a_func_a_day_in_python_numpy.argsort函数

numpy.argsort函数

标签： a_func_a_day_in_python

1. argsort函数简介

numpy.argsort(a, axis=-1, kind='quicksort', order=None)

返回按照数组值升序排序(默认)后的原始数组索引值.

Returns the indices that would sort an array.

Perform an indirect sort along the given axis using the algorithm specified by the kind keyword.

It returns an array of indices of the same shape as a that index data along the given axis in sorted order.

Parameters说明：

a : array_like，Array to sort.

axis : int or None, optional. Axis along which to sort. The default is -1 (the last axis). If None, the flattened array is used.

kind：{‘quicksort’, ‘mergesort’, ‘heapsort’}, optional. Sorting algorithm.

order : str or list of str, optional. When a is an array 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.

Returns说明：

index_array : ndarray, int. Array of indices that sort a along the specified axis. If a is one-dimensional, a[index_array] yields a sorted a.(参见进阶)

代码：

import numpy as np

x = np.array([3, 1, 2])
print "np.argsort(x): "
print np.argsort(x)

y = np.array([[0, 3], [2, 2]])
print "y: "
print y

print "np.argsort(y, axis=0): "
print np.argsort(y, axis=0)   # by axis 0, column

print "np.argsort(y, axis=1): "
print np.argsort(y, axis=1)   # by axis 1, row

# Sorting with keys:

z = np.array([(1, 0), (0, 1)], dtype=[('x', '<i4'), ('y', '<i4')])
print z

print "np.argsort(z, order=('x','y')): "
print np.argsort(z, order=('x','y'))

print ("np.argsort(z, order=('y','x')): "
print np.argsort(z, order=('y','x'))

输出：

np.argsort(x):
[1 2 0]
y:
[[0 3]
[2 2]]
np.argsort(y, axis=0):
[[0 1]
[1 0]]
np.argsort(y, axis=1):
[[0 1]
[0 1]]
[(1, 0) (0, 1)]
np.argsort(z, order=('x','y')):
[1 0]
np.argsort(z, order=('y','x')):
[0 1]

2. argsort函数简介之进阶

主要介绍如何使用argsort函数对数据进行排序.

import numpy as np

x = np.array([3, 1, 2])

print np.argsort(x)       # ascend
print np.argsort(-x)      # descend
print x[np.argsort(x)]    # get a sorted ascend 1d array by argsort
print x[np.argsort(-x)]   # get a sorted descend 1d array by argsort

输出：

[1 2 0]
[0 2 1]
[1 2 3]
[3 2 1]

另一种方式实现按降序排序：

import numpy as np

x = np.array([3, 1, 2])

a = x[np.argsort(x)]
print a
print a[::-1]

输出：

[1, 2, 3]
[3, 2, 1]

以上源码个人git链接：

https://github.com/humengdoudou/a_func_a_day_in_python/blob/master/test_argsort_20180402.py