Ticket #11657: trac_11657-zero-vector-speedup.patch

sage/modules/free_module_element.pyx

@@ -261 +261 @@
         (0, 0, 0, 0)
         sage: w.parent()
         Vector space of dimension 4 over Finite Field of size 7
+        
+    If you want a zero vector the fastest method is to call the
+    :meth:`~sage.modules.free_module.FreeModule_generic.zero_vector` 
+    method directly on a free module or vector space, as this 
+    constructor must do some type-checking.  Almost as fast as 
+    the ``zero_vector()`` method is the :func:`~sage.modules.free_module_element.zero_vector` 
+    constructor, which defaults to the integers. ::
+    
+        sage: (ZZ^5).zero_vector()
+        (0, 0, 0, 0, 0)
+        sage: zero_vector(ZZ, 5)
+        (0, 0, 0, 0, 0)
+        sage: z = zero_vector(5); z
+        (0, 0, 0, 0, 0)
+        sage: z.parent()
+        Ambient free module of rank 5 over
+        the principal ideal domain Integer Ring
 
     Here we illustrate the creation of sparse vectors by using a
     dictionary. ::
@@ -289 +306 @@
         Traceback (most recent call last):
         ...
         ValueError: cannot specify the degree of a vector as a negative integer (-4)
-
+        
+    It is an error to create a zero vector but not provide
+    a ring as the first argument.  ::
+    
+        sage: vector('junk', 20)
+        Traceback (most recent call last):
+        ...
+        TypeError: first argument must be base ring of zero vector, not junk
+        
     And it is an error to specify an index in a dictionary
     that is greater than or equal to a requested degree. ::
 
@@ -387 +412 @@
             if not maxindex < arg1:
                 raise ValueError("dictionary of entries has a key (index) exceeding the requested degree")
         # arg1 is now a legitimate degree
-        # replace it with a zero list or a dictionary, or a size-checked iterable
-        # so then down to just two arguments
+        # With no arg2, we can try to return a zero vector
+        #   else we size-check arg2 and slide it into arg1
         degree = arg1
         if arg2 is None:
-            arg1 = [0]*degree
+            if not is_Ring(arg0):
+                msg = "first argument must be base ring of zero vector, not {0}"
+                raise TypeError(msg.format(arg0))
+            return (arg0**degree).zero_vector()
         else:
             if not isinstance(arg2, dict) and len(arg2) != degree:
                 raise ValueError, "incompatible degrees in vector constructor"
@@ -596 +624 @@
         sage: zero_vector(-3)
         Traceback (most recent call last):
         ...
-        ValueError: cannot specify the degree of a vector as a negative integer (-3)
+        ValueError: degree of zero vector must be non-negative, not -3
 
     Garbage instead of a ring will be recognized as such. ::
 
@@ -611 +639 @@
         arg0 = sage.rings.integer_ring.IntegerRing()
     if not (sage.rings.integer.is_Integer(arg1) or isinstance(arg1,(int,long))):
         raise ValueError("constructing a zero vector requires the degree as an integer, not %s" % arg1)
+    if arg1 < 0:
+        msg = "degree of zero vector must be non-negative, not {0}"
+        raise ValueError(msg.format(arg1))
     if not is_Ring(arg0):
         raise ValueError("elements of a zero vector belong in a ring, not %s" % arg0)
-    return vector(arg0, arg1)
+    return (arg0**arg1).zero_vector()
 
 def random_vector(ring, degree=None, *args, **kwds):
     r"""