Understanding the Three Basic Structures Structure a basic

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 1

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 2

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

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

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 5

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

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

Understanding the Three Basic Structures (continued) • Dual-alternative if: contains two alternatives The Problem Pauedocode 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. 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 Flowchart Programming Logic and Design, Introductory, Fourth Edition 8

• 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 The Problem to the overtime pay amount which is overtime hours multiplied by 1 ½ time the regular pay amount. Pauedocode 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) FALSE path Flowchart Question TRUE path Programming Logic and Design, Introductory, Fourth Edition TRUE or FALSE path 9

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

Flowchart for a Decision ASK THE QUESTION IF condition THEN instruction 1 TRUE path if the questions answer is true (YES) instruction 2 as many instructions as needed as many structures (decision, sequential, looping) as needed ELSE instruction 1 instruction 2 as many instructions as needed as many structures (decision, sequential, looping) as needed ENDIF Continuation of the program (instructions and structures) IF condition THEN TRUE path if the questions answer is true (YES) as many instructions as needed as many structures (decision, sequential, looping) as needed ENDIF FALSELogic pathand if the questions answer false (NO) Programming Design, Introductory, Fourthis. Edition 11

Flowchart for a Decision CONDITIONS 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/structures ELSE IF X + 5 >= Z THEN instructions/structures ENDIF Nesting Programming Logic and Design, Introductory, Fourth Edition 12

Example 1 Flowchart for a Decision 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 start PAY = RATE * HOURS Data is put into ENDIF HOURS and RATE IF HOURS > 40 PAY= RATE * (40 + 1. 5 * (HOURS – 40)) PAY = RATE * HOURS Programming Logic and Design, Introductory, Fourth Edition end 13

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

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

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

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 17

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 18

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

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 20

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

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 22

Understanding the Three Basic Structures (continued) 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) • 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 26

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 question 2. Take action based on the answer 3. Return to ask the question again Programming Logic and Design, Introductory, Fourth Edition 27

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

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

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

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

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 32

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

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

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

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

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

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

Three Special Structures – Case, Do While, and Do Until • Many languages allow three additional structures: – case structure – do-while structure – do-until structure • Case Structure: – Decisions with more than two alternatives – Tests a variable against a series of values and takes action based on a match – Nested if-then-else statements will do what a case structure does Programming Logic and Design, Introductory, Fourth Edition 39

Three Special Structures – Case, Do While, and Do Until (continued) • Using nested if-then-else for multiple alternatives Programming Logic and Design, Introductory, Fourth Edition 40

Three Special Structures – Case, Do While, and Do Until (continued) • Using a case structure for multiple alternatives Programming Logic and Design, Introductory, Fourth Edition 41

Three Special Structures – Case, Do While, and Do Until (continued) • do-while and do-until loops – Question is asked at the end of the loop structure – Ensures that the loop statements are always used at least once Programming Logic and Design, Introductory, Fourth Edition 42

Three Special Structures – Case, Do While, and Do Until (continued) • do-while loop executes as long as the question’s answer is Yes or True Test checked at beginning May not be executed • do-until loop executes as long as the question’s answer is No or False (until it becomes Yes or True) Test checked at end of loop Will always execute loop at least once Programming Logic and Design, Introductory, Fourth Edition 43

Three Special Structures – Case, Do While, and Do Until (continued) • while loop with question at beginning is called a pretest loop • do-until with question at end are called posttest loops Programming Logic and Design, Introductory, Fourth Edition 44

Three Special Structures – Case, Do While, and Do Until (continued) Programming Logic and Design, Introductory, Fourth Edition 45

Three Special Structures – Case, Do While, and Do Until (continued) Programming Logic and Design, Introductory, Fourth Edition 46