Question

I was trying to write predicate divide(L,Len,Slist) which will be true when Slist can unify with a List of length Len allocated from List L. for example

divide([1,2,3,4,5,6,7],3,Slist).

Should give such answers

Slist=[1,2,3];
Slist=[2,3,4];
Slist=[3,4,5];
Slist=[4,5,6];
Slist=[5,6,7];

But i couldn’t find a better way then length(X,Len), sublist(L,X). but it does work too slow.
How should look divide predicate?

Answer 1

sublist/2 doesn’t seems to work as expected:

?- [library(dialect/sicstus/lists)].
% library(dialect/sicstus/lists) compiled into sicstus_lists 0,00 sec, 14 clauses
true.

?- L=[1,2,3,4,5,6], length(T, 3),sublist(T,L).
L = [1, 2, 3, 4, 5, 6],
T = [1, 2, 3] ;
L = [1, 2, 3, 4, 5, 6],
T = [1, 2, 4] ;
L = [1, 2, 3, 4, 5, 6],
T = [1, 2, 5] ;
....

You could use append/3 instead:

?- L=[1,2,3,4,5,6], length(T, 3), append(_, Q, L), append(T, _, Q).
L = [1, 2, 3, 4, 5, 6],
T = [1, 2, 3],
Q = [1, 2, 3, 4, 5, 6] ;
L = [1, 2, 3, 4, 5, 6],
T = [2, 3, 4],
Q = [2, 3, 4, 5, 6] ;
L = [1, 2, 3, 4, 5, 6],
T = [3, 4, 5],
Q = [3, 4, 5, 6] ;
L = [1, 2, 3, 4, 5, 6],
T = Q, Q = [4, 5, 6] ;
false.

I don’t think it’s very fast, just essential…