Topic # 09

Structuring System Requirements: Logic Modeling

Requirements Structuring as part of CIS Life Cycle

Objectives

1. Structured English as a tool for representing steps in logical processes 2. Decision tables and decision trees

Learning Objectives

As good as data flow diagrams are for identifying processes, they are not very good at showing the logic inside the processes. There are several methods to represent logic inside processes:

• Structured English • Decision Tables • Decision Trees • State-transition diagrams • Sequence diagrams • Activity diagrams

Introduction

Structured English • Structured English is a modified form of the English language used to specify the logic of information system processes.

Basic logic (programming) structures: • Sequence is represented by listing statements at the same level of

indentation. • Conditional statements are represented by BEGIN IF and END IF statements

and by SELECT CASE statements. • Repetition is represented by DO ... UNTIL and DO…WHILE statements.

• Action verbs represent main functions such as DO, BEGIN, READ, WRITE,

PRINT, SORT, MOVE, MERGE, ADD, SUBTRACT, MULTIPLY and DIVIDE. • Unlike regular English, Structured English does not use adjectives or

adverbs.

Structured English

Examples of Structured English

Example # 1: DO …UNTIL DO READ next Invoice-item-record FIND matching Inventory-record ADD Quantity-added from Invoice-item-record to Quantity-in- stock on Inventory-record UNTIL End-of-file

Example # 2: BEGIN IF …. ENF IF

BEGIN IF IF Employee–Type is Hourly AND Hours–Worked is >40 THEN CALCULATE hourly wage AND CALCULATE overtime

END IF

Examples of Structured EnglishExample # 3: SELECT CASE

SELECT CASE n CASE (describe)…….

(write instructions here) ….. CASE (describe)…….

(write instructions here) ….. CASE (describe)…….

(write instructions here) ….. ENF SELECT

Example:

Select Case n Case Is < -5 PRINT “n is less than -5" Case -4 To -1 PRINT "n is between -4 and -1" Case 0 PRINT "n is 0" Case 2, 4, 6, 8 PRINT "n is even" Case 1, 3, 5, 7, 9 PRINT "n is odd" Case Else PRINT "Only single-digit numbers are allowed" End Select

Structured English is used here to describe input and output.

Structured English is used here to describe arithmetic operations.

Structured English is used here to describe repetition.

Structured English is used here to describe decisions.

Structured English is used here to invoke procedures.

Current Logical DFD for Hoosier Burger’s Inventory Control System

DO READ next Invoice-item-record

FIND matching Inventory-record

ADD Quantity-added from

Invoice-item-record to Quantity-in-

stock on Inventory-record

UNTIL End-of-file

Decision Tables

• A decision table is a matrix representation of the logic of a decision, which specifies the possible conditions for the decision and the resulting actions. • The steps for creating a decision table are:

(1) name conditions and values each condition can assume; (2) name all possible actions that can occur; (3) list all possible rules; (4) define the actions for each rule; (5) simplify the decision table.

• To reduce the size and complexity of a decision table, use separate, linked decision tables, or use numbers that indicate sequence rather than Xs where rules and action stubs intersect.

Decision Tables

Decision Tables

Provided below is a sample decision table for the relatively simple process, Generate Payments.

Conditions/ Rules Courses of Action 1 2 3 4 Employee Type Salary Hourly Hourly Hourly Hours Worked - <40 40 >40 Pay Base Salary X Calculate hourly wage X X X Calculate overtime X Produce absence report X

Decision Trees

• A decision tree is a graphical representation of a decision situation in

which decision points (nodes) are connected together by arcs (one for each alternative on a decision) and terminate in ovals (the action which is the result of all of the decisions made on the path that leads to that oval). • Decision trees are organized in a hierarchical fashion, starting with the root node on the far left, and proceeding to subsequent decision nodes. All possible actions are listed in leaf nodes on the far right.

Decision Tree with Multiple Choices per Decision Point

Deciding Among Structured English, Decision Tables, and Decision Trees

BestBestThird BestChecking Consistency and Completeness

BestThird BestBestTransforming Conditions and Actions into Sequence

BestThird BestSecond BestDetermining Conditions and Actions

Decision TreesDecision TablesStructured English

Criteria

Deciding Between Decision Tables and Decision Trees

WorstBestEasier to manipulate

WorstBestMore compact

BestWorstMaking decisions

BestWorstPortraying simple rules

WorstBestPortraying complex logic

Decision TreesDecision TablesCriteria

Topic # 09

Structuring System Requirements: Logic Modeling: In-Classroom Exercise

Topic # 09

Structuring System Requirements: Logic Modeling: Homework assignment