Fix nth/2 to emit empty on index out of range

docs/content/manual/manual.yml

@@ -2832,10 +2832,8 @@ sections:
           The `first(expr)` and `last(expr)` functions extract the first
           and last values from `expr`, respectively.
 
-          The `nth(n; expr)` function extracts the nth value output by
-          `expr`.  This can be defined as `def nth(n; expr):
-          last(limit(n + 1; expr));`.  Note that `nth(n; expr)` doesn't
-          support negative values of `n`.
+          The `nth(n; expr)` function extracts the nth value output by `expr`.
+          Note that `nth(n; expr)` doesn't support negative values of `n`.
 
         examples:
           - program: '[first(range(.)), last(range(.)), nth(./2; range(.))]'

src/builtin.jq

@@ -163,7 +163,9 @@ def any(condition): any(.[]; condition);
 def all: all(.[]; .);
 def any: any(.[]; .);
 def last(g): reduce g as $item (null; $item);
-def nth($n; g): if $n < 0 then error("nth doesn't support negative indices") else last(limit($n + 1; g)) end;
+def nth($n; g):
+  if $n < 0 then error("nth doesn't support negative indices")
+  else label $out | foreach g as $item ($n + 1; . - 1; if . <= 0 then $item, break $out else empty end) end;
 def first: .[0];
 def last: .[-1];
 def nth($n): .[$n];

tests/jq.test

@@ -319,9 +319,13 @@ null
 "badness"
 [1]
 
-[first(range(.)), last(range(.)), nth(0; range(.)), nth(5; range(.)), try nth(-1; range(.)) catch .]
+[first(range(.)), last(range(.))]
 10
-[0,9,0,5,"nth doesn't support negative indices"]
+[0,9]
+
+[nth(0,5,9,10,15; range(.)), try nth(-1; range(.)) catch .]
+10
+[0,5,9,"nth doesn't support negative indices"]
 
 # Check that first(g) does not extract more than one value from g
 first(1,error("foo"))