One of the big names in ComputerScience.
It was him who discovered QuickSort in 1960, which is perhaps the single most widely used Algorithm ever.
He also described the formal logic system named after himself, Hoare logic or Hoare calculus, in his 1969 paper An axiomatic basis for computer programming. This system defines axioms which can be used to reason about computer programs, in order to do such things as formally proving their correctness.
He now works at the Microsoft Research Labs.
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