Programming pearls: Writing correct programs

In the late 1960s people were talking about the promise of programs that verify the correctness of other programs. Unfortunately, it is now the middle of the 1980s, and, with precious few exceptions, there is still little more than talk about automated verification systems. Despite unrealized expectations, however, the research on program verification has given us something far more valuable than a black box that gobbles programs and flashes “good” or “bad”—we now have a fundamental understanding of computer programming. The purpose of this column is to show how that fundamental understanding can help programmers write correct programs. But before we get to the subject itself, we must keep it in perspective. Coding skill is just one small part of writing correct programs. The majority of the task is the subject of the three previous columns: problem definition, algorithm design, and data structure selection. If you perform those tasks well, then writing correct code is usually easy.
Research and Advances

Programming pearls: Aha algorithms

The study of algorithms has much to offer the practicing programmer. After just a course or two on the subject, students take away algorithms for solving many important tasks and design techniques for attacking new problems. Advanced algorithmic tools can have a substantial impact on software systems. Next month, for instance, we'll study a system in which algorithm design helped reduce development time by 170 staff-years. As important as such tools are, algorithms can have an impact on a more common level of programming. In his book aha! Insight (from which I shamelessly stole my title), Martin Gardner describes the type of contribution I have in mind: “a problem that seems difficult may have a simple, unexpected solution.” Unlike the advanced methods of the field, the aha! insights of algorithms don't come only after extensive study; they're available to any programmer willing to think seriously before, during, and after coding. But enough generalities, on to the problems!

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