CIS 115 Lecture 8

There are 3 control structures common to most computer languages that determine the flow, or path of execution, of the code:

Sequential

Selection / Decisions

Repetition / Looping

Visual Basic repetition or looping statements Do While

▪ Repeat/Loop while true▪ Often called pretest loop (also has posttest

form) Do Until

▪ Repeat/Loop until true▪ Often called posttest loop (also has pretest

form) For...Next

▪ Repeat/Loop a specified number of times

Do While loop: repeat while test expression is true

The test expression is evaluated (pretest) When true, the statement(s)are executed The previous 2 steps are repeated as long

as the expression evaluates to true

When false, theloop ends andthe statement(s)are not executed

expression statement(s)

False

True

Do While expressionstatement(s)

Loop Syntax explanation:

Do, While, and Loop are keywords Do While statement marks the beginning of the

loop and the Loop statement marks the end The statement(s) to repeat are found between

these and called the body of the loop Expression – True/False value, variable,

function call or expression that serves as test for repetition

intCount = 1Do While (intCount <= 10) MsgBox (“The current number is: “ & intCount) intCount = intCount + 1Loop---------------------------------------------------------intNumGrades = 0, intSumGrades = 0strInput = InputBox(“Enter a grade(or -1 to stop)”) Do While (strInput <> “-1”)

intSumGrades = intSumGrades + Val(strInput)intNumGrades = intNumGrades + 1

strInput = InputBox(“Enter a grade(or -1 to stop)”)LoopdecAverage = intSumGrades / intNumGradesMsgBox(“Your average is: “ & decAverage)

A loop must have some way to end itself Something within the body of the loop

must eventually force the test expression to false

In the previous example The loop continues to repeat intCount increases by one for each repetition Finally (intCount <= 10) is false and the loop

ends If the test expression can never be false,

the loop will continue to repeat forever This is called an infinite loop

Counter-controlled loops repeat a specific number of times

see intCount from the previous example Counters are typically initialized before the loop

beginsintCount = 1

The counter is then modified in the body of the loop

intCount = intCount + 1

Event-controlled loops repeat until something happens in the loop body

to change the value of loop control variable Events can be: User Input, Expression results,

Function results, etc.

Sentinel Value For either control type, the test expression

will usually determine when to end the loop by making some comparison to a Sentinel value

The sentinel value serves as the test value or comparison criteria for ending the loop▪ In counted loops this is usually the number of

times to loop▪ In event controlled loops this is the state or value

that the event variable is compared to in the test expression

Counter A numeric variable that keeps track of or

counts: number of loop repetitions, items that have been processed, or some other occurrence within a loop

see intNumGrades from the previous example

Accumulator A numeric variable that is used to hold a

sub-total that is computed during multiple passes through a loop

see intSumGrades from the previous example

Do Until loop: repeat until test expression is true

The statement(s)are executedThe test expression is

evaluated (posttest)When false, the previous 2

steps are repeated until theexpression evaluates to true

When true, the loop ends andthe statement(s) are notexecuted again

expression

statement(s)

False

True

Dostatement(s)

Loop Until expression Syntax explanation:

Do, Loop, and Until are keywords Do statement marks the beginning of the loop

and the Loop Until statement marks the end The statement(s) to be repeated are found

between these and called the body of the loop Expression – True/False value, variable,

function call or expression that serves as test for repetition

intCount = 1Do

MsgBox (“The current number is: “ & intCount) intCount = intCount + 1Loop Until (intCount > 10)---------------------------------------------------------intNumGrades = 0, intSumGrades = 0Do strInput = InputBox(“Enter a grade(or -1 to stop)”)

If (strInput <> “-1”) ThenintSumGrades = intSumGrades + Val(strInput)intNumGrades = intNumGrades + 1

End IfLoop Until (strInput = “-1”)decAverage = intSumGrades / intNumGradesMsgBox(“Your average is: “ & decAverage)

Previous Do While loops are in pretest form The expression is evaluated first Then the loop body is executed only if expression is

true The body of a pretest loop may not be

executed at all (if initial value of expression is false)

Previous Do Until loops are in posttest form The body of the loop is executed first Then the expression is evaluated The body of a posttest loop is executed at

least once (even if initial value of expression is true)

