Programming Logic and Design, Introductory, Fourth Edition1 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 Edition2 Understanding the Three Basic Structures (continued) Sequence structure –A set of instructions, performed sequentially with no branching

Programming Logic and Design, Introductory, Fourth Edition3 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 Edition4 Understanding the Three Basic Structures (continued) 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

Programming Logic and Design, Introductory, Fourth Edition5 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. 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) The Problem Pauedocode Flowchart Question TRUE path TRUE or FALSE path FALSE path

Programming Logic and Design, Introductory, Fourth Edition6 Understanding the Three Basic Structures (continued) Single-alternative if Else clause is not required Null case: situation where nothing is done End If If

Programming Logic and Design, Introductory, Fourth Edition7 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 Edition8 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 Question TRUE (repeat) FALSE Check Condition here or here DO WHILE or DO UNTIL

Programming Logic and Design, Introductory, Fourth Edition9 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 Edition10 Understanding the Three Basic Structures (continued)

Programming Logic and Design, Introductory, Fourth Edition11 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 Edition12 Understanding the Three Basic Structures (continued)

Programming Logic and Design, Introductory, Fourth Edition13 Understanding the Three Basic Structures (continued)

Programming Logic and Design, Introductory, Fourth Edition14 Understanding the Three Basic Structures (continued)

Programming Logic and Design, Introductory, Fourth Edition15 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 Edition16 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 Edition17 Using the Priming Read (continued) Unstructured loop:

Programming Logic and Design, Introductory, Fourth Edition18 Using the Priming Read (continued) Structured but nonfunctional loop

Programming Logic and Design, Introductory, Fourth Edition19 Using the Priming Read (continued) Corrrect

Programming Logic and Design, Introductory, Fourth Edition20 Using the Priming Read (continued) Functional and structured loop

Programming Logic and Design, Introductory, Fourth Edition21 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 Edition22 Using the Priming Read (continued) What is wrong with this design?

Programming Logic and Design, Introductory, Fourth Edition23 Understanding the Reasons for Structure Advantages of structure: –Provides clarity –Professionalism –Efficiency –Ease of maintenance –Supports modularity

Programming Logic and Design, Introductory, Fourth Edition24 Understanding the Reasons for Structure (continued)

Programming Logic and Design, Introductory, Fourth Edition25 Recognizing Structure (continued) Next, pull up the flowline on the right side of B

Programming Logic and Design, Introductory, Fourth Edition26 Recognizing Structure (continued) Now pull up the flowline on the right side of D

Programming Logic and Design, Introductory, Fourth Edition27 Recognizing Structure (continued) Bring together the loose ends of D and of B

Programming Logic and Design, Introductory, Fourth Edition28 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 Edition29 Three Special Structures – Case, Do While, and Do Until (continued) Using nested if-then-else for multiple alternatives

Programming Logic and Design, Introductory, Fourth Edition30 Three Special Structures – Case, Do While, and Do Until (continued) Using a case structure for multiple alternatives

Programming Logic and Design, Introductory, Fourth Edition31 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 Edition32 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 Edition33 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 Edition34 Three Special Structures – Case, Do While, and Do Until (continued)

Programming Logic and Design, Introductory, Fourth Edition35 Three Special Structures – Case, Do While, and Do Until (continued)