1. CHAPTER 5A Loop Structure

2. Objectives: In this chapter, you will learn about:
The advantages of looping
Using a loop control variable
Nested loops
Avoiding common loop mistakes
Using a for loop
Common loop applications

3. 5.1 Advantages of Looping Loop Advantages
Set of instructions that is executed repetitively based on a condition
Advantages
Write one set of instructions to operate on multiple, separate sets of data
Allows processing on large sets of data, such as complex payroll and benefits processing
Example: while, do…while, for loop

4. 5.2 While loop with Loop Control Variable
Definite Loop with a Counter
Definite loop
Executes a definite, predetermined number of times
Counter-controlled loop
Program counts loop repetitions
Loop control variables altered by:
Incrementing
Decrementing

5. 5.2 While loop with Loop Control Variable
Definite Loop with a Counter

6. 5.2 While loop with Loop Control Variable
Indefinite Loop with a Sentinel Value
Indefinite loop
Performed a different number of times each time the program executes
The user decides how many times the loop executes
Use a sentinel value to terminate the loop
Sentinel
Sentinel is a preselected value that stops the execution of a program
The sentinel is assigned to a loop control variable

7. 5.2 While loop with Loop Control Variable
Indefinite Loop with a Sentinel Value

8. 5.2 While loop with Loop Control Variable
Indefinite Loop with a Sentinel Value

9. Questions: While Loop
Determine the output of the following pseudocode:
start
set number = 2
while number <= 50
output number
number = number * 2
endwhile
stop

10. Questions: While Loop
Write a pseudocode & flowchart to display the numbers from 1 to 10.

11. Questions: While Loop
Write a pseudocode & flowchart that displays the even numbers between 1 and 9.

12. Questions: While Loop
Write a pseudocode & flowchart to calculate and display the sum of the numbers from 1 to 10.

13. Questions: While Loop
Write a pseudocode & flowchart to calculate and display the sum of the numbers from 1 to 10. Calculate and display the average value of the sum of the numbers. Hint: Determine the sum from Determine the average value from the sum.

14. Questions: While Loop
Every day, a weather station receives 15 temperatures expressed in degrees Fahrenheit. A program is to be written that will accept each Fahrenheit temperature, convert it to Celsius and display the converted temperature to the screen. After 15 temperatures have been processed, the words “All temperatures processed” are to be displayed on the screen.

15. while loop structure Assignment:
Design an algorithm that print “hello” five times.
Design an algorithm that calculate the total from 0 to 10 and display the total results.
Design an algorithm that print 0,2,4,6,8,….,20.
Design an algorithm that print the sum of the even integers from 2 to 10.

16. 5.3 Nested Loop Nested loops loops within loops Outer loop
the loop that contains the other loop
Inner loop
the loop that is contained
Needed when values of two (or more) variables repeat to produce every combination of values

17. 5.3 Nested Loop
Example: Answer Sheet - without loop, 63 output statements

18. 5.3 Nested Loop
Example: Answer Sheet (cont’d)
“QUIT”

19. 5.3 Nested Loop
Example: Answer Sheet (cont’d)

20. 5.3 Nested Loop
Example: Answer Sheet (cont’d)

21. 5.3 Nested Loop

22. 5.5 Avoiding Common Loop Mistakes
Common Mistakes:
Neglecting to initialize the loop control variable
Neglecting to alter the loop control variable
Using the wrong comparison with the loop control variable
Including statements inside the loop that belong outside the loop

23. 5.5 Avoiding Common Loop Mistakes
Greeting Program
For example

24. 5.5 Avoiding Common Loop Mistakes
Missing Initialization

25. 5.5 Avoiding Common Loop Mistakes
Mistakes 1: Neglecting to initialize the loop control variable
Example: input name statement removed
Value of name unknown or garbage
Program may end before the user can enter any names
Or the greeting output will included garbage – for example, “ Hello 1ABr5”

26. 5.5 Avoiding Common Loop Mistakes
Not altering loop control variable

27. 5.5 Avoiding Common Loop Mistakes
Mistakes 2: Neglecting to alter the loop control variable
Remove input name instruction from end of the loop in the programe
User never enters a name after the first one
Inner loop executes infinitely
Always incorrect to create a loop that cannot terminate

28. 5.5 Avoiding Common Loop Mistakes
Wrong test

29. 5.5 Avoiding Common Loop Mistakes
Mistakes 3: Using the wrong comparison with the loop control variable
Programmers must use correct comparison
Seriousness depends on actions performed within a loop
Overcharge insurance customer by one month
Overbook a flight on airline application
Dispense extra medication to patients in pharmacy

30. 5.5 Avoiding Common Loop Mistakes
Mistakes 4: Including statements inside the loop that belong outside the loop
Example: Discount every item by 30 percent & produce 100 new price label stickers for each item
Inefficient because the same value (newPrice) is calculated 100 separate times for each price that is entered
Move outside the loop for efficiency

31. 5.5 Avoiding Common Loop Mistakes

32. 5.5 Avoiding Common Loop Mistakes

33. 5.5 Avoiding Common Loop Mistakes

34. 5.6 For Loop for statement or for loop is a definite loop
Provides three actions in one structure
Initializes
Evaluates
Alters

35. 5.6 For Loop Pseudocode: Step value
The amount by which a loop control variable changes
Can be positive or negative (incrementing or decrementing)
Default step value is 1
Programmer specifies a step value when each pass through the loop changes the loop control variable by a value other than 1

36. 5.6 For Loop Example: Display “Hello” four times
Above code accomplish 3 tasks:
count is initialized to 0
count is compared to the limit value 3
while count ≤ 3, loop body executes (output “Hello”)
Increases count by 1

37. 5.6 For Loop
Pre-test loop
the loop control variable is tested before each iteration
for loops and while loops are pre-test loops
Post-test loop
the loop control variable is tested after each iteration
do…while is a post-test loop

38. 5.6 For Loop

39. Questions: For Loop
Use for loop to write a pseudocode & flowchart to display the numbers from 1 to 10.

40. Questions: For Loop
Use for loop to write a pseudocode & flowchart to display the numbers 4,6,8,10,12,14,16,18,20

41. Questions: For Loop
Use a for loop to write a pseudocode & flowchart to calculate and display the sum of the numbers from 1 to 10.

42. Summary
Loops write one set of instructions that operate on multiple, separate sets of data
Three steps must occur in every loop
Initialize the loop control variable
Compare the variable to some value
Alter the variable that controls the loop
Nested loops
loops within loops
maintain two individual loop control variables
Alter each at the appropriate time

43. Summary Common mistakes made by programmers
Neglecting to initialize the loop control variable
Neglecting to alter 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 when the number of iterations is known