programming logic and design, introductory, fourth edition1 understanding the three basic structures...

Programming Logic and Design, Introductory, Fourth Edition 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


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.


Structure Theorem

1. Sequence Instruction

Instruction

Instruction

Instruction

2. Selection Instruction

?

Instruction Instruction

3. Repetition

Do While / Do Until

Instruction

Instruction

Instruction

Instruction

Instruction

Sequential instructions

IF … THEN … ELSECASE


Understanding the Three Basic Structures (continued)

• Sequence structure – A set of instructions, performed sequentially with no

branching



• 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


Flowchart for a SequenceSTART

END

Input sales amount from customer

Computer total amount

Sales amount x .06

Print report

Save in file

Sale report

Sale data

Sale

Instructions follow each other sequentially

Sequential instructions

Hard DrivePrinted Report


Flowchart for a Decision

Decision or selection structure flowchart:

IF --- THEN --- ELSE

CASE statement Question

Answer is “NO” (false)

Answer is “YES” (true)



• Dual-alternative if: contains two alternatives

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

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

Pauedocode

Flowchart



• Single-alternative if: contains one alternativeIf 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.

Total pay = regular hours multiplied by regular payIF 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 IFPrint to printer the total pay amount (action if true or false)

The Problem

Pauedocode

FlowchartQuestion TRUE path

TRUE or FALSE path

FALSE path



• Single-alternative if

• Else clause is not required

• Null case: situation where nothing is done

End If

If


Flowchart for a DecisionASK THE QUESTION

IF condition THEN instruction1 instruction2 as many instructions as needed as many structures (decision, sequential, looping) as neededELSE instruction1 instruction2 as many instructions as needed as many structures (decision, sequential, looping) as neededENDIFContinuation of the program (instructions and structures)

TRUE path if the questions answer is true (YES)

FALSE path if the questions answer is false (NO)

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)


Flowchart for a DecisionCONDITIONS

A < B (A & B are the same data type (numeric or alphanumeric)

X + 5 >= 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/structuresELSE IF X + 5 >= Z THEN instructions/structures ENDIFENDIF

Nesting


Somewhere before this decision, data is placed in the variables HOURS and RATE IF HOURS > 40 THEN PAY = RATE * (40 + 1.5 * (HOURS – 40))ELSE PAY = RATE * HOURSENDIF

Flowchart for a Decision

Data is put into HOURS and RATE

start

end

IFHOURS > 40

PAY = RATE * HOURS

PAY= RATE * (40 + 1.5 * (HOURS – 40))

Example 1


Nested Decisions (IF – THEN – ELSE)Example 2

ENDIF

ENDIF


Example 3

Range Check

ENDIF

ENDIF

ENDIF


Example 4

ENDIF

ENDIF

ENDIF



• 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



• 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

QuestionTRUE (repeat)

FALSE

Check Condition here or here

DO WHILE or DO UNTIL


Flowchart for a Loop

Loop or repetition structure flowchart:

Ask a questionAnswer is “Yes” Execute the loop

Answer is “NO” Exit the loop

Question



• 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



• 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



• Each structure has one entry and one exit point• Structures attach to others only at entry or exit points


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:1. First ask a question2. Take action based on the answer3. Return to ask the question again


Using the Priming Read (continued)• Unstructured loop:


Using the Priming Read (continued)

• Structured but nonfunctional loop



• Corrrect


Using the Priming Read (continued)• Functional and structured loop



• Priming read sets up the process so the loop can be structured

• To analyze a flowchart’s structure, try writing pseudocode for it



• What is wrong with this design?


Understanding the Reasons for Structure

• Advantages of structure:– Provides clarity– Professionalism– Efficiency– Ease of maintenance– Supports modularity


Understanding the Reasons for Structure (continued)


Recognizing Structure (continued)

• Next, pull up the flowline on the right side of B



• Now pull up the flowline on the right side of D



• Bring together the loose ends of D and of B

programming logic and design, introductory, fourth edition1 understanding the three basic structures...

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