Using a constrained variable with `length/2`

by

What’s probably more useful than a slightly less nondeterministic length/2 is a proper list-length constraint. You can find an ECLiPSe implementation of it here, called len/2. With this you get the following behaviour:

?- N :: 1..3, len(Xs, N).
N = N{1 .. 3}
Xs = [_431|_482]               % note it must contain at least one element!
There is 1 delayed goal.
Yes (0.00s cpu)

You can then enumerate the valid lists either by enumerating N:

?- N :: 1..3, len(Xs, N), indomain(N).
N = 1
Xs = [_478]
Yes (0.00s cpu, solution 1, maybe more)
N = 2
Xs = [_478, _557]
Yes (0.02s cpu, solution 2, maybe more)
N = 3
Xs = [_478, _557, _561]
Yes (0.02s cpu, solution 3)

or by generating lists with good old standard length/2:

?- N :: 1..3, len(Xs, N), length(Xs, _).
N = 1
Xs = [_488]
Yes (0.00s cpu, solution 1, maybe more)
N = 2
Xs = [_488, _555]
Yes (0.02s cpu, solution 2, maybe more)
N = 3
Xs = [_488, _555, _636]
Yes (0.02s cpu, solution 3)

More Related Contents:

  1. Most general higher-order constraint describing a sequence of integers ordered with respect to a relation
  2. reversible “binary to number” predicate
  3. Prolog map procedure that applies predicate to list elements
  4. What is the difference between ‘ and ” in Prolog?
  5. Definition of a path/trail/walk
  6. Definition of Reflexive Transitive Closure
  7. Delete vowels in a list
  8. Reification of term equality/inequality
  9. Prolog successor notation yields incomplete result and infinite loop
  10. Find powers of 2 in a list Prolog
  11. Stack overflow in Prolog DCG grammar rule: how to handle large lists efficiently or lazily
  12. Einsteins Riddle Prolog
  13. Safer type tests in Prolog
  14. Complexity of ISO Prolog predicates
  15. Faster implementation of verbal arithmetic in Prolog
  16. Explanation of a Prolog algorithm to append two lists together
  17. Meaning of instantiation mode indicators in arguments of Prolog predicates
  18. Shuffle in prolog
  19. Splitting a list of integers into a list of positive integers and a list of negative integers
  20. Collect all “minimum” solutions from a predicate
  21. Converting Terms to Atoms preserving variable names in YAP prolog
  22. Knowing when to use cut in prolog
  23. ‘if’ in prolog?
  24. Should I avoid tail recursion in Prolog and in general?
  25. Prolog append with cut operator
  26. Understanding Difference Lists
  27. Define graph in Prolog: edge and path, finding if there is a path between two vertices
  28. PROLOG: Determining if elements in list are equal if order does not matter
  29. Features of good Prolog code? [closed]
  30. GNU Prolog assert error

Leave a Comment