CODEWITHSUNDEEP
listprolog
Nicke Manarin
Nicke Manarin

Reputation: 3358

Check if list is ordered

I'm trying to compare the elements of a list of integer to see if they are ordered (or not). I'm using Amzi!

I gave a few attempts, but nothing works... :(

ordered([X]).
ordered([N|[N1|L]]) :- N <= N1, ordered([N1|L]).

ordered_([X]).
ordered_([Head,Head1|Tail]) :- Head <= Head1, ordered_([Head1|Tail]).

Both return no if this list is entered:

ordered([1,2,3,4]).
ordered_([1,2,3,4]).

I understand that I need to compare the head with the head of the tail.

Upvotes: 2

Views: 8897

Answers (3)

Kryštof Řeh&#225;ček
Kryštof Řeh&#225;ček

Reputation: 2483

More efficient alternative to most upvoted solution, comparing three instead of appending the bigger element back to the list

is_sorted([]).
is_sorted([X, Y, Z|T]) :- X =< Y, Y =< Z, is_sorted(T).
is_sorted([X, Y|T]) :- X =< Y, is_sorted(T).

Upvotes: 0

CapelliC
CapelliC

Reputation: 60004

a compact alternative:

ordered(L) :- \+ ( append(_,[A,B|_], L), A > B ).

Upvotes: 2

Nicholas Carey
Nicholas Carey

Reputation: 74197

Doesn't seem like it should be any more complex than

ordered( []      ) .
ordered( [_]     ) .
ordered( [X,Y|Z] ) :- X =< Y , ordered( [Y|Z] ) .

Arithmetic comparison predicates are covered here: http://www.amzi.com/manuals/amzi/pro/ref_math.htm#MathematicalComparisons

Use @=</2 or compare/3 to check the ordering of things in the standard order of terms: http://www.amzi.com/manuals/amzi/pro/ref_manipulating_terms.htm#StandardOrder

Upvotes: 5

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