1. Understanding the Three Basic Structures
Structure: a basic unit of programming logic
Any program can be constructed from only three basic types of structures
Sequence
Selection
Loop
Programming Logic and Design, Introductory, Fourth Edition

2. Control Structures Sequence –in sequential order.
The simplest of control structures – start at the beginning and continue in sequential order.
Repetition – repeat statements more than once
Also called a loop, it needs a stop condition, i.e, the program will continue to loop until some condition is met.
Selection – selectively execute statements
Called a branch, it requires a condition to determine when to execute statements.
Programming Logic and Design, Introductory, Fourth Edition

3. Structure Theorem 1. Sequence 3. Repetition 2. Selection ? Instruction
Sequential instructions
Do While / Do Until
Instruction
2. Selection
Instruction ?
IF … THEN … ELSE CASE
Programming Logic and Design, Introductory, Fourth Edition

4. Understanding the Three Basic Structures (continued)
Sequence structure
A set of instructions, performed sequentially with no branching
Programming Logic and Design, Introductory, Fourth Edition

5. Understanding the Three Basic Structures (continued)
Selection structure
Asks a question, then takes one of two possible courses of action based on the answer
Also called a decision structure or an if-then-else
Programming Logic and Design, Introductory, Fourth Edition

6. Flowchart for a Sequence
START
Instructions follow each other sequentially
Input sales amount from customer
Sale
Sequential instructions
Computer total amount
Sales amount x .06
Print report
Sale report
Hard Drive
Printed Report
Sale data
Save in file
END
Programming Logic and Design, Introductory, Fourth Edition

7. Flowchart for a Decision
Decision or selection structure flowchart:
IF --- THEN --- ELSE
CASE statement
Question
Answer is “NO” (false)
Answer is “YES” (true)
Programming Logic and Design, Introductory, Fourth Edition

8. Understanding the Three Basic Structures (continued)
Dual-alternative if: contains two alternatives
If the hours an employee has worked is greater than 40 hours then calculate their pay as regular hours multiplied by their regular time pay mount added to the overtime pay amount which is overtime hours multiplied by 1 ½ time the regular pay amount.
The Problem
IF the hours worked is more than 40 THEN (question) total pay will be = regular pay amount plus overtime hours multiplied by 1 ½ times regular pay amount (action if true) ELSE total pay is regular hours times regular pay amount (action if false) END IF
Pauedocode
Flowchart
Programming Logic and Design, Introductory, Fourth Edition

9. Understanding the Three Basic Structures (continued)
Single-alternative if: contains one alternative
If the hours an employee has worked is greater than 40 hours then calculate their pay as regular hours multiplied by their regular time pay mount added to the overtime pay amount which is overtime hours multiplied by 1 ½ time the regular pay amount.
The Problem
Total pay = regular hours multiplied by regular pay IF the hours worked is more than 40 THEN (question) total pay will be = total pay amount plus overtime hours multiplied by 1 ½ times regular pay amount (action if true) END IF Print to printer the total pay amount (action if true or false)
Pauedocode
FALSE path
TRUE path
Question
Flowchart
Programming Logic and Design, Introductory, Fourth Edition
TRUE or FALSE path

10. Understanding the Three Basic Structures (continued)
Single-alternative if
Else clause is not required
Null case: situation where nothing is done If
End If
Programming Logic and Design, Introductory, Fourth Edition

11. Flowchart for a Decision
ASK THE QUESTION
IF condition THEN instruction instruction as many instructions as needed as many structures (decision, sequential, looping) as needed ELSE instruction instruction as many instructions as needed as many structures (decision, sequential, looping) as needed ENDIF Continuation of the program (instructions and structures)
TRUE path if the questions answer is true (YES)
IF condition THEN as many instructions as needed as many structures (decision, sequential, looping) as needed ENDIF
TRUE path if the questions answer is true (YES)
FALSE path if the questions answer is false (NO)
Programming Logic and Design, Introductory, Fourth Edition

12. Flowchart for a Decision
CONDITIONS
A < B (A & B are the same data type (numeric or alphanumeric)
X >= Z (X and Z are numeric data types)
E < 5 (E is a numeric data type)
F > 10 (F is a numeric data type)
IF A < B THEN instructions/structures ELSE IF X + 5 >= Z THEN instructions/structures ENDIF ENDIF
Nesting
Programming Logic and Design, Introductory, Fourth Edition

