numpy.partition

numpy.partition(a, kth, axis=-1, kind='introselect', order=None)

Return a partitioned copy of an array.

Creates a copy of the array with its elements rearranged in such a way that the value of the element in kth position is in the position it would be in a sorted array. All elements smaller than the kth element are moved before this element and all equal or greater are moved behind it. The ordering of the elements in the two partitions is undefined.

New in version 1.8.0.

Parameters:

a : array_like

Array to be sorted.

kth : int or sequence of ints

Element index to partition by. The kth value of the element will be in its final sorted position and all smaller elements will be moved before it and all equal or greater elements behind it. The order all elements in the partitions is undefined. If provided with a sequence of kth it will partition all elements indexed by kth of them into their sorted position at once.

axis : int or None, optional

Axis along which to sort. If None, the array is flattened before sorting. The default is -1, which sorts along the last axis.

kind : {‘introselect’}, optional

Selection algorithm. Default is ‘introselect’.

order : list, optional

When a is a structured array, this argument specifies which fields to compare first, second, and so on. This list does not need to include all of the fields.

Returns:

partitioned_array : ndarray

Array of the same type and shape as a.

See also

ndarray.partition

Method to sort an array in-place.

argpartition

Indirect partition.

sort

Full sorting

Notes

The various selection 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 available algorithms have the following properties:

kind	speed	worst case	work space	stable
‘introselect’	1	O(n)	0	no

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

Examples

>>> a = np.array([3, 4, 2, 1])
>>> np.partition(a, 3)
array([2, 1, 3, 4])

>>> np.partition(a, (1, 3))
array([1, 2, 3, 4])