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On the proof of correctness of a calendar program

Programming Languages J.J. Horning Editor On the Proof of Correctness of a Calendar Program Leslie Lamport SRI International A formal specification is given for a simple calendar program, and the derivation and proof of correctness of the program are sketched. The specification is easy to understand, and its correctness is manifest to humans. Key Words and Phrases: program specification, program verification, inductive assertions CR Category: 5.24 Introduction In [1], Geller introduced a method for proving the correctness of a program based upon the concept of test data, and illustrated it with a simple calendar program. The program accepts as input a pair of dates in the same year, and computes the number of days between those dates. Geller presented two proofs for this program: one using his method, the other using the customary inductive assertion method. He made the following valid criticism of the assertional proof: We also have less confidence in the proof that our output assertion actually ensures that the program computes values that correspond with the way real calendars behave. The formal specific a t i o n . . , could easily be off by a constant of 1 or 2. The example of http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Communications of the ACM Association for Computing Machinery

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