A Do While loop Repeats as long as its test expression is true Ends when its test expression becomes false Usually a pretest loop,

but also has a postest form:

A Do Until loop Repeats as long as its test expression is

false Ends when its test expression becomes true Usually a posttest loop,

but also has a pretest form:

Do Until expressionstatement(s)

Loop

Dostatement(s)

Loop While expression

Do strInputPW = InputBox(“Enter Password”)

Loop Until (strInputPW = strSavedPW)

strInputPW = InputBox(“Enter Password”)Do While (strInputPW <> strSavedPW)

strInputPW = InputBox(“Enter Password”)Loop

---------------------------------------------------------

Do While (decBalance < 0)intDep = InputBox(“Negative balance, enter amount to deposit”)decBalance += decDep

Loop

For Next loop: used for counted loops that repeat a specific number of times

The Counter variable is setinitially to the StartValue

After each loop iteration, the Step Value is added tothe value of the countervariable. (if there is no stepvalue, 1 is added)

Iteration continues until theEndValue is exceeded

Counter >EndValue?

statement(s)False

True

SetCounter

to StartValue

IncrementCounter

For Counter = StartValue To EndValue[Step Value]statement[s]

Next [Counter]

Syntax explanation: For, To, and Next are keywords Counter – Variable to track/control number of

iterations StartValue is initial value of counter EndValue is counter value at final iteration Step (optional) – determines counter increment

amount for each iteration of the loop (if not specified the default is +1; if specified can be positive – add or count up, or negative – subtract or count down

For intCount = 1 to 10 Step 1msgBox (“The current number is: “ & intCount)

Next----------------------------------------------------------For intCount = 100 to 0 Step -5msgBox (“The current number is: “ & intCount)

Next----------------------------------------------------------intSum = 0intNum = Val(InputBox(“Enter number of grades to avg”))For intCount = 1 to intNum Step 1strInput = InputBox(“Enter a grade”)intSum = intSum + Val(strInput)

NextdecAverage = intSum / intNumMsgBox(“Your average is: “ & decAverage)

In some cases it is convenient to end a loop before the test condition would end it

The following statements accomplish this Exit Do (used in Do While or Until loops) Exit For (used in For Next loops)

Use this capability with caution It bypasses normal loop termination Makes code more difficult to debug

Do While (true)strInput = InputBox(“Enter a grade(or -1 to stop)”)If (strInput = “-1”) Then

Exit DoEnd IfintSumGrades = intSumGrades + Val(strInput)intNumGrades = intNumGrades + 1

LoopdecAverage = intSumGrades / intNumGradesMsgBox(“Your average is: “ & decAv

The body of a loop can contain any type of VB statements including another loop

When a loop is found within the body of another loop, it’s called a nested loop

strOutput = “”For intTensPlace = 0 to 4 Step 1For intOnesPlace = 0 to 9 Step 1

strOutput &= (intTensPlace & intOnesPlace & “ “)Next

NextMsgBox(“I can count from 0 to 49: ” & strOutput)----------------------------------------------------------For hours = 0 To 24lblHours.Text = hoursFor minutes = 0 To 59

lblMinutes.Text = minutesFor seconds = 0 To 59

lblSeconds.Text = secondsNext seconds

Next minutesNext hours

Have the user input a positive number via input box. Use a validation loop to prompt for input until valid. Display message indicating when valid input is received.

Have the user input numbers via inputbox until the value “stop” is entered. Display the sum of all the input numbers.

Have the user input an integer. Display the sum of all the integers from 1 through the input integer.

Have the user input a yearly deposit amount and a yearly interest rate. Use a loop to determine the min number of years it will take to reach at least 1 million $ (for each year – add dep first, then compute and add int). Display the number of years and the final value. Modify to have the user also enter a number of years and display the final value after that number of years.

Write a VB application to have the user input via textbox an integer from 1 t0 10 (inclusive) to specify the height and base of a right triangle. Display the triangle in a list box using strings made up of the ‘#’ character. Create and display strings - one char at a time - using nested loops. Input:4 Output:

##########

Write a VB application to have the user input a binary number via input box. Use a validation loop ensure the input is a valid binary number. When valid, convert the number to a decimal value (use a function) and report the results via message box.

Lab 7 and Homework 7 Visual Basic – Looping See handout for details and due date Questions?