Triple( num As Single)Rem Triples a numberRem Triples a numberPicture1.Print num;Picture1.Print num;Let num = num*3Let num = num*3Picture1.Print num;Picture1.Print num;End SubEnd Sub


►Example 1: Program Example 1: Program

Sub Command1_Click( )Sub Command1_Click( )Dim amt As SingleDim amt As SinglePicture1.ClsPicture1.ClsLet amt=2Let amt=2Picture1.Print amt;Picture1.Print amt;Call Triple(amt)Call Triple(amt)Picture1.Print amtPicture1.Print amtEnd SubEnd Sub


►Example 1: Result Example 1: Result

2 2 6 62 2 6 6

=> Variable was passed by reference in => Variable was passed by reference in Example 1Example 1

Variable Passing by Val!Variable Passing by Val!



Sub Command1_Click( )Sub Command1_Click( )Dim amt As SingleDim amt As SinglePicture1.ClsPicture1.ClsLet amt=2Let amt=2Picture1.Print amt;Picture1.Print amt;Call Triple((amt))Call Triple((amt)) ‘extra parentheses‘extra parenthesesPicture1.Print amtPicture1.Print amtEnd SubEnd Sub



2 2 6 22 2 6 2

=> Variable was passed by value in Example 2=> Variable was passed by value in Example 2


►Example 3: SubroutineExample 3: Subroutine

Sub Triple( ByVal num As Single)Sub Triple( ByVal num As Single)Rem Triples a numberRem Triples a numberPicture1.Print num;Picture1.Print num;Let num = num*3Let num = num*3Picture1.Print num;Picture1.Print num;End SubEnd Sub



Sub Command1_Click( )Sub Command1_Click( )Dim amt As SingleDim amt As SinglePicture1.ClsPicture1.ClsLet amt=2Let amt=2Picture1.Print amt;Picture1.Print amt;Call Triple(amt)Call Triple(amt)Picture1.Print amtPicture1.Print amtEnd SubEnd Sub



2 2 6 22 2 6 2

=> Variable was passed by value in Example 3=> Variable was passed by value in Example 3

By default: Variable passing by referenceBy default: Variable passing by reference To be precise you may use ByRef as in:To be precise you may use ByRef as in:

Sub Triple( ByRef num As Single)Sub Triple( ByRef num As Single)

Subprograms: Local Subprograms: Local Variables Variables

► Example 1Example 1Sub Command1_Click( )Sub Command1_Click( )Picture1.ClsPicture1.ClsCall ThreeCall ThreeCall ThreeCall ThreeEnd SubEnd Sub

Sub Three( )Sub Three( )Dim num As SingleDim num As SinglePicture1.Print num Picture1.Print num Let num=3Let num=3Picture1.Print numPicture1.Print numEnd SubEnd Sub


► Example 1: ResultExample 1: Result

0 3 0 30 3 0 3

=> => The variable num is local to the The variable num is local to the Subprogram;Subprogram;

Lives only for the time the Subprogram is Lives only for the time the Subprogram is called;called;

Initialized each timeInitialized each time


► Example 2Example 2

Sub Command1_Click( )Sub Command1_Click( ) Sub Trivial ( )Sub Trivial ( )Dim x As SingleDim x As Single Dim x As SingleDim x As SinglePicture1.ClsPicture1.Cls Picture1. Print x;Picture1. Print x;Let x = 2Let x = 2 Let x = 3Let x = 3Picture1.Print x;Picture1.Print x; Picture1.Print x;Picture1.Print x;Call TrivialCall Trivial End SubEnd SubPicture1.Print x;Picture1.Print x;Call TrivialCall TrivialPicture1.Print x;Picture1.Print x;End SubEnd Sub



2 0 3 2 0 3 22 0 3 2 0 3 2

=> => The variable x has separate identities in the The variable x has separate identities in the Event Event procedure and in the subroutineprocedure and in the subroutine

Subprograms: Form-Level Subprograms: Form-Level Variables Variables

► Example 1Example 1Dim num1 As Single, num2 As SingleDim num1 As Single, num2 As Single

Sub Command1_Click( )Sub Command1_Click( ) Sub AddAndIncrement ( Sub AddAndIncrement ( ))Let num1 =2Let num1 =2 Picture1.Print Picture1.Print num1+num2num1+num2Let num2 =3Let num2 =3 Let num1= num1+1Let num1= num1+1Picture1.ClsPicture1.Cls Let num2=num2+1Let num2=num2+1Call AddAndIncrementCall AddAndIncrement End SubEnd SubPicture1.PrintPicture1.PrintPicture1.Print num1Picture1.Print num1Picture1.Print num2Picture1.Print num2End SubEnd Sub


