vb6(branching,loops) .ppt

Tuesday, April 11, 2023 1

VISUAL BASIC 6 (cont.)

1. BRANCHING

2. LOOPS


BRANCHING• If …Then Statement• Syntax:

If (condition) Then

(reaction)

End If

If there is only one reaction to the condition, the statement can also be expressed without the End If


If..Then..Else Statement • Syntax: If condition Then statement Else other statement End If If the original condition is met, then all the code

within the first statement is executed. The optional Else section specifies an alternative statement that will be executed if the condition is false.


• The If-Else statement can be extended to the following form: (called nested if)

If condition Then statement ElseIf condition Then other statement ElseIf condition Then other statement ... Else condition Then another statement End If


Select Case • If you have a lot of conditional statements,

using If..Then..Else could be very messy. For multiple conditional statements, it is better to use Select Case

• The data type specified in expression must match that of Case values. (see syntax below)


Select Case expression Case value1

Block of one or more VB statements Case value2 Block of one or more VB Statements Case value3 . . . Case Else Block of one or more VB Statements

End Select


• Example 1: Select Case grade Case "A"

result.Caption="High Distinction" Case “B+" result.Caption="Distinction" Case "B" result.Caption="Credit" Case "C" result.Caption="Pass" Case Else result.Caption="Fail" End Select

• *Please note that grade is a string, so all the case values such as "A" are of String data type.


• Example 2:

Select Case mark

Case Is >= 85 comment.Caption = "Excellence"

Case Is >= 70 comment.Caption = "Good"

Case Is >= 60

comment.Caption = "Above Average"


Case Is >= 50

comment.Caption = "Average"

Case Else

comment.Caption = "Need to work harder"

End Select

• * Note we use the keyword Is here to impose the conditions. This is generally used for numeric data.


LOOPS

• Visual Basic allows a procedure to be repeated as many times as long as the processor could support. This is generally called looping

Visual Basic has two main types of loops: (i) for… next loops (ii) Do… loops


For..Next Loops

• The syntax of a For..Next loop has three components: a counter, a range, and a step. A basic for..next loop appears as follows:

For counter=start_value to end_value (Step increment)

One or more VB statements Next

start_value and end_value can be any numbers, variables or expressions that produce suitable value.


• Example: (a) For counter=1 to 10 display.Text=counter

Next (b) For counter=1 to 1000 step 10 counter=counter+1

Next (c) For counter=1000 to 5 step -5 counter=counter-10 Next


Do Loops

• Do loops are a bit more flexible then For loops, but should generally only be used when necessary. Do loops come in the following formats:

• Do while

• Do until

• Loop while

• Loop until


• While loops (both do while and loop while) will continue to execute as long as a certain conditional is true. An Until loop will loop as long as a certain condition is false, on the other hand. The only difference between putting either While or Until in the Do section or the Loop section, is that Do checks when the loop starts, and Loop check when the loops ends.


Endless loop: Do..Loop• The endless loop is a loop which never

ends and the statements inside are repeated forever.

• Syntax:

Do

Do_Something

Loop


Do While…Loop • This loop has a condition at the beginning.

The statements are repeated as long as the condition is met. If the condition is not met at the very beginning then the statements inside the loop are never executed.

Do While condition

Block of one or more VB statements

Loop


X =1

Do While X <= 5

Print X

X = X + 1

Loop


Do Until…loop• Syntax: Do Until condition

Block of one or more VB statements Loop

Example: X =1 Do Until X > 5 Print X X = X + 1 Loop


Do…Loop Until

• This loop has a condition at the end and the statements are repeated until the condition is met. Since the check is at the end the statements are at least executed once

Do Block of one or more VB statements

Loop Until condition


X =1

Do

Print X

X = X + 1

Loop Until X = 6


Do…Loop While• Syntax: Do

Block of one or more VB statements Loop While condition

Example X =1 Do Print X X = X + 1 Loop while X <= 5


Do..Exit Do..Loop (loop with condition in the middle )

• Sometimes you need to first make a calculation and exit the loop when a certain criterion is met. However when the criterion is not met there is something else to be done. Hence you need a loop where the exit condition is in the middle


• Syntax:

Do


(condition)

Exit Do

(end condition)


Loop


• Example:

X = 1Do Print X X = X + 1 If X > 5 Then Exit Do End If Loop


NESTED LOOPS

• This is when one or more loops are inside the other.• They may of the same kind or different. Example: For i = 1 to 10 For j = 1 to 10 k = i*j print k Next j Next i

vb6(branching,loops) .ppt

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