1. Objectives In this chapter, you will learn about:
Counter-controlled and sentinel controlled loops
Nested loops
Using a for loop
Common loop applications
Loop control structure in RAPTOR
repeat until true rather than repeat while true

2. Understanding the Advantages of Looping
control structure that repeats a set of actions (loop body) while some condition remains true or until some condition becomes true.

3. Loop Control Structure in RAPTOR
In RAPTOR: you repeat the body of the loop UNTIL the condition evaluates to true.
RAPTOR lets you structure your loop as one of three types:
Pre-Test loop ( while / for )
Mid-Test loop illegal in our book
Post-Test loop ( do … while )
Our book only shows us pre-test loops.
RAPTOR Loop Symbol
Note: In most languages (including Java) you repeat the loop body while the condition remains true

4. Figure 5-1 The loop structure (pre-test loop)
Our book calls this a while loop

5. Looping Definite and Indefinite Loops
Looping may be achieved using either a definite loop or an indefinite loop
A definite loop is also referred to as a counter-controlled loop
An indefinite loop is also referred to as a sentinel-controlled loop
For a definite loop, the loop body will be executed a specific number of times.
For an indefinite loop, the number of times the body of the loop should be executed can be different for each run of a program

6. Using a Counter-Controlled while loop
As long as a boolean expression (the condition) remains true, the while loop’s body executes.
RAPTOR keep executing UNTIL the condition becomes true
Essential Steps:
Create and initialize a loop control variable (lcv)
Determine the upper or lower limit for the lcv
Determine the step (increment or decrement) for the lcv
Determine the boolean expression (condition) that will control the loop
Each iteration of the loop, update the lcv

7. Using a Definite (Counter-Controlled) Loop
Must the condition be true or false for the loop body to be executed?
In RAPTOR, what would the condition be?
How many times is “Hello” displayed?
lcv is: count
Figure A counter-controlled while loop

8. Using an Indefinite Loop (Sentinel-Controlled) with a Sentinel Value
May be performed a different number of times each time the program executes
Essential steps:
Identify a sentinel value (value outside the range of valid input data) that will be used as the loop exit condition
name = QUIT //where QUIT is a named constant sentinel value
End_of_input //RAPTOR sentinel module expression
Java  hasNext( ) == false //Java method to evaluate end of input condition
Each iteration of the loop get a new input value to compare to the sentinel value.

9. Must the condition be true or false for the loop body to be executed?
In RAPTOR, what would the condition be?
Figure An indefinite while loop that displays “Hello” as long as the user wants to continue

10. Understanding the Loop in a Program’s Mainline Logic
Three steps that should occur in every properly functioning loop
Initialize the variable that will control the loop ( sentinel or counter value )
Test the condition to determine whether the loop body executes
Update (aka alter) the loop control variable
increment/decrement the lcv
get a new input value to compare to the sentinel value

11. Nested Loops Nested loops: Outer loop: Inner loop:
loop within another loop
Outer loop:
loop that contains the other loop
Inner loop:
loop that is contained
Needed when values of two (or more) variables repeat to produce every combination of values

12. Figure 5-8 Flowchart and pseudocode for AnswerSheet program
Must the condition be true or false for the loop body to be executed?
In RAPTOR, what would the condition be?
RAPTOR / Java
<= versus >
= versus !=
> versus <=
“flip” the operator!
Figure Flowchart and pseudocode for AnswerSheet program

13. Common Loop Mistakes
Neglecting to initialize the loop control variable
Neglecting to update the loop control variable
Loop executes one too many or one too few times
operator: < or <= OR > or >=
number of iterations = Last used – First used + 1
Including statements inside the loop that belong outside the loop

14. Figure Incorrect logic for greeting program because the loop control variable initialization is missing

15. Programming Logic & Design, Sixth Edition
Figure Incorrect logic for greeting program because the loop control variable is not altered
Programming Logic & Design, Sixth Edition

16. Programming Logic & Design, Sixth Edition
For a
sentinel-controlled loop
the only operators you should use in the condition are = or !=
Figure Incorrect logic for greeting program because the wrong test is used in the condition
Programming Logic & Design, Sixth Edition

17. Avoiding Common Loop Mistakes (continued)
including statements inside the loop that belong outside the loop
Example:
discount every item by 30 percent
Inefficient because the same value is calculated 100 separate times for each price that is entered
Move outside loop for efficiency

