Index mapping

Index mapping is a computer science term (also known as a "trivial hash function") that is used to describe the mapping of raw data, used directly as in array index, for an array.

Examples

The technique can be most effective for mapping data with a small range. If the array encompasses all combinations of input, a range check is not required.

C example

This example of a C function - returning TRUE if a month (x) contains 30 days (otherwise FALSE), illustrates the concept succinctly

 if ((unsigned)x > 11) return 0;                    /*x>12?*/                                                 
 static const int T[12] ={0,0,0,0,1,0,1,0,0,1,0,1}; /* 0-based table 'if 30 days =1,else 0'  */
 return T[x];                                       /* return with boolean 1 = true, 0=false */

See also

• Multiway branch
• lookup table

External links

• A Superoptimizer Analysis of Multiway Branch Code Generation by Roger Anthony Sayle