numpy.sum
-
numpy.sum(a, axis=None, dtype=None, out=None, keepdims=<no value>, initial=<no value>, where=<no value>)
[source] -
Sum of array elements over a given axis.
Parameters: -
a : array_like
-
Elements to sum.
-
axis : None or int or tuple of ints, optional
-
Axis or axes along which a sum is performed. The default, axis=None, will sum all of the elements of the input array. If axis is negative it counts from the last to the first axis.
New in version 1.7.0.
If axis is a tuple of ints, a sum is performed on all of the axes specified in the tuple instead of a single axis or all the axes as before.
-
dtype : dtype, optional
-
The type of the returned array and of the accumulator in which the elements are summed. The dtype of
a
is used by default unlessa
has an integer dtype of less precision than the default platform integer. In that case, ifa
is signed then the platform integer is used while ifa
is unsigned then an unsigned integer of the same precision as the platform integer is used. -
out : ndarray, optional
-
Alternative output array in which to place the result. It must have the same shape as the expected output, but the type of the output values will be cast if necessary.
-
keepdims : bool, optional
-
If this is set to True, the axes which are reduced are left in the result as dimensions with size one. With this option, the result will broadcast correctly against the input array.
If the default value is passed, then
keepdims
will not be passed through to thesum
method of sub-classes ofndarray
, however any non-default value will be. If the sub-class’ method does not implementkeepdims
any exceptions will be raised. -
initial : scalar, optional
-
Starting value for the sum. See
reduce
for details.New in version 1.15.0.
-
where : array_like of bool, optional
-
Elements to include in the sum. See
reduce
for details.New in version 1.17.0.
Returns: -
sum_along_axis : ndarray
-
An array with the same shape as
a
, with the specified axis removed. Ifa
is a 0-d array, or ifaxis
is None, a scalar is returned. If an output array is specified, a reference toout
is returned.
See also
-
ndarray.sum
- Equivalent method.
-
add.reduce
- Equivalent functionality of
add
. -
cumsum
- Cumulative sum of array elements.
-
trapz
- Integration of array values using the composite trapezoidal rule.
Notes
Arithmetic is modular when using integer types, and no error is raised on overflow.
The sum of an empty array is the neutral element 0:
>>> np.sum([]) 0.0
For floating point numbers the numerical precision of sum (and
np.add.reduce
) is in general limited by directly adding each number individually to the result causing rounding errors in every step. However, often numpy will use a numerically better approach (partial pairwise summation) leading to improved precision in many use-cases. This improved precision is always provided when noaxis
is given. Whenaxis
is given, it will depend on which axis is summed. Technically, to provide the best speed possible, the improved precision is only used when the summation is along the fast axis in memory. Note that the exact precision may vary depending on other parameters. In contrast to NumPy, Python’smath.fsum
function uses a slower but more precise approach to summation. Especially when summing a large number of lower precision floating point numbers, such asfloat32
, numerical errors can become significant. In such cases it can be advisable to usedtype=”float64”
to use a higher precision for the output.Examples
>>> np.sum([0.5, 1.5]) 2.0 >>> np.sum([0.5, 0.7, 0.2, 1.5], dtype=np.int32) 1 >>> np.sum([[0, 1], [0, 5]]) 6 >>> np.sum([[0, 1], [0, 5]], axis=0) array([0, 6]) >>> np.sum([[0, 1], [0, 5]], axis=1) array([1, 5]) >>> np.sum([[0, 1], [np.nan, 5]], where=[False, True], axis=1) array([1., 5.])
If the accumulator is too small, overflow occurs:
>>> np.ones(128, dtype=np.int8).sum(dtype=np.int8) -128
You can also start the sum with a value other than zero:
>>> np.sum([10], initial=5) 15
-
© 2005–2019 NumPy Developers
Licensed under the 3-clause BSD License.
https://docs.scipy.org/doc/numpy-1.17.0/reference/generated/numpy.sum.html