553344

=> => The variable num1 and num2 known to allThe variable num1 and num2 known to all subprograms subprograms



► Example 2Example 2

Dim pi As SingleDim pi As Single

Sub Form_Load( )Sub Form_Load( )Let pi = 3.14159Let pi = 3.14159 End SubEnd Sub

Sub Command1_Click ( )Sub Command1_Click ( )Picture1.ClsPicture1.ClsPicture1.Print “Circle area is:” pi*5^2Picture1.Print “Circle area is:” pi*5^2End Sub End Sub


Circle area is: 78.53975Circle area is: 78.53975

=> => The variable pi is initialized in Form_load The variable pi is initialized in Form_load eventevent


Functions Functions

Functions Functions ►General Format : SubroutineGeneral Format : Subroutine

Sub Sub SubprogrammeNameSubprogrammeName ( (list of parameterslist of parameters))statementsstatementsEnd SubEnd Sub

►General Format : FunctionsGeneral Format : FunctionsSub Sub FunctionNameFunctionName ( (list of parameterslist of parameters) As ) As datatypedatatypestatementsstatementsFunctionName = expressionFunctionName = expressionEnd FunctionEnd Function

Functions Functions ►Examples:Examples:

Function FtoC (t As Single) As SingleFunction FtoC (t As Single) As SingleFtoC = (5/9)*(t-32) FtoC = (5/9)*(t-32) End FunctionEnd Function

Function FirstName$ (nom As String)Function FirstName$ (nom As String)Dim firstSpace As IntegerDim firstSpace As IntegerRem Extract the first name from the full name nomRem Extract the first name from the full name nomLet firstSpace = Instr(nom, “ ”)Let firstSpace = Instr(nom, “ ”)FirstName$ = Left$(nom, firstSpace-1)FirstName$ = Left$(nom, firstSpace-1)End FunctionEnd Function

Functions Functions ►Using function in a programUsing function in a program

Sub Command1_ClickSub Command1_ClickDim x As SingleDim x As SinglePicture1.Print FtoC(212)Picture1.Print FtoC(212)x = FtoC(32)x = FtoC(32)Picture1.Print xPicture1.Print xx = FtoC(81+32)x = FtoC(81+32)Picture1.Print xPicture1.Print xEnd SubEnd Sub

Functions Functions (Contd.)(Contd.) ►Result:Result:

100100

00

4545

► Practice ProblemPractice ProblemFunction Cider (g As Single, x As Single) As SingleFunction Cider (g As Single, x As Single) As SingleCider=g*xCider=g*xEnd FunctionEnd Function

Sub DisplayNumOfGallons(galPerBu, apples As Single)Sub DisplayNumOfGallons(galPerBu, apples As Single)Picture1.ClsPicture1.ClsPicture1.Print “You can make”; Cider(galPerBu, apples);Picture1.Print “You can make”; Cider(galPerBu, apples);Picture1.Print “gallons of cider.”Picture1.Print “gallons of cider.”End SubEnd Sub

Sub GetData (gallonsPerBushel As Single, apples As Sub GetData (gallonsPerBushel As Single, apples As Single)Single)

Let gallonsPerBushel =3Let gallonsPerBushel =3Let apples =9Let apples =9End SubEnd Sub

Functions Functions (Contd.)(Contd.)

Functions Functions (Contd.)(Contd.) ►Practice ProblemPractice Problem

Sub Command1_ClickSub Command1_ClickRem How many gallons of apple cider can we Rem How many gallons of apple cider can we make make Dim gallonsPerBushel As Single, apples as SingleDim gallonsPerBushel As Single, apples as SingleCall GetData(gallonPerBushel, apples)Call GetData(gallonPerBushel, apples)Call DisplayNumOfGallons(gallonsPerBushel, Call DisplayNumOfGallons(gallonsPerBushel, apples) apples) End SubEnd Sub

Functions Functions (Contd.)(Contd.) ►Practice Problem: ResultPractice Problem: Result

You can make 27 gallons of cider You can make 27 gallons of cider

Modular DesignModular Design

►Top-Down DesignTop-Down Design ►Structured ProgrammingStructured Programming

SequencesSequences DecisionsDecisions LoopsLoops

Note:Note:

►Read: Schneider (Section 4.2,4.3, 4.4)Read: Schneider (Section 4.2,4.3, 4.4)►Read comments and do practice Read comments and do practice

problemsproblems of these sectionsof these sections►Attempt questions of ExercisesAttempt questions of Exercises

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Introduction to Computing Dr. Nadeem A Khan. Lecture 18.

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