mars.tensor.sort#

mars.tensor.sort(a, axis=-1, kind=None, parallel_kind=None, psrs_kinds=None, order=None, return_index=False, **kw)[source]#

Return a sorted copy of a tensor.

Parameters

  • a (array_like) – Tensor to be sorted.

  • axis (int or None, optional) – Axis along which to sort. If None, the tensor is flattened before sorting. The default is -1, which sorts along the last axis.

  • kind ({'quicksort', 'mergesort', 'heapsort', 'stable'}, optional) – Sorting algorithm. The default is ‘quicksort’. Note that both ‘stable’ and ‘mergesort’ use timsort or radix sort under the covers and, in general, the actual implementation will vary with data type. The ‘mergesort’ option is retained for backwards compatibility. Note that this argument would not take effect if a has more than 1 chunk on the sorting axis.

  • parallel_kind ({'PSRS'}, optional) – Parallel sorting algorithm, for the details, refer to: http://csweb.cs.wfu.edu/bigiron/LittleFE-PSRS/build/html/PSRSalgorithm.html

  • psrs_kinds (list with 3 elements, optional) – Sorting algorithms during PSRS algorithm.

  • order (str or list of str, optional) – When a is a tensor with fields defined, this argument specifies which fields to compare first, second, etc. A single field can be specified as a string, and not all fields need be specified, but unspecified fields will still be used, in the order in which they come up in the dtype, to break ties.

  • return_index (bool) – Return indices as well if True.

Returns

sorted_tensor – Tensor of the same type and shape as a.

Return type

Tensor

See also

Tensor.sort

Method to sort a tensor in-place.

argsort

Indirect sort.

lexsort

Indirect stable sort on multiple keys.

searchsorted

Find elements in a sorted tensor.

partition

Partial sort.

Notes

The various sorting algorithms are characterized by their average speed, worst case performance, work space size, and whether they are stable. A stable sort keeps items with the same key in the same relative order. The four algorithms implemented in NumPy have the following properties:

kind

speed

worst case

work space

stable

‘quicksort’

1

O(n^2)

0

no

‘heapsort’

3

O(n*log(n))

0

no

‘mergesort’

2

O(n*log(n))

~n/2

yes

‘timsort’

2

O(n*log(n))

~n/2

yes

Note

The datatype determines which of ‘mergesort’ or ‘timsort’ is actually used, even if ‘mergesort’ is specified. User selection at a finer scale is not currently available.

All the sort algorithms make temporary copies of the data when sorting along any but the last axis. Consequently, sorting along the last axis is faster and uses less space than sorting along any other axis.

The sort order for complex numbers is lexicographic. If both the real and imaginary parts are non-nan then the order is determined by the real parts except when they are equal, in which case the order is determined by the imaginary parts.

quicksort has been changed to an introsort which will switch heapsort when it does not make enough progress. This makes its worst case O(n*log(n)).

‘stable’ automatically choses the best stable sorting algorithm for the data type being sorted. It, along with ‘mergesort’ is currently mapped to timsort or radix sort depending on the data type. API forward compatibility currently limits the ability to select the implementation and it is hardwired for the different data types.

Timsort is added for better performance on already or nearly sorted data. On random data timsort is almost identical to mergesort. It is now used for stable sort while quicksort is still the default sort if none is chosen. For details of timsort, refer to CPython listsort.txt. ‘mergesort’ and ‘stable’ are mapped to radix sort for integer data types. Radix sort is an O(n) sort instead of O(n log n).

Examples

>>> import mars.tensor as mt
>>> a = mt.array([[1,4],[3,1]])
>>> mt.sort(a).execute()                # sort along the last axis
array([[1, 4],
       [1, 3]])
>>> mt.sort(a, axis=None).execute()     # sort the flattened tensor
array([1, 1, 3, 4])
>>> mt.sort(a, axis=0).execute()        # sort along the first axis
array([[1, 1],
       [3, 4]])

Use the order keyword to specify a field to use when sorting a structured array:

>>> dtype = [('name', 'S10'), ('height', float), ('age', int)]
>>> values = [('Arthur', 1.8, 41), ('Lancelot', 1.9, 38),
...           ('Galahad', 1.7, 38)]
>>> a = mt.array(values, dtype=dtype)       # create a structured tensor
>>> mt.sort(a, order='height').execute()                
array([('Galahad', 1.7, 38), ('Arthur', 1.8, 41),
       ('Lancelot', 1.8999999999999999, 38)],
      dtype=[('name', '|S10'), ('height', '<f8'), ('age', '<i4')])

Sort by age, then height if ages are equal:

>>> mt.sort(a, order=['age', 'height']).execute()       
array([('Galahad', 1.7, 38), ('Lancelot', 1.8999999999999999, 38),
       ('Arthur', 1.8, 41)],
      dtype=[('name', '|S10'), ('height', '<f8'), ('age', '<i4')])