Returning a true value instead of integer value in prolog -

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i have simple function below takes 2 lists ( same size) , variable stores result.

my intention compare first list's head second 1 , increase result one, , return it.

but instead true / false.

myfunction( [], [], 0). myfunction([h1|t1], [h2|t2], n) :-          h1 > h2 -> myfunction(t1, t2, n1 + 1); myfunction(t1, t2, n1), n n1 .

you're treating prolog imperative/procedural language, doesn't work way. should read through prolog tutorials and/or books.

in prolog, define predicate defines relation. when define predicate variables, saying that, these variables related in such , such way if.... if prolog succeeds in verifying relation true, predicate response "true" or "yes". otherwise, responds "false" or "no".

here's rework of you're trying do:

my_predicate([], [], 0).

this relation "base case" , says the count of cases corresponding values in first list greater second when lists empty 0.

my_predicate([h1|t1], [h2|t2], n) :-     my_predicate(t1, t2, n1),     (  h1 > h2     -> n n1 + 1     ;  n = n1     ).

this relation says n count of cases corresponding values in first list greater second if n1 count of cases tails, , n n1 + 1 if current head greater, otherwise n n1.

if want make tail recursive efficiency, can use accumulator:

my_predicate(l1, l2, n) :-     my_predicate(l1, l2, 0, n).  my_predicate([], [], n, n). my_predicate([h1|t1], [h2|t2], a, n) :-     (  h1 > h2     -> a1 + 1     ;  a1 =     ),     my_predicate(t1, t2, a1, n).

note above definition of my_predicate/3 assumes want failure if lists different length. if don't want failure in cases, need redefine base case(s).


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