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A lookup (table) is an array that replaces run-time 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. Over time the definition has become less strict, any kind of searching with a key value can be considered a lookup.

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Optimize the FFT calculation in C++ without creating even/odd arrays

Precalculate the roots of unity How to optimize this to use a lookup table or precomputed values. Yes, just do that. …
G. Sliepen's user avatar
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20 votes

Sine function calculated via look-up table and linear interpolation

You only need to store values for \$\left[0, \frac{1}{2}\pi\right)\$ You can exploit the structure of the sine function, and only store values in the table for inputs between 0 and \$\frac{1}{2}\pi\$. …
G. Sliepen's user avatar
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