Loopless algorithm

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In computational combinatorics, a loopless algorithm or loopless imperative algorithm is an imperative algorithm that generates successive combinatorial objects, such as partitions, permutations, and combinations, in constant time and the first object in linear time.[1][2] The objects must be immediately available in simple form without requiring any additional steps.[1]

A loopless functional algorithm is a functional algorithm that takes the form unfoldr step • prolog where step takes constant time and prolog takes linear time in the size of the input.[3][4] The standard function unfoldr is a right-associative Bird unfold.[3]


  1. ^ a b Ehrlich, G. (July 1973). "Loopless algorithms for generating permutations, combinations, and other combinatorial configuration". Journal of the ACM. New York, N.Y.: ACM. 20 (3): 500–513. doi:10.1145/321765.321781. ISSN 0004-5411. 
  2. ^ Knuth, D. (February 2005). Volume 4, Fascicle 2: Generating All Tuples and Permutations. The Art of Computer Programming. Upper Saddle River, N.J.: Addison–Wesley Professional. ISBN 0-201-85393-0. 
  3. ^ a b Bird, R. (July 2006). Loopless functional algorithms. International Conference on Mathematics of Program Construction (MPC 06). Heidelberg, Germany: Springer. doi:10.1007/11783596. 
  4. ^ Snape, J. (September 2005). Loopless Functional Algorithms. Master's thesis. Oxford, U.K.: University of Oxford. OCLC 63162239.