Unranking Combinations Lexicographically: an efficient new strategy compared with others Cyann Donnot, Antoine Genitrini, Yassine Herida To cite this version: Cyann Donnot, Antoine Genitrini, Yassine Herida. Unranking Combinations Lexicographically: an efficient new strategy compared with others. 2020. hal-02462764 HAL Id: hal-02462764 https://hal.sorbonne-universite.fr/hal-02462764 Preprint submitted on 31 Jan 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Unranking Combinations Lexicographically: an efficient new strategy compared with others Cyann Donnot1, Antoine Genitrini2, and Yassine Herida1 1 Full-time master student at Sorbonne University 2 Sorbonne University, LIP6.
[email protected] Abstract. We propose a comparative study of four algorithms dedi- cated to the lexicographic unranking of combinations. The three first ones are algorithms from the literature. We give some key ideas how to analyze them in average and describe their strengths and weaknesses. We then introduce a new algorithm based on a new strategy inside the classical factorial numeral system (or factoradics). We conclude the pa- per with an experimental study, in particular when n = 10000; k = 5000, our algorithm is 36 (resp. 53) times as fast as our C++ implementation of Matlab’s (resp.