It is common to use a so-called “lagged argument” to benefit from first-argument indexing: list_min([L|Ls], Min) :- list_min(Ls, L, Min). list_min([], Min, Min). list_min([L|Ls], Min0, Min) :- Min1 is min(L, Min0), list_min(Ls, Min1, Min). This pattern is called a fold (from the left), and foldl/4, which is available in recent SWI versions, lets you write … Read more
iso_dif/2 is much simpler to implement than a comparison: The built-in \= operator is available You now exactly what arguments to provide to\= Definition Based on your comments, the safe comparison means that the order won’t change if variables in both subterms are instanciated. If we name the comparison lt, we have for example: lt(a(X), … Read more
Use assertz/1 or asserta/1 instead. GNU-Prolog does not provide assert/1 because only asserta/1 and assertz/1 are defined in the standard. Note that while asserta/1 always had one clear interpretation meaning add the clause at the beginning, the meaning of assertz/1 was more difficult to resolve since “add a clause at the end” does not completely … Read more
SWI-Prolog has several different representation of what you might call “strings”. List of character codes (Unicode); List of chars (one-letter atoms); Strings, which are “atomic” objects, and can be manipulated only with the built-in predicates for strings; And finally, of course, atoms. You should read the documentation, but for now, you have at least two … Read more
Ad 1: It is impossible to define reverse/2 as a (directly edit thx to @repeat: tail) recursive predicate – unless you permit an auxiliary predicate. Ad 2: palindrome(X) :- reverse(X,X). But the easiest way is to define such predicates with DCGs: iseq([]) –> []. iseq([E|Es]) –> iseq(Es), [E]. reverse(Xs, Ys) :- phrase(iseq(Xs), Ys). palindrome(Xs) :- … Read more
The major difficulty in writing good Prolog code lies in not only understanding but also adequately transmitting the intention or purpose of a program. In contrast to other programming languages there are several quite different kinds of Prolog code often within the same program. By confusing such levels bugs and problems ensue: Pure, monotonic code. … Read more
As a starting point, let’s take the second implementation of equal_elements/2 by @CapelliC: equal_elements([], []). equal_elements([X|Xs], Ys) :- select(X, Ys, Zs), equal_elements(Xs, Zs). Above implementation leaves useless choicepoints for queries like this one: ?- equal_elements([1,2,3],[3,2,1]). true ; % succeeds, but leaves choicepoint false. What could we do? We could fix the efficiency issue by using … Read more
Question 1: Prolog systems are not always able to decide whether or not a clause will apply prior to executing it. The precise circumstances are implementation dependent. That is, you cannot rely on that decision in general. Systems do improve here from release to release. Consider as the simplest case: ?- X = 1 ; … Read more
bet(N, M, K) :- N =< M, K = N. bet(N, M, K) :- N < M, N1 is N+1, bet(N1, M, K). In action: $ swipl ?- [bet]. % bet compiled 0.00 sec, 1,064 bytes true. ?- bet(1,5, K). K = 1 n K = 2 n K = 3 n K = 4 … Read more
Cycles in the graph. You need to track what nodes you’re visiting, and check them. Try this, using a helper predicate with an accumulator to track the visited nodes: path(A,B) :- % two nodes are connected, if walk(A,B,[]) % – if we can walk from one to the other, . % first seeding the visited … Read more