Program Design Language (PDL) - Computer Programming Techniques - Lecture Slides, Slides of Computer Engineering and Programming

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Program Design

Language (PDL)

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Steps in Building a Routine

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Example of Bad PDL

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Example of Good PDL

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Design the Routine

Example: ReadErrorMessage()

Spec:

RecordErrorMessage() takes an error code as an input argument and outputs an error message corresponding to the code. It‟s responsible for handling invalid codes. If t h e p r o g r a m i s o p e r a t i n g i n t e r a c t i v e l y , RecordErrorMessage() prints the message to the user. If it‟s operating in batch mode, RecordErrorMessage() logs the message to a message file. After outputting the message, RecordErrorMessage() returns a status variable indicating w h e t h e r i t s u c c e e d e d o r f a i l e d.

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Design the Routine

 Check the prerequisites

• check if the job of the routine is well defined and fits into the architecture

 Define the problem that the routine will solve

• State the problem in enough detail
• State the Input, Output, Internal data, Error handling

 Name the routine

• Use precise, unambiguous name (like ReadErrorMessage() )
• Avoid names with the words „Handle‟, „Process‟ or other vague words

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Design the Routine

 Think about the data

• If data manipulation is the main purpose, think about it before the logic.

 Check the PDL

• Make sure you really understand PDL. Otherwise, how can you understand the code written from it?

 Iterate

• Try as many ideas as you can in PDL before start coding.
• Refine until each PDL statement can be translated into one or a few lines of code

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Turning PDL into Code

 Write the routine declaration

 Added the header comment

 Turn the PDL into comments

 Fill in the code below each comment.

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Example of a Header Comment

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Example of PDL

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Example of Expressing PDL Comments

as Code

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Complete Routine

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Complete Routine