Structured Programming: Retrospect and Prospect

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Structured ProgrammingRetrospect and Prospect

• Harlan D.Mills,IBM corp.

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Djikstras Article

• Published in 1969.
• Based on results from Corrado Boehm Jacopini
• Proposed to restrict program control logic to
  three forms Sequence ,Selection , iteration
• Impact of structured programming
• Programming regarded as an activity of
  restructuring specifications into programs
• Structured programming made testing easy.
• Structured programming has reduced much of the
  unnecessary complexity

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Early controversies

• goto statement viewed as foundation of stored
  program concept
• Problem with goto modularity
• Controversy No one believed that three
  constructs proposed by Djikstra would implement
  complex programs
• Explanation These constructs are compared to
  And Or Not in hardware implementation which can
  build any combinational circuit.

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Early Implementations

• The New York Times Project
• First major implementation of Structured
  Programming
• Used Several New Techniques
• Top-down development, Step-wise refinement,
  Hierarchical modularity, function verification.
• Results
• reliability was good
• Definite error rate low
• Productivity 5 times better

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Structure theorem

• States that A structured program exists for any
  problem that permits a Flow chartable solution
• Structure theorem permits management by exception
  in program design standards.

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Top-Down Corollary

• Lines of a structured program can be written
  chronologically so that every line can be
  verified by reference only to lines already
  written ,and not to lines yet to be written.
• Natural hierarchy defined among instructions.
• Misunderstandings
• Doesnt claim thinking to be done Top-Down.
• Effective in later phases of program design.

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Language problems

• Algol like languages had problem with converting
  to structured programming ,so they had to modify
  language definitions.
• Assembly language was easy to adapt to
  structured programming through use of
  Macroassemblers (intermediate)

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Mathematical correctness

• A program will be correct with respect to its
  specification if only if, for every initial
  value permissible by the specification ,the
  program will produce a final value that
  correspond to that initial value in the
  specification.
• Mathematical Notation
• Domain(f r) Domain(r)
• fFunction f defined by program P
• rspecification

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Program Correctness Proofs

• Two different approaches are used by industry and
  universities
• Axiomatic verification Universities
• Only applicable to small projects
• Least efficient
• Functional verification Industry
• Requires training
• Applicable to any project
• Cleanroom software development

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Conclusion

• Structured programming can
• Reduce Unnecessary complexity
• Increase Human expectations