【Python】numpy中argsort函数的使用
1、概述
argsort()函数在模块numpy.core.fromnumeric中。
函数形式是：
help (numpy.argsort)
Help on function argsort in module numpy.core.fromnumeric:
argsort(a, axis=-1, kind='quicksort', order=None)
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
----------
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
-------
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`.argsort()函数返回的是数组值从小到大的索引值。
排序后会产生一个新的数组，不改变原有的数组。
2、使用
一维数组
a = array([1,4,9,3,2,3,8])
b = a.argsort()
print(a)
print(b)
output:
[1 4 9 3 2 3 8]
[0 4 3 5 1 6 2]x = array([[0, 3], [1, 2]])
print(x)
# [[0 3]
# [1 2]]
b = argsort(x) #默认按最后一维（列）排序
print(b)
# [[0 1]
# [0 1]]
c = argsort(x, axis=0) #按行排序
print(c)
# [[0 1]
# [1 0]]
d = argsort(x, axis=1) #按列排序
print(d)
# [[0 1]
# [0 1]]其他使用示例：
x = array([1,3,2])
print(x)
# [1 3 2]
b = argsort(x) #按升序排列
print(b)
#[0 2 1]
c = argsort(-x) #按降序排列
print(c)
# [1 2 0]
d = x[argsort(x)] #排序后的数组
print(d)
# [1 2 3]3、argsort()[num]使用
z = argsort(x)[num] 与 y = arfsort(x) z = y[sum]结果相同
x = array([1,3,2])
y = argsort(x)
print(y[1])
# 2
z = argsort(x)[1]
print(z)
# 2y = argsort(x)
print(y[-1])
# 1
z = argsort(x)[-1]
print(z)
# 1