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Structured Analysis Lec 3 - Structured Structured Systems...

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Structured Systems Analysis: Transform Descriptions EGA/ERLJ IE 154
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Objectives for Today ring2 Know the need for Transform Descriptions ring2 Know how to describe minispecs and policies ring2 Draw and improve decision trees ring2 Draw and improve decision tables
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Introduction ring2 We are looking at certain techniques which are used in the design of programs - particularly within the context of the design of software systems . ring2 Remember that when we are talking here about programs and software, we do not generally mean packaged software (the software you can buy in shops). ring2 The larger part of the software in the world is custom-built , and typically the result of analysis and design methodologies.
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Diagram 1
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Decomposition of DFDs ring2 By a process of decomposition the system is first modeled in a structured and comprehensible form; and then that model is transformed into a set of transform descriptions . ring2 Each of these transform descriptions is an algorithm - whether or not it describes a process which is going to be computerized. ring2 Where a process is suitable for implementation in software, we now have a specification (minispecs) for that software component - ie: a specification for a (small) piece of program code.
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Review ring2 A functional primitive is a bubble which: circle6 Cannot be partitioned or leveled further, that is, it has no internal data flows, or circle6 has a single input and a single output, or circle6 is “suitable small” and can be described in a page or less
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Transform Descriptions ring2 In the structured specification, the logic of processes is defined only at the functional primitive level. ring2 Process logic or policy descriptions are done by making a minispec for each functional primitive. ring2 Each minispec usually covers a single page of the description of the process
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Objectives for Today ring2 Know the need for Transform Descriptions ring2 Know how to describe minispecs and policies ring2 Draw and improve decision trees ring2 Draw and improve decision tables
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How do we describe Minispecs? ring2 A natural language such as English is imprecise, verbose, redundant, and full of implications and connotations. A natural language is not ideal for specification purposes. circle6 “If the total value of a given sales order exceeds the threshold value of the given customer, the sales order is rejected, otherwise it is accepted”
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How do we describe Minispecs? ring2 Programming languages on the other hand are much too strict or specific for the purpose of defining the policy or decisions in a process. ring2 At this level, a programming language is too implementation dependent to be ideal for the task circle6 if GetVal(sales_order) >= CredLim(Cust_N){ sales_order.status = “rejected” else sales_order.status = “accepted” End If }
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Tools Used to Describe Policy 1. Decision Trees 2. Decision Tables 3. Structured English
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Objectives for Today ring2 Know the need for Transform Descriptions ring2 Know how to describe minispecs and policies ring2 Draw and improve decision trees ring2 Draw and improve decision tables
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