1. Program Design Language (PDL)
Slides by: Noppadon Kamolvilassatian
Source: Code Complete by Steve McConnell, Chapter 4

2. Steps in Building a Routine

3. PDL Overview
PDL (Program Design Language) is for designing at a slightly higher level that the code itself.
Use English-like (or Thai-like) statements that precisely describe specific operations
Write PDL at the level of intent
Write PDL at a low enough level that generating code from it will be nearly automatic

4. Example of Bad PDL

5. Example of Good PDL

6. PDL Benefits PDL makes reviews easier
PDL supports the idea of iterative refinement: architecture->interface & PDL->source code
PDL makes changes easier
PDL minimizes commenting effort
PDL will be turned into comments for source code
PDL is easier to maintain that other forms of design documentation

7. 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 the program is operating interactively, 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 whether it succeeded or failed.

8. 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

9. Design the Routine Decide how to test the routine
Think about testing cases, the method to use, etc.
Think about efficiency
In general, refining overall design helps a lot more.
Research the algorithms and data structures
Don’t “reinvent the wheel”
Write the PDL
Begin with the purpose of the routine, followed by the algorithm

10. Design the Routine Think about the data Check the PDL Iterate
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

11. Turning PDL into Code Write the routine declaration
Added the header comment
Turn the PDL into comments
Fill in the code below each comment.

12. Turning PDL into Code Check the code informally Clean up the leftovers
Repeat steps as needed
In some cases, you may need to move back to design.
Formally review the code
Until you are sure that you understand why the code works!
The main difference between hobbyists and professional programmers grows out of moving from superstition into understanding.

13. Example of a Header Comment

14. Example of PDL

15. Example of Expressing PDL Comments as Code

16. Example of Expressing PDL Comments as Code

17. Complete Routine

18. Complete Routine

19. Complete Routine