13. Flowchart for a Decision
Example 1
Somewhere before this decision, data is placed in the variables HOURS and RATE IF HOURS > 40 THEN PAY = RATE * ( * (HOURS – 40)) ELSE PAY = RATE * HOURS ENDIF
Data is put into HOURS and RATE
start
end
IF HOURS > 40
PAY = RATE * HOURS
PAY= RATE * ( * (HOURS – 40))
Programming Logic and Design, Introductory, Fourth Edition

14. Nested Decisions (IF – THEN – ELSE) Example 2
ENDIF
ENDIF
Programming Logic and Design, Introductory, Fourth Edition

15. Example 3 ENDIF ENDIF ENDIF Range Check
Programming Logic and Design, Introductory, Fourth Edition

16. Example 4 ENDIF ENDIF ENDIF
Programming Logic and Design, Introductory, Fourth Edition

17. Understanding the Three Basic Structures (continued)
Loop structure
Repeats a set of actions based on the answer to a question
Also called repetition or iteration
Question is asked first in the most common form of loop
Programming Logic and Design, Introductory, Fourth Edition

18. Understanding the Three Basic Structures (continued)
Loop structure
WHILE testcondition (check if testcondition is true) do however many instructions are required (testcondition is true) END LOOP (end of loop – go back to beginning and check condition) Continue with whatever processing is necessary
DO WHILE or DO UNTIL
Check Condition here or here
Question
TRUE (repeat)
FALSE
Programming Logic and Design, Introductory, Fourth Edition

19. Flowchart for a Loop Loop or repetition structure flowchart:
Ask a question Answer is “Yes” Execute the loop
Answer is “NO” Exit the loop
Question
Programming Logic and Design, Introductory, Fourth Edition

20. Understanding the Three Basic Structures (continued)
All logic problems can be solved using only these three structures
Structures can be combined in an infinite number of ways
Stacking: attaching structures end-to-end
End-structure statements
Indicate the end of a structure
endif: ends an if-then-else structure
endwhile: ends a loop structure
Programming Logic and Design, Introductory, Fourth Edition

21. Understanding the Three Basic Structures (continued)
Programming Logic and Design, Introductory, Fourth Edition

22. Understanding the Three Basic Structures (continued)
Any individual task or step in a structure can be replaced by a structure
Nesting: placing one structure within another
Indent the nested structure’s statements
Block: group of statements that execute as a single unit
Programming Logic and Design, Introductory, Fourth Edition

23. Understanding the Three Basic Structures (continued)
Programming Logic and Design, Introductory, Fourth Edition

24. Understanding the Three Basic Structures (continued)
Programming Logic and Design, Introductory, Fourth Edition

25. Understanding the Three Basic Structures (continued)
Programming Logic and Design, Introductory, Fourth Edition

26. Understanding the Three Basic Structures (continued)
Each structure has one entry and one exit point
Structures attach to others only at entry or exit points
Programming Logic and Design, Introductory, Fourth Edition

27. Using the Priming Read Priming read (or priming input):
Reads the first input data record
Outside the loop that reads the rest of the records
Helps keep the program structured
Analyze a flowchart for structure one step at a time
Watch for unstructured loops that do not follow this order:
First ask a question
Take action based on the answer
Return to ask the question again
Programming Logic and Design, Introductory, Fourth Edition

28. Using the Priming Read (continued)
Unstructured loop:
Programming Logic and Design, Introductory, Fourth Edition

29. Using the Priming Read (continued)
Structured but nonfunctional loop
Programming Logic and Design, Introductory, Fourth Edition

30. Using the Priming Read (continued)
Corrrect
Programming Logic and Design, Introductory, Fourth Edition

31. Using the Priming Read (continued)
Functional and structured loop
Programming Logic and Design, Introductory, Fourth Edition

32. Using the Priming Read (continued)
Priming read sets up the process so the loop can be structured
To analyze a flowchart’s structure, try writing pseudocode for it
Programming Logic and Design, Introductory, Fourth Edition

33. Using the Priming Read (continued)
What is wrong with this design?
Programming Logic and Design, Introductory, Fourth Edition

34. Understanding the Reasons for Structure
Advantages of structure:
Provides clarity
Professionalism
Efficiency
Ease of maintenance
Supports modularity
Programming Logic and Design, Introductory, Fourth Edition

35. Understanding the Reasons for Structure (continued)
Programming Logic and Design, Introductory, Fourth Edition

36. Recognizing Structure (continued)
Next, pull up the flowline on the right side of B
Programming Logic and Design, Introductory, Fourth Edition

37. Recognizing Structure (continued)
Now pull up the flowline on the right side of D
Programming Logic and Design, Introductory, Fourth Edition

38. Recognizing Structure (continued)
Bring together the loose ends of D and of B
Programming Logic and Design, Introductory, Fourth Edition