Lesson 7 COP1006 McManus 1

      Flowchart Symbols The Loop Logic Structure Incrementing/ Decrementing Accumulating While/WhileEnd Repeat/Until      Automatic Counter Loop (For) Nested Loops Indicators Algorithm Instructions Recursion COP1006 McManus 2

   ◦ Decision True/False/Else Process Assign Assign Process Decision COP1006 McManus 3

 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 pay As long as there are items still remaining on the list the loop will continue.

◦ Otherwise known as iteration .

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  Known as known.

definite

because the number of iterations to be performed at runtime is ◦ Also known as Counter-controlled Repetition Uses a variable called a execute.

counter to control the number of times a set of statements should COP1006 McManus 5

 Known as iterations at runtime is not known before the loop begins executing.

◦

indefinite

Uses a sentinel value ( or a dummy value because the number of signal value , a flag 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 COP1006 McManus 6

 ◦ ◦ ◦ one of the three types of program control structures Sequence Selection Repetition COP1006 McManus 7

   Initialization Condition (the test) Increment (or Decrement)  The Accumulator, although used in frequently in loops, is NOT part of the generic looping process COP1006 McManus 8

  

The Initialization

◦ set to an initial value usually zero but not all the time ◦ ◦ ◦ Examples: Count = 0 Count = 1 Count = 100 COP1006 McManus 9

 

The Test or Condition

◦ tested before the start of each loop repetition, called the iteration or pass.

◦ ◦ ◦ ◦ Examples: Count < 3 Count <= 3 Count > 5 Count >= 5 Count = 10 Count <> 10 COP1006 McManus 10

 

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 COP1006 McManus 11

   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 COP1006 McManus 12

 ◦ ◦ ◦ 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 evaluated before while which is each repetition of the loop body.

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Set counter To 0 While counter < final_value statements Increment counter by 1 While-end Init ?

false Stmt true Stmt COP1006 McManus 15

x = 3; Count = 0; while Count < 3 do begin X = X * 2; Print (X); Count = Count + 1 end; {while Count} How many times does the loop execute?

What is displayed?

6 12 24 Pascal Code COP1006 McManus 16

x = 3 Count = 0 While Count < 3 X = X * 2 Print X Count = Count + 1 End While VB Code COP1006 McManus 17

} int X = 3; int count = 0; While (count < 3) { X = X * 2 cout << X << endl; Count = Count + 1; Print statement C++ Code COP1006 McManus 18

 ◦ ◦

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 COP1006 McManus 19

 ◦ 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 Sum Read next value into counter value While-end; {while} COP1006 McManus 20

 ◦ Boolean Flag-Controlled Loops  Executes until the event being monitored occurs. A program flag occurs.

, or flag , is a Boolean variable whole value (True or False) signals whether a particular event  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 Template

Initialize flag to False while not statements..

Reset flag flag to True if the event being monitored occurs while-end; {while} COP1006 McManus 22

  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.

Init Counter < 5 true false Stmt Stmt COP1006 McManus 23

x = 3 Count = 3 Do Until Count < 1 X = X * 2 Print X Count = Count - 1 Loop COP1006 McManus 24

 ◦ ◦ 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 Template Repeat statements increment counterVariable Until testCondition Notice that the statements are executed before the condition to end the loop counter = 1 statements counter < 5 False

Action occurs before Test

True COP1006 McManus 26

   ◦ 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 is greater than the

final initial

value, unless the increment value is negative.

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 Pseudocode Example For counter = initialvalue To finalvalue Step 1 statements… Next counter ‘to increment For counter = finalvalue statements… Next counter To initialvalue Step -1 ‘to decrement COP1006 McManus 28

 Note that the “To” is equivalent to “while less than or equal to” For counter = 1 to 5 Step 1 counter = 1 Print counter Next counter

(implicit) (implicit)

counter <= 5 False True Print counter counter = counter + 1

(implicit)

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 ◦ 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.

x True False y True False COP1006 McManus 30

Initialize outer loop While 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} COP1006 McManus 31

Dim outercounter As Integer Dim innercounter As Integer outercounter = 1 While Wend outercounter innercounter = 1 While Wend innercounter ‘outercounter <= 3 innercounter = innercounter + 1 ‘innercounter <= 3 outercounter = outercounter + 1 VB Code COP1006 McManus 32

  ◦ 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) COP1006 McManus 34

 ◦ 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 COP1006 McManus 36

Or twisted tails COP1006 McManus 37

   Occurs when a function calls itself from within the body of the function Also known as “procedural iteration” Classic Examples of Recursion ◦ Factorial ◦ Fibonacci COP1006 McManus 38

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

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Private Function Factorial(ByRef y As Double) _ As Double ‘2 nd double defines Factorial If y <= 1 Then Factorial = 1 ' Base case Else Factorial = y * Factorial(y - 1) ' Recursive step End If End Function The 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. COP1006 McManus 41

Female Male COP1006 McManus 42

   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). COP1006 McManus 43

 The number of clockwise spirals and the number of counterclockwise spirals formed by the seeds of certain varieties of flowers COP1006 McManus 44

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 Fibonacci Numbers Each 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)

A Shortened Form

: Fib (1) = 1 Fib (2) = 1 Fib (

n

) = Fib (

n

- 1) + Fib (

n

- 2) Fib(3) = Fib(2) + Fib(1) = 1 + 1 = 2 Fib(4) = Fib(3) + Fib(2) = 2 + 1 = 3 Fib(5) = Fib(4) + Fib(3) = 3 + 2 = 5 Fib(6) = Fib(5) + Fib(4) = 5 + 3 = 8 COP1006 McManus 46

Chapter 8 COP1006 McManus 47

   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 COP1006 McManus 48

 The Setup List1.AddItem “England” List1.AddItem “Germany” List1.AddItem “Spain” List1.AddItem “Italy” ‘adds items to a ‘Windows list box VB Code COP1006 McManus 49

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.

VB Code COP1006 McManus 50

Select Case List1.ListIndex

Case 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 VB Code COP1006 McManus 51

Select Case Age Case 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 COP1006 McManus 52

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