diff --git a/CHANGES b/CHANGES
index e70bc97..03a125c 100644
--- a/CHANGES
+++ b/CHANGES
@@ -19,9 +19,14 @@
 
 - added :meth:`jinja2.environment.TemplateStream.dump`.
 
-- added support for implicit string literal concatenation.
+- added missing support for implicit string literal concatenation.
   ``{{ "foo" "bar" }}`` is equivalent to ``{{ "foobar" }}``
 
+- `else` is optional for conditional expressions.  If not given it evaluates
+  to `false`.
+
+- improved error reporting for undefined values by providing a position.
+
 Version 2.0rc1
 --------------
 (no codename, released on June 9th 2008)
diff --git a/jinja2/compiler.py b/jinja2/compiler.py
index 3edc5f8..a90526c 100644
--- a/jinja2/compiler.py
+++ b/jinja2/compiler.py
@@ -622,6 +622,13 @@
             bool(frame.accesses_caller)
         ))
 
+    def position(self, node):
+        """Return a human readable position for the node."""
+        rv = 'line %d' % node.lineno
+        if self.name is not None:
+            rv += ' in' + repr(self.name)
+        return rv
+
     # -- Statement Visitors
 
     def visit_Template(self, node, frame=None):
@@ -835,8 +842,11 @@
             self.writeline('l_%s = environment.undefined(%r %% '
                            'included_template.__name__, '
                            'name=%r)' %
-                           (alias, 'the template %r does not export '
-                            'the requested name ' + repr(name), name))
+                           (alias, 'the template %%r (imported on %s) does '
+                           'not export the requested name %s' % (
+                                self.position(node),
+                                repr(name)
+                           ), name))
             self.outdent()
             if frame.toplevel:
                 var_names.append(alias)
@@ -908,10 +918,11 @@
         if 'loop' not in aliases and 'loop' in find_undeclared(
            node.iter_child_nodes(only=('else_', 'test')), ('loop',)):
             self.writeline("l_loop = environment.undefined(%r, name='loop')" %
-                "'loop' is undefined. the filter section of a loop as well " \
-                "as the else block doesn't have access to the special 'loop' "
-                "variable of the current loop.  Because there is no parent "
-                "loop it's undefined.")
+                ("'loop' is undefined. the filter section of a loop as well "
+                 "as the else block doesn't have access to the special 'loop'"
+                 " variable of the current loop.  Because there is no parent "
+                 "loop it's undefined.  Happened in loop on %s" %
+                 self.position(node)))
 
         self.writeline('for ', node)
         self.visit(node.target, loop_frame)
@@ -1330,13 +1341,20 @@
         self.write(')')
 
     def visit_CondExpr(self, node, frame):
+        def write_expr2():
+            if node.expr2 is not None:
+                return self.visit(node.expr2, frame)
+            self.write('environment.undefined(%r)' % ('the inline if-'
+                       'expression on %s evaluated to false and '
+                       'no else section was defined.' % self.position(node)))
+
         if not have_condexpr:
             self.write('((')
             self.visit(node.test, frame)
             self.write(') and (')
             self.visit(node.expr1, frame)
             self.write(',) or (')
-            self.visit(node.expr2, frame)
+            write_expr2()
             self.write(',))[0]')
         else:
             self.write('(')
@@ -1344,7 +1362,7 @@
             self.write(' if ')
             self.visit(node.test, frame)
             self.write(' else ')
-            self.visit(node.expr2, frame)
+            write_expr2()
             self.write(')')
 
     def visit_Call(self, node, frame, forward_caller=False):
diff --git a/jinja2/nodes.py b/jinja2/nodes.py
index 56daae4..effa6d4 100644
--- a/jinja2/nodes.py
+++ b/jinja2/nodes.py
@@ -499,6 +499,11 @@
     def as_const(self):
         if self.test.as_const():
             return self.expr1.as_const()
+
+        # if we evaluate to an undefined object, we better do that at runtime
+        if self.expr2 is None:
+            raise Impossible()
+
         return self.expr2.as_const()
 
 
diff --git a/jinja2/parser.py b/jinja2/parser.py
index 1e43ed8..d365d4c 100644
--- a/jinja2/parser.py
+++ b/jinja2/parser.py
@@ -307,8 +307,10 @@
         expr1 = self.parse_or()
         while self.stream.skip_if('name:if'):
             expr2 = self.parse_or()
-            self.stream.expect('name:else')
-            expr3 = self.parse_condexpr()
+            if self.stream.skip_if('name:else'):
+                expr3 = self.parse_condexpr()
+            else:
+                expr3 = None
             expr1 = nodes.CondExpr(expr2, expr1, expr3, lineno=lineno)
             lineno = self.stream.current.lineno
         return expr1
diff --git a/jinja2/runtime.py b/jinja2/runtime.py
index 0417f61..d6c5553 100644
--- a/jinja2/runtime.py
+++ b/jinja2/runtime.py
@@ -197,14 +197,19 @@
 class LoopContext(object):
     """A loop context for dynamic iteration."""
 
