Merge pull request #174 from ichiban/nth0

add nth0/3 and rename nth/3 -> nth1/3
ichiban · Mar 19, 2022 · 08fb802 · 08fb802
2 parents 830c647 + 2d0ae53
commit 08fb802
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diff --git a/README.md b/README.md
@@ -224,7 +224,8 @@ for sols.Next() {
 | List Processing      | `append(List1, List2, List3)`                    |      | Succeeds if `List3` is the concatination of `List1` and `List2`.                                                                                                                                                | Prolog                                                                                   |
 |                      | `member(Elem, List)`                             |      | Succeeds if `Elem` is a member of `List`.                                                                                                                                                                       | Prolog                                                                                   |
 |                      | `length(List, Length)`                           |      | Succeeds if `Length` is the length of `List`.                                                                                                                                                                   | Prolog                                                                                   |
-|                      | `nth(N, List, Elem)`                             |      | Succeeds if `Elem` is the `N`-th element of `List`.                                                                                                                                                             | Prolog                                                                                   |
+|                      | `nth0(N, List, Elem)`                            |      | Succeeds if `Elem` is the `N`-th element of `List`, counting from 0.                                                                                                                                            | Prolog                                                                                   |
+|                      | `nth1(N, List, Elem)`                            |      | Succeeds if `Elem` is the `N`-th element of `List`, counting from 1.                                                                                                                                            | Prolog                                                                                   |
 | Term Expansion       | `expand_term(In, Out)`                           |      | Unifies `Out` with an expanded term for `In`.                                                                                                                                                                   | [Go](https://pkg.go.dev/github.com/ichiban/prolog/engine#State.ExpandTerm)               |
 | Environment Variable | `environ(Key, Value)`                            |      | Succeeds if an environment variable `Key` has a value `Value`.                                                                                                                                                  | [Go](https://pkg.go.dev/github.com/ichiban/prolog/engine#Environ)                        |
 | DCG                  | `phrase(GRBody, S0, S)`                          |      | Succeeds if a different list `S0-S` satisfies the grammar rule `GRBody`.                                                                                                                                        | Go                                                                                       |

diff --git a/bootstrap.pl b/bootstrap.pl
@@ -238,12 +238,19 @@
 length([], 0).
 length([_|Xs], N) :- length(Xs, L), N is L + 1.
 
-:- built_in(nth/3).
-nth(1, [Elem|_], Elem) :- !.
-nth(N, [_|Rest], Elem) :-
+:- built_in(nth0/3).
+nth0(0, [Elem|_], Elem) :- !.
+nth0(N, [_|Rest], Elem) :-
+  N > 0,
+  M is N - 1,
+  nth0(M, Rest, Elem).
+
+:- built_in(nth1/3).
+nth1(1, [Elem|_], Elem) :- !.
+nth1(N, [_|Rest], Elem) :-
   N > 1,
   M is N - 1,
-  nth(M, Rest, Elem).
+  nth1(M, Rest, Elem).
 
 :- built_in(maplist/2).
 maplist(_Cont, []).