Finding the αn-th largest element


We describe an algorithm for selecting the αn-th largest element (where 0<α<1), from a totally ordered set ofn elements, using at most (1+(1+o(1))H(α))·n comparisons whereH(α) is the binary entropy function and theo(1) stands for a function that tends to 0 as α tends to 0. For small values of α this is almost the best possible as there is a lower bound of about (1+H(α))·n comparisons. The algorithm obtained beats the global 3n upper bound of Schönhage, Paterson and Pippenger for α<1/3.

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Dor, D., Zwick, U. Finding the αn-th largest element. Combinatorica 16, 41–58 (1996).

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Mathematics Subject Classification (1991)

  • 68 Q 25
  • 68 Q 20