1. Programming Logic and Design Seventh Edition
Chapter 3
Understanding Structure

2. Objectives Sequence Selection Repetition ( aka looping and iteration )
In this chapter, you will learn about:
The three basic control structures:
Sequence
Selection
Repetition ( aka looping and iteration )
The need for structure
Using a priming input

3. Three Basic Control Structures Sequence, Selection, Repetition/Iteration/Looping
Connector
Use where flowlines come together in a flowchart

4. RAPTOR Flowchart Symbols
Sequence Control Structures
Selection and Repetition
Control Structures

5. Understanding the Three Basic Control Structures (continued)
There is a distinction between sequence and a sequence structure. The book doesn't make this distinction which is unfortunate.
On the right, you see 3 sequence structures that will execute in sequence (or sequentially). To the right of that, you see 3 selection structures that will execute in a sequence (sequentiall).
You enter a control structure through an entry point and exit from an exit point.
Control structures can be stacked and nested.
Stacked control structures are always executed in sequence.
Figure 3-2
Sequence control structure
Sequential execution of control structures

6. Understanding the Three Basic Control Structures (continued)
expression
usually referred to as the condition
null case
No structure to be executed
Figure Single-alternative if selection structure

7. Understanding the Three Basic Control Structures (continued)
Single-alternative if
Else clause is not required
If the condition does not evaluate to true, the structure(s) are not executed and control passes to the next structure
null case
Situation where nothing is done
This would be the "empty" else branch of a selection structure
if employee belongs to dentalPlan then
deduct $40 from employeeGrossPay
endif

8. Understanding the Three Basic Control Structures (continued)no
yes
structure
structure
Figure 3-3 Dual-alternative if selection structure

9. Understanding the Three Basic Control Structures (continued)
Dual-alternative if
Contains two alternatives
If-then-else structure
Pseudocode keywords:
if, then, else, endif
if someCondition [is true] then
statement_1
else
statement_2
endif
structures

10. Understanding the Three Basic Control Structures (continued)
Some languages have an "else if" keyword
The spelling of the keyword in languages varies (elseif / elsif / elif)
Python uses the elif keyword
Java and C++ do not have an "else-if" keyword
Usage:
if x == 1 then
print 1
elif x == 2 then
print 2
... (additional elif statements)
else
print "no match for x"
endif

11. Understanding the Three Basic Structures (continued)
Loop structure
Repeats a set of actions based on the answer to a question
Loop body
Also called repetition or iteration
Question is asked first in the most common form of a loop
while (pre-test) or do … while (post-test)
RAPTOR lets you put the test anywhere you like!

12. Understanding the Three Basic Control Structures (continued)
Entry Point
Connector
When using drawing software such as Visio
Figure Loop structure (pre-test form – while loop)

13. Understanding the Three Basic Control Structures (continued)
Loop structure [ pre-test loop ]
while testCondition is true
do someStatement
endwhile
while count < 11
printCount() //module call
count += 1

14. Understanding the Three Basic Control Structures (continued)
All logic problems can be solved using only these three control structures
Structures can be combined in an infinite number of ways
Stacking
Attaching structures end-to-end at their entry/exit points
Nesting
discussed a few slides later…
End-structure statements
Indicate the end of a structure
endif ends an if-then-else structure
endwhile ends a pre-test loop structure
enddo ends a post-test loop structure

15. Understanding the Three Basic Control Structures (continued)
Figure Structured flowchart and pseudocode with three stacked structures

16. Understanding the Three Basic Control Structures (continued)
Any individual task or step can be inserted by creating a new structure or an existing structure can be replaced by a different structure
Nesting
Placing one structure within another
Indent the nested structure’s statements
Block [ aka compound statement ]
Group of statements that execute as a single unit
Java and C++ use { } to enlcose these
Python relies on indentation of the block (suite)

17. Understanding the Three Basic Control Structures (continued)
block or compound statement
Figure Flowchart and pseudocode showing nested structures [ sequence nested within a selection ]

18. Understanding the Three Basic Control Structures (continued)
entry point
and
exit point
for a structure
structures have ONE of each
Figure Flowchart and pseudocode showing nested structures
a loop nested within a sequence, nested within a selection

19. Understanding the Three Basic Structures (continued)
selection
loop
Note:
The diamond flowchart symbol is used for both selection and loop structures.
How do you know if you are looking at a selection or loop structure?
A selection structure only has flowlines merging at the exit point for the structure
A loop structure will always have flowlines merging above the test expression and the body of the loop
Figure Flowchart and pseudocode for
selection structure with nested sequence and loop structures

20. Understanding the Three Basic Control Structures (continued)
Rules for creating a "structured" flowchart
Include only combinations of the three basic control structures (sequence, selection, loop)
Each of the structures has a single entry point and a single exit point
Structures can be stacked (connected) to one another only at their entry and exit points
Ideally whenever flowchart lines come together a connector is used (if drawn manually)
Selection and Loop structures can have nested structures

21. Priming input (or priming read)
Using a Priming Input
Priming input (or priming read)
Does the first input operation outside of the loop that inputs the rest of the data
Inside the body of the loop the input operation is usually the last operation performed in the body of the loop
Note: it is not always necessary to do this… Some languages can “look ahead” to see if there is any data that can be input

22. Using a Priming Input (continued)
RAPTOR would let you do this because there are languages that support a “mid-test” condition.
Our book only considers pre-test and post-test loops as structured.
Figure 3-16
Programming Logic & Design, Sixth Edition

23. Using a Priming Input (continued)
Priming input sets up the process so the loop can be structured
To analyze a flowchart’s structure, try writing pseudocode for it
start
get inputNumber //priming input
while not eof
calculatedAnswer = inputNumber * 2
print calculatedAnswer
get inputNumber
endwhile
stop

24. A priming input is OUTSIDE the loop structure.
It precedes the structure.
The input structure INSIDE the loop is usually the last structure nested in the loop.
Figure number-doubling problem

25. Programming Logic & Design, Sixth Edition
Figure Structured but incorrect solution to the number-doubling problem
Programming Logic & Design, Sixth Edition

26. Recognizing Structure
Any set of instructions can be expressed in structured format
Any task to which you can apply rules can be expressed logically using sequence, selection, and loop control structures

27. Flowchart Status
Flowcharts fall into one of the 2 following categories:
structured
unstructured

28. Recognizing Structure (continued)
This is a no no!
Understandable
but not
Structured
Figure 3-21 Example 3

29. Recognizing Structure (continued)
What are the structures?
Are they:
stacked?
nested?
structured?
Figure 3-20 Example 2
Programming Logic & Design, Sixth Edition

30. Recognizing Structure (continued)
Nested selection structures in RAPTOR
Programming Logic and Design, Seventh Edition

31. Summary Spaghetti code Three basic control structures Priming input
Snarled program logic
Three basic control structures
Sequence, selection, and looping
Combined by stacking and/or nesting
Priming input
Statement that reads the first input data record