1. (Originally Unix, now also MS-DOS) A program that processes an input data stream into an output data stream in some well-defined way, and does no I/O to anywhere else except possibly on error conditions; one designed to be used as a stage in a pipeline (see plumbing). Compare sponge.

2. (functional programming) A higher-order function which takes a predicate and a list and returns those elements of the list for which the predicate is true. In Haskell:

filter p [] = [] filter p (x:xs) = if p x then x : rest else rest where rest = filter p xs

See also: filter promotion.

(01 Jul 2002)

1. A porous substance through which a liquid or gas is passed in order to separate it from contained particulate matter or impurities.

Synonyms: filtrum.

2. To use or to subject to the action of a filter.

3. In diagnostic or therapeutic radiology, a plate made of one or more metals such as aluminum and copper that, placed in the x-or gamma-ray beam, permits passage of a greater proportion of higher energy radiation and attenuation of lower and less desirable energy radiation, raising the average energy or hardening the beam.

4. A device used in spectrophotometric analysis to isolate a segment of the spectrum.

5. A mathematical algorithm applied to image data for the purpose of enhancing image quality, usually by suppression of high spatial frequency noise.

Origin: Mediev. L. Filtro, pp. -atus, to strain through felt, fr. Filtrum, felt

(05 Mar 2000)

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