Lesson 7

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Flowchart Symbols

The Loop Logic Structure

Incrementing/Decrementing

Accumulating While/WhileEnd Repeat/Until

Automatic-Counter Loop (For)

Nested Loops Indicators Algorithm

Instructions Recursion

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Decision◦ True/False/Else

Process Assign

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Assign

Process

Decision

Allows the programmer to specify that an action is to be repeated based on the truth or falsity of some condition.While there are more timecards to be processed

Obtain time from each card and calculate payAs long as there are items still remaining on the

list the loop will continue.◦ Otherwise known as iteration.

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Known as definite because the number of iterations to be performed at runtime is known.

Also known as Counter-controlled Repetition ◦ Uses a variable called a counter to control the

number of times a set of statements should execute.

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Known as indefinite because the number of iterations at runtime is not known before the loop begins executing.◦ Uses a sentinel value (signal value, a flag value,

or a dummy value to indicate “end of data entry.” When using a sentinel

Choose a sentinel that will not naturally exist within the range of data being used.

Ex. 999-99-9999 for Social Security Numbers

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one of the three types of program control structures ◦ Sequence◦ Selection ◦ Repetition

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Initialization Condition (the test) Increment (or Decrement)

The Accumulator, although used in frequently in loops, is NOT part of the generic looping process

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The Initialization set to an initial value

◦ usually zero but not all the time Examples:

◦ Count = 0 ◦ Count = 1◦ Count = 100

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The Test or Condition◦ tested before the start of each loop repetition,

called the iteration or pass. Examples:

◦ Count < 3 Count = 10◦ Count <= 3 Count <> 10◦ Count > 5◦ Count >= 5

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The Increment (or Decrement)◦ updates the variable during each iteration◦ must be part of the loop body (usually the last

line) Examples:

◦ Count = Count + 1◦ Count = Count - 1

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Variables used to store values being computed in increments during the execution of a loop.

Not part of the Looping Process◦ But quite often an integral addition to the process◦ Very often looks like an increment, which is part of the

looping process Examples:

◦ Sum = Sum + 1◦ TotalGrade = TotalGrade + CurrentGrade

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The Three Most Common Loop-control statements: ◦ the while, ◦ the for, and ◦ the repeat

But there are lots of other loop variations dependent on the language.

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The while statement ◦ The most versatile of the loops◦ The loop body contains the instructions to be

repeated.◦ The loop-repetition condition is the Boolean

expression after the reserved word while which is evaluated before each repetition of the loop body.

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Set counter To 0While counter < final_value

statementsIncrement counter by 1

While-end

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?

Init

Stmt

true

false

Stmt

x = 3; How many times does theCount = 0; loop execute?while Count < 3 do begin What is displayed? X = X * 2; 6 Print (X); 12 Count = Count + 1 24 end; {while Count}

Pascal Code

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x = 3Count = 0While Count < 3 X = X * 2 Print X Count = Count + 1 End While

VB Code

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int X = 3;int count = 0;While (count < 3)

{ X = X * 2 cout << X << endl; Print statement Count = Count + 1;}

C++ Code

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A Sentinel-Controlled Loop◦Controlled by a sentinel value.◦Examples

end-of-file marker (EOF)end-of-line marker (EOL)999999999 for SSN 999999 for date

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A Sentinel-Controlled Loop ◦ Pseudocode Template

Initialize Sum to 0 Read the first value into counter variable While counter variable is not sentinel do

Add counter value to SumRead next value into counter value

While-end; {while}

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Boolean Flag-Controlled Loops◦ Executes until the event being monitored occurs.

A program flag, or flag, is a Boolean variable whole value (True or False) signals whether a particular event occurs.

The flag should initially be set to False and reset to True when the event occurs.

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Boolean Flag-Controlled Loops◦Pseudocode TemplateInitialize flag to Falsewhile not flag

statements.. Reset flag to True if the event being monitored

occurs while-end; {while}

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Logical Opposite of the While Loop

Unlike the While/While-End, the Do/Until tests for falsity.◦ Should be used when the

question being asked is more naturally asked in the negative.

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Counter < 5

Init

Stmt

false

true

Stmt

x = 3Count = 3Do Until Count < 1 X = X * 2 Print X Count = Count - 1Loop

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Similar to the While/While-End structure◦ In the While Loop

loop-continuation is tested at the beginning of the loop before the body of the loop is performed.

◦ In the Repeat Until Loop loop-continuation is tested after the loop body is

performed, thus executing the loop body at least once.

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Pseudocode TemplateRepeat

statementsincrement counterVariable

Until testCondition Notice that the

statements are executed before thecondition to end the loop

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counter = 1

counter < 5

True

False

statements

ActionoccursbeforeTest

Combines all of the Loop Process components into one statement.

Notes on the Counter:◦ Can be used non-destructively but must not be

modified (destructively) within the loop body.◦ Should be a local variable.

The loop body will not be executed if initial is greater than the final value, unless the increment value is negative.

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Pseudocode ExampleFor counter = initialvalue To finalvalue Step 1

statements… ‘to incrementNext counter

For counter = finalvalue To initialvalue Step -1statements… ‘to decrement

Next counter

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Note that the “To” is equivalent to “while less than or equal to”

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counter = 1

Print countercounter =

counter + 1counter

<= 5

True

False

(implicit) (implicit)

(implicit)

For counter = 1 to 5 Step 1

Print counter

Next counter

Loops can be nested just as if statements.◦ Cannot use the same counter-control variable for

the inner loop as is used for the outer loop.

