【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]

二维数组

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)
# 2

y = argsort(x)
print(y[-1])
# 1

z = argsort(x)[-1]
print(z)
# 1