18. Figure 5-13 Inefficient way to produce 100 discount price stickers for differently priced items

19. Figure 5-14 Improved discount sticker-making program

20. Using a for Loop for statement or for loop is a definite loop
specifically, it is a pre-test loop
can be used in pseudocode or a Java program
Remember, keywords like while, for, do have no meaning in a flowchart!
Puts all of the loop control expressions in the for loop header
Initialize
Test
Update
Takes the form: [initial, final]
for loopControlVariable = initialValue to finalValue [step stepValue]
do something
endfor

21. Using a for Loop (continued)
Example pseudocode for loop
for count = 0 to 3 step 1 [0,3]
output “Hello”
end for
Initializes count to 0
Checks count against the limit value 3 (test)
If evaluation is true, for statement body prints the label
Executes 4 times ( last=3, first=0, equals 4 )
Increases count by 1 (update)
while loop:
count = 0
while count <= or while count < 4
Java for loop: for ( count=0; count <= 3; count += 1 )

22. Using a for Loop (continued)
while statement could be used in place of for statement
Step value:
number used to increase (decrease) the loop control variable on each pass through a loop
Programming languages can:
Require a statement that indicates the step value
Have a step value default of 1
Specify step value when each pass through the loop changes the loop control variable by a value other than 1

23. Common Loop Applications
Using a loop to count and accumulate totals
Examples
Business reports often include totals
List of real estate sold and total value
Count of number of records processed
keep track of number of A’s, B’s, C’s, etc.
Counters and Accumulators:
Counter usually is incremented by one each iteration
Accumulator has some value added to it each iteration
Running sum

24. Common Loop Applications
Accumulate total real estate prices
Declare numeric variable at beginning to be used as an accumulator
Initialize the accumulator to 0
Read each transaction’s data record
Add its value to accumulator variable
Keep reading records until eof is encountered

25. Common Loop Applications (continued)
5 records processed
Figure Month-end real estate sales report

26. Figure 5-17 Flowchart and pseudocode for real estate sales report program

27. Common Loop Applications (continued)
Use a loop to validate data
When prompting a user for data, there is no guarantee that
Validate data:
make sure data falls within an acceptable range
Example: user enters birth month
If number is less than 1 or greater than 12
Display error message and stop the program
Assign default value for the month
Reprompt the user for valid input

28. Figure 5-18 Reprompting a user once after an invalid month is entered
Programming Logic & Design, Sixth Edition

29. Programming Logic & Design, Sixth Edition
This test checks to see if the user entered a value too small or too large, that is, if the value is OUtSIDE of the valid range.
In RAPTOR, the test would be month >= LOW_MONTH and month <= HIGH_MONTH
Figure Reprompting a user using a loop after an invalid month is entered
Programming Logic & Design, Sixth Edition

30. Common Loop Applications (continued)
Validating a data type
Validating data requires a variety of methods
isNumeric() isChar() or isWhitespace()
RAPTOR has a built-in procedure Is_number
Your program may:
Accept user data as a string
Use a method to convert to correct to data type

31. Common Loop Applications (continued)
Figure Checking data for correct type

32. RAPTOR test for invalid data
Programming Logic & Design, Seventh Edition

33. Common Loop Applications (continued)
Validating reasonableness and consistency of data
Many data items can be checked for reasonableness
Good defensive programs try to foresee all possible inconsistencies and errors

34. Summary Use a loop to: Three steps must occur in every loop
repeat a sequence of statements (RAPTOR symbols)
Three steps must occur in every loop
Initialize input variable or loop control variable
Test variable (evaluate condition)
Update the variable that controls the loop
Nested loops:
loops within loops

35. Summary (continued) Common mistakes made by programmers
Neglecting to initialize loop control variable
Neglecting to update the loop control variable
Using the wrong comparison with the loop control variable
Including statements inside the loop that belong outside the loop
Most computer languages support a for statement
for loop used with definite loop
number of iterations is known
while loop used with indefinite loop
number of iterations is not known

36. Summary (continued) for loop expressions: Accumulator Counter
Initialize
Test
Update
Accumulator
variable that accumulates values creating a running sum
Counter
Variable used to keep track of the number of occurrences of something (records, customers processed, etc.)
Loops are often used to ensure user data is valid