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x yTrue

False False

True

Initialize outer loopWhile outer loop test {while}

statements... Initialize inner loop

While inner loop test {while} Inner loop processing and Update inner loop variable

while-end {inner while} statements... Update outer loop variable

while-end {outer while}

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Dim outercounter As IntegerDim innercounter As Integeroutercounter = 1 While outercounter <= 3

innercounter = 1While innercounter <= 3 innercounter = innercounter + 1Wend ‘innercounteroutercounter = outercounter + 1

Wend ‘outercounterVB Code

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Assertions about the characteristics of a loop that always must be true for a loop to execute properly.

The assertions are true on loop entry, at the start of each loop iteration, and on exit from the loop.◦ They are not necessarily true at each point within

the body of the loop.

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when the loop is skipped entirely (zero iteration loop)

when the loop body is executed just once When the loop executes some normal

number of times When the loop fails to exit (infinite loop)

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Verify the Algorithm◦ Test the value of the algorithm

before the loop during the loop, and after the loop.

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Beware of infinite loops Beware of off-by-one Loop Errors

◦ Executes the loop one too many times◦ Executes the loop one too few times

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Or twisted tails

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Occurs when a function calls itself from within the body of the function

Also known as “procedural iteration” Classic Examples of Recursion

◦ Factorial◦ Fibonacci

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How it works:If X = 3, the chain of recursive calls would be as

follows:Factorial(3) 3 * Factorial(2) 3 * (2 * Factorial(1) )

Precondition x 0Postcondition

Returns the product 1 * 2 * 3 * …* x for x > 1Returns 1 when X is 0 or 1.

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Private Function Factorial(ByRef y As Double) _As Double ‘2nd double defines Factorial

If y <= 1 Then Factorial = 1 ' Base case Else Factorial = y * Factorial(y - 1) ' Recursive step End IfEnd FunctionThe Base Case is also known as the Stopping Case.

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Investigated (in the year 1202) was about how fast rabbits could breed in ideal circumstances.

Suppose a newly-born pair of rabbits, one male, one female, are put in a field. Rabbits are able to mate at the age of one month so that at the end of its second month a female can produce another pair of rabbits. Suppose that our rabbits never die and that the female always produces one new pair (one male, one female) every month from the second month on.

The puzzle that Fibonacci posed was...

How many pairs will there be in one year?

At the end of the first month, they mate, but there is still one only 1 pair.

At the end of the second month the female produces a new pair, so now there are 2 pairs of rabbits in the field.

At the end of the third month, the original female produces a second pair, making 3 pairs.

At the end of the fourth month, the original female has produced yet another new pair, the female born two months ago produces her first pair also, making 5 pairs.

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FemaleMale

We can make another picture showing the Fibonacci numbers 1,1,2,3,5,8,13,21

If we start with two small squares of size 1 next to each other.

On top of both of these draw a square of size 2 (=1+1).

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The number of clockwise spirals and the number of counterclockwise spirals formed by the seeds of certain varieties of flowers

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Fibonacci NumbersEach Fibonacci number is the sum of the two preceding Fibonacci numbers. The Fibonacci series defined recursively:

Fibonacci(0) = 0

Fibonacci(1) = 1

Fibonacci(2) = 1

Fibonacci(n) = Fibonacci(n-1) + Fibonacci(n - 2)

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A Shortened Form:Fib (1) = 1 Fib (2) = 1Fib (n) = Fib (n - 1) + Fib (n - 2)

Fib(3) = Fib(2) + Fib(1) = 1 + 1 = 2Fib(4) = Fib(3) + Fib(2) = 2 + 1 = 3Fib(5) = Fib(4) + Fib(3) = 3 + 2 = 5Fib(6) = Fib(5) + Fib(4) = 5 + 3 = 8

Chapter 8

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Used for menu-driven programs and event-driven programs

Menu ◦ A list of options that a program based on case

logic can perform Event-driven

◦ Order is dictated by the user, not the programmer, by which button is clicked on

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The SetupList1.AddItem “England” ‘adds items to aList1.AddItem “Germany” ‘Windows list boxList1.AddItem “Spain”List1.AddItem “Italy”

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VB Code

Label3.Text = List1.Text‘Sets the caption for the Label ‘above the output textbox

‘Next, the appropriate language phrase is displayed in the textbox based on the Country selected.

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VB Code

Select Case List1.ListIndexCase 0

Label4.Caption = “Hello, programmer”Case 1

Label4.Caption = “Hallo, programmierer”Case 2

Label4.Caption = “Hola, programador”Case 3

Label4.Caption = “Ciao, programmatori”End Select

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VB Code

Select Case AgeCase 16

LabelAge.Caption = “You can drive now! Parents Beware!”Case 18

LabelAge.Caption = “You can vote now!”Case 21

LabelAge.Caption = “You can drink wine with your meals.”Case 65

LabelAge.Caption = “Time to retire and have fun!”Case Else

LabelAge.Caption = “You’re a great age! Enjoy it!End Select

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