Merge from documentation editor.

1 files changed, 128 insertions, 2 deletions

diff --git a/numpy/doc/reference/ufuncs.py b/numpy/doc/reference/ufuncs.py
index a7f349aa9..4819e5268 100644
--- a/numpy/doc/reference/ufuncs.py
+++ b/numpy/doc/reference/ufuncs.py
@@ -1,9 +1,135 @@
 """
-
 ===================
 Universal Functions
 ===================
 
-Placeholder for ufunc documentation.
+Ufuncs are, generally speaking, mathematical functions or operations that are
+applied element-by-element to the contents of an array. That is, the result
+in each output array element only depends on the value in the corresponding
+input array (or arrays) and on no other array elements. Numpy comes with a
+large suite of ufuncs, and scipy extends that suite substantially. The simplest
+example is the addition operator: ::
+
+ >>> np.array([0,2,3,4]) + np.array([1,1,-1,2])
+ array([1, 3, 2, 6])
+
+The unfunc module lists all the available ufuncs in numpy. Additional ufuncts
+available in xxx in scipy. Documentation on the specific ufuncs may be found
+in those modules. This documentation is intended to address the more general
+aspects of unfuncs common to most of them. All of the ufuncs that make use of
+Python operators (e.g., +, -, etc.) have equivalent functions defined
+(e.g. add() for +)
+
+Type coercion
+=============
+
+What happens when a binary operator (e.g., +,-,\\*,/, etc) deals with arrays of
+two different types? What is the type of the result? Typically, the result is
+the higher of the two types. For example: ::
+
+ float32 + float64 -> float64
+ int8 + int32 -> int32
+ int16 + float32 -> float32
+ float32 + complex64 -> complex64
+
+There are some less obvious cases generally involving mixes of types
+(e.g. uints, ints and floats) where equal bit sizes for each are not
+capable of saving all the information in a different type of equivalent
+bit size. Some examples are int32 vs float32 or uint32 vs int32.
+Generally, the result is the higher type of larger size than both
+(if available). So: ::
+
+ int32 + float32 -> float64
+ uint32 + int32 -> int64
+
+Finally, the type coercion behavior when expressions involve Python
+scalars is different than that seen for arrays. Since Python has a
+limited number of types, combining a Python int with a dtype=np.int8
+array does not coerce to the higher type but instead, the type of the
+array prevails. So the rules for Python scalars combined with arrays is
+that the result will be that of the array equivalent the Python scalar
+if the Python scalar is of a higher 'kind' than the array (e.g., float
+vs. int), otherwise the resultant type will be that of the array.
+For example: ::
+
+  Python int + int8 -> int8
+  Python float + int8 -> float64
+
+ufunc methods
+=============
+
+Binary ufuncs support 4 methods. These methods are explained in detail in xxx
+(or are they, I don't see anything in the ufunc docstring that is useful?).
+
+**.reduce(arr)** applies the binary operator to elements of the array in sequence. For example: ::
+
+ >>> np.add.reduce(np.arange(10))  # adds all elements of array
+ 45
+
+For multidimensional arrays, the first dimension is reduced by default: ::
+
+ >>> np.add.reduce(np.arange(10).reshape(2,5))
+     array([ 5,  7,  9, 11, 13])
+
+The axis keyword can be used to specify different axes to reduce: ::
+
+ >>> np.add.reduce(np.arange(10).reshape(2,5),axis=1)
+ array([10, 35])
+
+**.accumulate(arr)** applies the binary operator and generates an an equivalently
+shaped array that includes the accumulated amount for each element of the
+array. A couple examples: ::
+
+ >>> np.add.accumulate(np.arange(10))
+ array([ 0,  1,  3,  6, 10, 15, 21, 28, 36, 45])
+ >>>  np.multiply.accumulate(np.arange(1,9))
+ array([    1,     2,     6,    24,   120,   720,  5040, 40320])
+
+The behavior for multidimensional arrays is the same as for .reduce(), as is the use of the axis keyword).
+
+**.reduceat(arr,indices)** allows one to apply reduce to selected parts of an array.
+It is a difficult method to understand. See the documentation at:
+
+**.outer(arr1,arr2)** generates an outer operation on the two arrays arr1 and arr2. It will work on multidimensional arrays (the shape of the result is the
+concatenation of the two input shapes.: ::
+
+ >>> np.multiply.outer(np.arange(3),np.arange(4))
+ array([[0, 0, 0, 0],
+        [0, 1, 2, 3],
+        [0, 2, 4, 6]])
+
+Output arguments
+================
+
+All ufuncs accept an optional output array. The array must be of the expected output shape. Beware that if the type of the output array is of a
+different (and lower) type than the output result, the results may be silently
+truncated or otherwise corrupted in the downcast to the lower type. This usage
+is useful when one wants to avoid creating large temporary arrays and instead
+allows one to reuse the same array memory repeatedly (at the expense of not
+being able to use more convenient operator notation in expressions). Note that
+when the output argument is used, the ufunc still returns a reference to the
+result.
+
+ >>> x = np.arange(2)
+ >>> np.add(np.arange(2),np.arange(2.),x)
+ array([0, 2])
+ >>> x
+ array([0, 2])
+
+and & or as ufuncs
+==================
+
+Invariably people try to use the python 'and' and 'or' as logical operators
+(and quite understandably). But these operators do not behave as normal
+operators since Python treats these quite differently. They cannot be
+overloaded with array equivalents. Thus using 'and' or 'or' with an array
+results in an error. There are two alternatives:
+
+ 1) use the ufunc functions logical_and() and logical_or().
+ 2) use the bitwise operators & and \\|. The drawback of these is that if
+    the arguments to these operators are not boolean arrays, the result is
+    likely incorrect. On the other hand, most usages of logical_and and
+    logical_or are with boolean arrays. As long as one is careful, this is
+    a convenient way to apply these operators.
 
 """