Question

I’m trying to compare two lists by length and set the output to true or false.

min(List1, List2, output) :-
     length(List1, N),
     length(List2, M),
       output is N<M.

But I keep getting errors, what’s the syntax for the lists?

Answer 1

A couple of problems here; @Enigmativity is right in that you need to make Output variable, but is/2 isn’t defined over the operator < (it’s used to evaluate arithmetic expressions such as +, or those that are user defined).

Instead, consider the following:

min(List1, List2, Output) :-
    length(List1, N),
    length(List2, M),
    (N < M ->
        Output = 'true'
    ;   Output = 'false'
    ).

Here, N < M is a logical test which either succeeds or fails. If N < M is true, implication -> directs the interpreter to bind the Output variable to the atom 'true', else to 'false', indicating the length relationship between the lists that you’ve asked for. You can bind anything here as you like, not just atoms.

Executing this gives:

?- min([1,2,3,4],[1,2],Output).
Output = false.

If you want min/3 to simply return the smaller of the two input lists, you can try:

min(List1, List2, Output) :-
    length(List1, N),
    length(List2, M),
    (N < M ->
        Output = List1
    ;   Output = List2
    ).

Executing this on the same example gives:

?- min([1,2,3,4],[1,2],X).
X = [1, 2].