按列对 python 数组/recarray 进行排序

Question

我有一个相当简单的问题，关于如何按给定列对整个数组/记录数组进行排序。例如，给定数组：

import numpy as np
data = np.array([[5,2], [4,1], [3,6]])

我想按要返回的第一列对数据进行排序：

array([[3,6], [4,1], [5,2]])

Answer 1

使用 data[np.argsort(data[:, 0])]，其中 0 是要排序的列索引：

In [27]: import numpy as np

In [28]: data = np.array([[5,2], [4,1], [3,6]])

In [29]: col = 0

In [30]: data=data[np.argsort(data[:,col])]
Out[30]: 
array([[3, 6],
       [4, 1],
       [5, 2]])

Answer 2

您正在寻找 operator.itemgetter

>>> from operator import itemgetter, attrgetter

>>> sorted(student_tuples, key=itemgetter(2))
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]

>>> sorted(student_objects, key=attrgetter('age'))
[('dave', 'B', 10), ('jane', 'B', 12), ('john', 'A', 15)]

即

In [7]: a
Out[7]: [[5, 2], [4, 1], [3, 6]]

In [8]: sorted(a, key=operator.itemgetter(0))
Out[8]: [[3, 6], [4, 1], [5, 2]]

Answer 3

这有点棘手：

data[data[:,0].argsort()]

# data[:,n] -- get entire column of index n
# argsort() -- get the indices that would sort it
# data[data[:,n].argsort()] -- get data array sorted by n-th column

我在这里找到了这个食谱：

链接

http://mathesaurus.sourceforge.net/matlab-numpy.html

Answer 4

要对第二列进行排序，请使用 itemgetter

>>> from operator import itemgetter
>>> data = [[5,2], [4,1], [3,6]]
>>> sorted(data)
[[3, 6], [4, 1], [5, 2]]
>>> sorted(data,key=itemgetter(1))
[[4, 1], [5, 2], [3, 6]]
>>> 

Answer 5

这是一个适用于切片的扩展：

import numpy as np
x = np.array([[9, 1, 2],
              [5, 3, 4],
              [0, 5, 6]])

按行排序：

x[:, x[1,:].argsort()] # Sort by second row

array([[1, 2, 9]
       [3, 4, 5]
       [5, 6, 0]])

按列排序：

x[x[:,0].argsort(), :] # Sort by first column

array([[0, 5, 6],
       [5, 3, 4],
       [9, 1, 2]])