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

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

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

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

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

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

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

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

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

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

Flowchart for a Decision ASK THE QUESTION IF condition THEN instruction1 instruction2 as many instructions as needed as many structures (decision, sequential, looping) as needed ELSE instruction1 instruction2 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

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 ENDIF Nesting Programming Logic and Design, Introductory, Fourth Edition

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 * (40 + 1.5 * (HOURS – 40)) ELSE PAY = RATE * HOURS ENDIF Data is put into HOURS and RATE start end IF HOURS > 40 PAY = RATE * HOURS PAY= RATE * (40 + 1.5 * (HOURS – 40)) Programming Logic and Design, Introductory, Fourth Edition

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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