Loop Blocks Chapter 6

Part A

Program Blocks 1.Actions- commands,

messages, methods2.Branches- decisions to

be made3.Loops- actions to be

repeated

Loop Blocks Loops have 4 parts

1.a beginning2.an end3.body4.test (Boolean)

Two Types of Loops

1.When you know how many times to execute (definite)

2.When you keep executing until the user says to stop (indefinite)

Do While..Loop  Do while (<Boolean expression>)  do something

  do something  Want to do it again?

Loop Notice that the test is at the beginning of

the loop. This means that the commands in the loop may never be executed! 

Do.. Loop While Do

do something  do something  Want to do it again?

Loop While (Boolean expression)Notice that the test is at the end of the loop.

This means that the commands in the loop must be executed at least once.

Some Problems with Loops

• "off by one” error•no exit condition (infinite loop - use Control-Break to stop!)

Example 1intCount = 1Do while (intCount < 10)  

  intCount = intCount + 1Loop

MsgBox Str(intCount)

Example 2intCount = 1Do    intCount = intCount + 1Loop While (intCount < 10)MsgBox Str(intCount)

Example 3intCount = 1Do while (intCount <= 10)  

intCount = intCount + 1LoopMsgBox Str(intCount)

Example 4intCount = 1Do 

  intCount = intCount + 1 Loop While (intCount <= 10)

MsgBox Str(intCount)

Example 5intCount = 1Do while (intCount > 10)  

intCount = intCount + 1Loop

MsgBox Str(intCount)

Example 6intCount = 1Do   

intCount = intCount + 1Loop While (intCount > 10)

MsgBox Str(intCount)

Class Exercise Write a program that allows the user

to input an integer into a text box and test the integer to see if it is a prime number. Prime numbers are numbers that are only divisible by 1 and themselves. After clicking a Check button, the program uses a message box to tell the user if the number is Prime or Not Prime

The Interface

The Objects •frmPrime- the form•txtInput- text box for input•cmdCheck•cmdEnd•lblEnterIntegerLabel

AlgorithmStart "the number at 1“Keep Looping

increment "the number"While integer not divisible by

"the number"

CodeDim intTestNum, intDivisor As IntegerintTestNum = Val(txtInput.Text)intDivisor = 1Do

intDivisor = intDivisor + 1Loop While (intTestNum Mod intDivisor <>

0)

Is it Prime?If (loop ended when intDivisor = intTestNum)

the number is primeelse

its not

VB Codeif (intDivisor = intTestNum) then

MsgBox "It is Prime"Else

MsgBox "It is NOT Prime"End If

Error CheckingThe main Do loop does not

have to be executed if intTestNum <= 1

•If < 1 prime NOT defined•If = 1 it is Prime by definition

Nesting BlocksWe will place the main DO loop inside

an If..Then so that it is ONLY executed if intTestNum > 1. Otherwise we will give an error message

If ("the test number" > 1) execute the loop

Elsegive an error message

VB CodeIf (intTestNumber > 1) then

DointDivisor = intDivisor + 1

Loop While (intTestNum Mod intDivisor <> 0)

ElseMsgBox "Please Enter a positive integer"

End If

Using Input Boxes InputBox CommandInputBox <message prompt>, <caption>

InputBox FunctionstrName = InputBox (<message prompt>,

<caption>)

Note the use of parentheses in function!

Homework • Modify Review 2 (page 6-2) to

have the user to input the integer using a horizontal scroll bar rather that a text box

• Do Review 4, a & b (page 6-8)• Exercises 1, 2, and 3 (a & b)