-    def __init__(self, iterable, enforce_length=False, recurse=None):
-        self._iterable = iterable
-        self._next = iter(iterable).next
-        self._length = None
+    def __init__(self, iterable, recurse=None):
+        self._iterator = iter(iterable)
         self._recurse = recurse
         self.index0 = -1
-        if enforce_length:
-            len(self)
+
+        # try to get the length of the iterable early.  This must be done
+        # here because there are some broken iterators around where there
+        # __len__ is the number of iterations left (i'm looking at your
+        # listreverseiterator!).
+        try:
+            self._length = len(iterable)
+        except (TypeError, AttributeError):
+            self._length = None
 
     def cycle(self, *args):
         """Cycles among the arguments with the current loop index."""
@@ -213,7 +218,7 @@
         return args[self.index0 % len(args)]
 
     first = property(lambda x: x.index0 == 0)
-    last = property(lambda x: x.revindex0 == 0)
+    last = property(lambda x: x.index0 + 1 == x.length)
     index = property(lambda x: x.index0 + 1)
     revindex = property(lambda x: x.length - x.index0)
     revindex0 = property(lambda x: x.length - x.index)
@@ -237,18 +242,13 @@
     @property
     def length(self):
         if self._length is None:
-            try:
-                # first try to get the length from the iterable (if the
-                # iterable is a sequence)
-                length = len(self._iterable)
-            except TypeError:
-                # if that's not possible (ie: iterating over a generator)
-                # we have to convert the iterable into a sequence and
-                # use the length of that.
-                self._iterable = tuple(self._iterable)
-                self._next = iter(self._iterable).next
-                length = len(tuple(self._iterable)) + self.index0 + 1
-            self._length = length
+            # if was not possible to get the length of the iterator when
+            # the loop context was created (ie: iterating over a generator)
+            # we have to convert the iterable into a sequence and use the
+            # length of that.
+            iterable = tuple(self._iterator)
+            self._iterator = iter(iterable)
+            self._length = len(iterable) + self.index0 + 1
         return self._length
 
     def __repr__(self):
@@ -272,7 +272,7 @@
     def next(self):
         ctx = self.context
         ctx.index0 += 1
-        return ctx._next(), ctx
+        return ctx._iterator.next(), ctx
 
 
 class Macro(object):
diff --git a/tests/test_forloop.py b/tests/test_forloop.py
index b8d3ae6..f7e4d68 100644
--- a/tests/test_forloop.py
+++ b/tests/test_forloop.py
@@ -116,6 +116,12 @@
     assert tmpl.render(table=['ab', 'cd']) == '[1|1][1|2][2|1][2|2]'
 
 
+def test_reversed_bug(env):
+    tmpl = env.from_string('{% for i in items %}{{ i }}{% if not loop.last %}'
+                           ',{% endif %}{% endfor %}')
+    assert tmpl.render(items=reversed([3, 2, 1])) == '1,2,3'
+
+
 def test_loop_errors(env):
     tmpl = env.from_string(LOOPERROR1)
     raises(UndefinedError, tmpl.render)
diff --git a/tests/test_syntax.py b/tests/test_syntax.py
index 4d28dbc..29b3974 100644
--- a/tests/test_syntax.py
+++ b/tests/test_syntax.py
@@ -8,7 +8,7 @@
 """
 from py.test import raises
 from jinja2 import Environment, DictLoader
-from jinja2.exceptions import TemplateSyntaxError
+from jinja2.exceptions import TemplateSyntaxError, UndefinedError
 
 
 CALL = '''{{ foo('a', c='d', e='f', *['b'], **{'g': 'h'}) }}'''
@@ -124,6 +124,14 @@
     assert tmpl.render() == '0'
 
 
+def test_short_conditional_expression(env):
+    tmpl = env.from_string('<{{ 1 if false }}>')
+    assert tmpl.render() == '<>'
+
+    tmpl = env.from_string('<{{ (1 if false).bar }}>')
+    raises(UndefinedError, tmpl.render)
+
+
 def test_filter_priority(env):
     tmpl = env.from_string(FILTERPRIORITY)
     assert tmpl.render() == 'FOOBAR'