Loop and repetition

Today Passing values to and back from Sub procedures Option Buttons Do While Loops For Loops Population Growth

Passing value to a sub procedure Private Sub Add(num1 As Single, num2 As Single ) Call Add(x,y) The values of x and y are copied to num1 and num2

Passing value(s) back from sub procedure As in the previous example, if the variable num1 is changed in the Add sub procedure, will the variable x be changed? The value of the arguments will be copied back to the caller sub procedure. This provides a tunnel for outputting several results. –Private Sub Phasor(real as single, image as single, modulus as single, phase as single) –Call Phasor(x,y,norm,angle)

Complex number to its phasor form Private Sub Phasor(real as single, image as single, modulus as single, phase as single) ‘convert a complex number to its ‘phasor form modulus = sqr(real^2 + image^2) phase=atn(image/real) End Sub

What is really happened in the memory VB is not really copying back and forth. When passing the argument to the sub prodedure, it is actually the memory block of the variable that is passed. It is called passing by reference. (for more information, read book pp )

Option Buttons Have value of true (clicked) or false (unclicked) Use If statements to evaluate Only one can be true If button.Value Then –Tests for TRUTH (selected) Read pp

Using Option Buttons

Code If Fahrbutton.Value Then statements Elseif Celsiusbutton.Value Then statements Else statements End If

Code for Program Private Sub Convert_Click() Outpic.Cls 'Decare DegC and DegF as singles Dim DegC As Single, DegF As Single If (Fahrbutton.Value = True) Then 'Get value for Celsius Temp DegF = Val(inBox.Text) 'Calculate DegF from DegC DegC = (DegF - 32) / 1.8 'Output answer Outpic.Print DegF; " degrees F equals" Outpic.Print DegC; " degrees C. " Else … End If End Sub

Example Start Get a number Divide the Number By 2 Is the quotient Equal to 1? Print The Remainder to the left of previous remains No Print 1 To the left of Previous remainders End Yes Output number If number Equal to 1 or 0 Yes No

Convert the first decision to code IF number <> 0 OR number <> 1 THEN Do the conversion part END IF Call OutputNumber(number)

Convert second decision to code The “NO” branch includes the decision itself. IF quotient <> 1 THEN quotient = number \ 2 reminder = number mod 2 number = quotient call PrintReminder() IF quotient <> 1 THEN quotient = number \ 2 reminder = number mod 2 number = quotient call PrintReminder() IF quotient <> 1 THEN ………..

Math operator \ and mod The \ (backward slash) operator is used to divide two numbers and return an integer result. – 5 \ 2 = 2, 10 \ 3 = 3 –5 / 2 =2.5, 10 / 3 = The mod operator will return the reminder of the division. –5 mod 2 =1, 10 mod 3 =1

Two solutions Use Goto key word for unconditional jumping. –Using goto is a bad habit for programming. Use loop structure –Do while …loop –For…Next

Repetition Along with decisions (if-then-else), repetition is the other key to making programs working Do procedure over and over

Do While Do While condition Code goes here Loop When condition becomes false, loop ends

Code fragment for the example Do While quotient <> 1 quotient = number \ 2 reminder = number mod 2 number = quotient call PrintReminder() Loop

Exponential Population Growth Population increases by a reproductive FACTOR each year (r)

Exponential Growth A bacteria divides in two at each event. How many bacteria are there after four reproductive events? 1 * 2  2 * 2  4 * 2  8 * 2  16

Population Growth r = growth factor or rate ( growth ) N = population ( pop ) Change in population = growth rate * population Population = Population + Change pop = pop + growth * pop

Population Growth Two variables –pop for population –growth for growth rate Repeat equation until pop = 1000 –pop = pop + growth * pop

Do While Loop Flowchart Start Declare Variables Pop as Single Counter as Integer Do While Pop < Finish Initialize Variables Pop = 10 Read growth from Text1.Text Clear Picture1 Pop = Pop + growth * pop Output Population Pop < is FALSE T

Sub Procedure Private Sub Command1_Click() Dim pop As Single, growth As Single pop = 10 growth = Val(Text1.Text) Picture1.Cls Do While pop < pop = pop + growth * pop Picture1.Print pop Loop End Sub

For-Next Loops For-Next loops are used to do an operation a specific number of times Want to do it ten times –use a For-Next Want to do it until number > 10000? –Use a Do While

Syntax of For-Next Loop Dim counter As Integer For counter = 1 to 10 statements go here Next counter

Population Growth Instead of until population = 10000, let’s just let it run for 10 times and see what the maximum number we get is

For Loop Flowchart Start Declare Variables Pop as Single Counter as Integer For Counter = 1 to 10 Finish Initialize Variables Pop = 10 Read growth from Text1.Text Clear Picture1 Pop = Pop + growth * pop Output Population Counter > 10 Next Counter (Counter = Counter + 1)

Sub Procedure Private Sub Command1_Click() Dim pop As Single, growth As Single Dim counter As Integer pop = 10 growth = Val(Text1.Text) Picture1.Cls For counter = 1 To 10 pop = pop + growth * pop Picture1.Print pop Next counter End Sub