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Structured Programming: Retrospect and Prospect

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Structured Programming: Retrospect and Prospect. Harlan D.Mills,IBM corp. Djikstra's Article ... Based on results from Corrado Boehm & Jacopini ... – PowerPoint PPT presentation

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Title: Structured Programming: Retrospect and Prospect


1
Structured ProgrammingRetrospect and Prospect
  • Harlan D.Mills,IBM corp.

2
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

3
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.

4
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

5
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.

6
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.

7
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)

8
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

9
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

10
Conclusion
  • Structured programming can
  • Reduce Unnecessary complexity
  • Increase Human expectations
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