Elementary Process Descriptions

When to use Data flow diagrams should be decomposed until each process occurs: – In one place – At one time – Can be completed by (logically) one person Elementary Process Descriptions are used to describe that lowest level

How to describe Narrative – Easy to write and to read – Long-winded – Sometimes ambiguous Mathematical formulae e.g. Z notation – Precise – Compact – Unambiguous – Difficult to write and to read Decision Tables – More later Decision Trees – More later Structured English – Harder to write than narrative – Shorter than narrative – Less ambiguous than narrative

Narrative for Worked Example A warehouse receives orders from its customers which are processed as follows. Any order of £500 or more from a credit-worthy customer attracts a discount of 5% whereas orders if less than £500 attract a discount of 3%. Other circumstances must be referred to the supervisor for a decision. (so is it 3% for creditworthy customers only or 3% for everyone ordering less than £500?)

Decision Tree for Worked example Credit OK? Order >= £500? Refer to super Disc= 3% Disc= 5% Yes No Yes

Decision Trees Graphical representation Includes sequence of actions and of evaluation of conditions Scaleable i.e. suitable for small simple scenarios or large complex scenarios

Decision Table for Worked Example (1) Rules CONDITIONS1234 Order >= £500?YYNN Credit OK?YNYN ACTIONS Give 3% discountX Give 5% discountX Refer to super.XX

Decision Tables Easy to use Easy to understand Show cause and effect (of combinations of circumstances) Can ensure all possible combinations have been considered Do not show time sequence Unwieldy if many conditions present

Structured English for Worked Example (1) If credit satisfactory then if order>= 500 then discount=5% else discount=3% endif else print “refer-to-super.” endif Other statements available: Case Do..while Repeat....until

Structured English Shorter than narrative Less ambiguous Ready to convert to program code More thought needed to write