From: nobuyoshi nakada <nobuyoshi.nakada@...>
Date: 2005-07-11T14:29:24+09:00
Subject: Re: accessing index inside map

Hi,

At Mon, 11 Jul 2005 14:09:41 +0900,
Ryan Leavengood wrote in [ruby-talk:147729]:
> > What about Enumerator#with_index?
> > 
> >   [1,2,3,4,5,6].map.with_index {|x,i|[2,5].include?(i) ? x : x*2}
> 
> I like that, personally. In fact each_with_index could become 
> each.with_index (though we obviously need the old one for backwards 
> compatibility.)

Indeed.

  $ ./ruby -renumerator -e 'p (1..6).to_enum.with_index{|x,i|p [x,i]}'
  [1, 0]
  [2, 1]
  [3, 2]
  [4, 3]
  [5, 4]
  [6, 5]
  1..6

A patch based on the current implementation.


Index: ext/enumerator/enumerator.c
===================================================================
RCS file: /cvs/ruby/src/ruby/ext/enumerator/enumerator.c,v
retrieving revision 1.3.2.2
diff -U2 -p -r1.3.2.2 enumerator.c
--- ext/enumerator/enumerator.c	4 Nov 2004 01:20:50 -0000	1.3.2.2
+++ ext/enumerator/enumerator.c	11 Jul 2005 05:25:15 -0000
@@ -162,4 +162,27 @@ enumerator_each(obj)
 }
 
+static VALUE
+enumerator_with_index_i(val, memo)
+    VALUE val, *memo;
+{
+    val = rb_yield_values(2, val, INT2FIX(*memo));
+    ++*memo;
+    return val;
+}
+
+static VALUE
+enumerator_with_index(obj)
+    VALUE obj;
+{
+    VALUE memo = 0;
+
+    obj = (VALUE)rb_node_newnode(NODE_MEMO,
+				 rb_ivar_get(obj, id_enum_obj),
+				 rb_to_id(rb_ivar_get(obj, id_enum_method)),
+				 rb_ivar_get(obj, id_enum_args));
+    return rb_iterate((VALUE (*)_((VALUE)))enumerator_iter, obj,
+		      enumerator_with_index_i, (VALUE)&memo);
+}
+
 void
 Init_enumerator()
@@ -183,4 +206,5 @@ Init_enumerator()
     rb_define_method(rb_cEnumerator, "initialize", enumerator_initialize, -1);
     rb_define_method(rb_cEnumerator, "each", enumerator_each, 0);
+    rb_define_method(rb_cEnumerator, "with_index", enumerator_with_index, 0);
 
     sym_each		= ID2SYM(rb_intern("each"));


-- 
Nobu Nakada