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to allow some normal test on the array's contents also to function as a termination test. For example, a highly optimized routine for finding a value in an array might artificially place a copy of the value to be searched for after the last slot of the array, thus allowing the main search loop to search for the value without having to check at each pass whether the end of the array had been reached. 3. [among users of optimizing compilers] Any technique, usually exploiting knowledge about the compiler, that blocks certain optimizations. Used when explicit mechanisms are not available or are overkill. Typically a hack: "I call a dummy procedure there to force a flush of the optimizer's register-coloring info" can be expressed by the shorter "That's a fence procedure". |
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fencepost error n. 1. A problem with the discrete equivalent of a boundary condition, often exhibited in programs by iterative loops. From the following problem: "If you build a fence 100 feet long with posts 10 feet apart, how many posts do you need?" (Either 9 or 11 is a better answer than the obvious 10.) For example, suppose you have a long list or array of items, and want to process items m through n; how many items are there? The obvious answer is n - m, but that is off by one; the right answer is n - m + 1. A program that used the 'obvious' formula would have a fencepost error in it. See also zeroth and off-by-one error, and note that not all off-by-one errors are fencepost errors. The game of Musical Chairs involves a catastrophic off-by-one error where N people try to sit in N - 1 chairs, but it's not a fencepost error. Fencepost errors come from counting things rather than the spaces between them, or vice versa, or by neglecting to consider whether one should count one or both ends of a row. 2. [rare] An error induced by unexpected regularities in input values, which can (for instance) completely thwart a theoretically efficient binary tree or hash table implementation. (The error here involves the difference between expected and worst case behaviors of an algorithm.) |
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fepped out /fept owt/ adj. The Symbolics 3600 LISP Machine has a Front-End Processor called a 'FEP' (compare sense 2 of box). When the main processor gets wedged, the FEP takes control of the keyboard and screen. Such a machine is said to have fepped out or dropped into the fep. |
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FidoNet n. A worldwide hobbyist network of personal computers which exchanges mail, discussion groups, and files. Founded in 1984 and originally consisting only of IBM PCs and compatibles, FidoNet now includes such diverse machines as Apple ][s, Ataris, Amiags, and Unix systems. FidoNet has grown rapidly and in early 1996 has approximately 38000 nodes. |
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