Adding nthcdr as accessor.

* eval.c (eval_init): Register nthcdr function.

* lib.c (nthcdr): New function.

* lib.h (nthcdr): Declared.

* share/txr/stdlib/place.tl (nthcdr): New defplace.

* txr.1: Documented.

1 files changed, 13 insertions, 0 deletions

diff --git a/lib.c b/lib.c
index 4d0c901b..6a662fbc 100644
--- a/lib.c
+++ b/lib.c
@@ -498,6 +498,19 @@ val last(val list)
   return nullocp(p) ? list : deref(p);
 }
 
+val nthcdr(val pos, val list)
+{
+  cnum n = c_num(pos);
+
+  if (n < 0)
+    uw_throwf(error_s, lit("nthcdr: negative index ~s given"), pos, nao);
+
+  while (n-- > 0)
+    list = cdr(list);
+
+  return list;
+}
+
 loc ltail(loc cons)
 {
   while (cdr(deref(cons)))