In computer science, a lookup table is a data structure, usually an array or associative array, often used to replace a runtime computation with a simpler array indexing operation. The savings in terms of processing time can be significant, since retrieving a value from memory is often faster than undergoing an 'expensive' computation or input/output operation. The tables may be precalculated and stored in static program storage, calculated (or "pre-fetched") as part of a programs initialization phase (memoization), or even stored in hardware in application-specific platforms. Lookup tables are also used extensively to validate input values by matching against a list of valid (or invalid) items in an array and, in some programming languages, may include pointer functions (or offsets to labels) to process the matching input.
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“In this sad state, God Tender Bowells run
Out streams of Grace: And he to end all strife
The Purest Wheate in Heaven, his deare-dear Son
Grinds, and kneads up into this Bread of Life.
Which Bread of Life from Heaven down came and stands
Disht on thy Table up by Angells Hands.”
—Edward Taylor (1645–1729)