Numpy.argsort-看不出什么问题 [英] Numpy.argsort - can't see what's wrong
问题描述
我正在尝试使用argsort函数对一个numpy数组进行排序.
I am trying to sort a numpy array using the argsort function.
不幸的是，这不起作用，我不明白为什么:(
Unfortunately this is not working and I can't see why :(
代码是:
import numpy as np
distance = np.array([38.26, 33.01, 32.33, 30.77, 37.96, 44.37, 32.72, 36.56,
27.77, 33.62, 42.85, 34.6 , 32.04, 27.49, 49.64, 51.85,
44.37, 38.26, 46.93, 40.45, 40.72, 39.7 , 34.12, 36.9 ,
34.6 , 34. , 36.56, 39.29, 38.6 , 32.33, 32.65, 40.72,
43.85, 47.89, 33.62, 35.24, 42.5 , 36.97, 28.36, 37.57,
37.25, 25.54, 29.6 , 37.25, 40.45, 32.04, 40.45, 31.4 ,
41.78, 35.89, 59.24, 51.2 , 57.22, 35.54, 50.09, 40.33,
50.58, 29.77, 51.97, 34.33, 29. , 43.81, 40.84, 45.62,
39.77, 54.5 , 40.36, 40.93, 43.28, 37.61, 45.05, 45.05,
45.94, 45.05, 49.37, 52.56, 54.08, 53.89, 44.41, 39.25,
36.01, 36.01, 40.93, 43.29, 38.16, 47.56, 54.5 , 44.98,
40.36, 36.5 , 37.01, 46.21, 40.4 , 30.29, 38.65, 41.49,
40.9 , 46.85, 32.26, 40.33, 50.58, 40.93, 59.41, 48.1 ,
51.25, 66.76, 30.26, 61.7 , 51.14, 64.8 , 52.49, 48.25,
55.24, 38.74, 41.48, 51.2 , 51.25, 73.73, 66.05, 40.84,
57.85, 39.2 , 67.13, 46.98, 55.78, 62.08, 46.28, 46.21,
48.8 , 60.84, 62.6 , 76.85, 48.8 , 47.53, 43.97, 68.29,
51.25, 50.57, 45. , 57.22, 54.5 , 57.22, 40.93, 56.48,
55.78, 53.89, 45.94, 51.25, 50. , 43.81])
要调试它，我将距离数组与argsort一起打印:
To debug it, I am printing the distance array together with the argsort:
print np.array(zip(distance, distance.argsort()))
我得到的是这个.它看起来是错误的，例如30.77小于32.33，但是32.33被标记为8，而30.77被标记为38.
What I get is this. It looks wrong, because for example 30.77 is smaller than 32.33, but 32.33 is marked as 8 and 30.77 as 38. What am I doing wrong?
[[ 38.26 41. ]
[ 33.01 13. ]
[ 32.33 8. ]
[ 30.77 38. ]
[ 37.96 60. ]
[ 44.37 42. ]
[ 32.72 57. ]
[ 36.56 106. ]
[ 27.77 93. ]
[ 33.62 3. ]
[ 42.85 47. ]
[ 34.6 45. ]
[ 32.04 12. ]
[ 27.49 98. ]
[ 49.64 2. ]
[ 51.85 29. ]
[ 44.37 30. ]
[ 38.26 6. ]
[ 46.93 1. ]
[ 40.45 9. ]
[ 40.72 34. ]
[ 39.7 25. ]
[ 34.12 22. ]
[ 36.9 59. ]
[ 34.6 24. ]
[ 34. 11. ]
[ 36.56 35. ]
[ 39.29 53. ]
[ 38.6 49. ]
[ 32.33 80. ]
[ 32.65 81. ]
[ 40.72 89. ]
[ 43.85 26. ]
[ 47.89 7. ]
[ 33.62 23. ]
[ 35.24 37. ]
[ 42.5 90. ]
[ 36.97 43. ]
[ 28.36 40. ]
[ 37.57 39. ]
[ 37.25 69. ]
[ 25.54 4. ]
[ 29.6 84. ]
[ 37.25 0. ]
[ 40.45 17. ]
[ 32.04 28. ]
[ 40.45 94. ]
[ 31.4 113. ]
[ 41.78 121. ]
[ 35.89 79. ]
[ 59.24 27. ]
[ 51.2 21. ]
[ 57.22 64. ]
[ 35.54 99. ]
[ 50.09 55. ]
[ 40.33 88. ]
[ 50.58 66. ]
[ 29.77 92. ]
[ 51.97 46. ]
[ 34.33 44. ]
[ 29. 19. ]
[ 43.81 20. ]
[ 40.84 31. ]
[ 45.62 62. ]
[ 39.77 119. ]
[ 54.5 96. ]
[ 40.36 67. ]
[ 40.93 101. ]
[ 43.28 142. ]
[ 37.61 82. ]
[ 45.05 114. ]
[ 45.05 95. ]
[ 45.94 48. ]
[ 45.05 36. ]
[ 49.37 10. ]
[ 52.56 68. ]
[ 54.08 83. ]
[ 53.89 149. ]
[ 44.41 61. ]
[ 39.25 32. ]
[ 36.01 134. ]
[ 36.01 5. ]
[ 40.93 16. ]
[ 43.29 78. ]
[ 38.16 87. ]
[ 47.56 138. ]
[ 54.5 73. ]
[ 44.98 71. ]
[ 40.36 70. ]
[ 36.5 63. ]
[ 37.01 146. ]
[ 46.21 72. ]
[ 40.4 127. ]
[ 30.29 91. ]
[ 38.65 126. ]
[ 41.49 97. ]
[ 40.9 18. ]
[ 46.85 123. ]
[ 32.26 133. ]
[ 40.33 85. ]
[ 50.58 33. ]
[ 40.93 103. ]
[ 59.41 111. ]
[ 48.1 132. ]
[ 51.25 128. ]
[ 66.76 74. ]
[ 30.26 14. ]
[ 61.7 148. ]
[ 51.14 54. ]
[ 64.8 137. ]
[ 52.49 100. ]
[ 48.25 56. ]
[ 55.24 108. ]
[ 38.74 115. ]
[ 41.48 51. ]
[ 51.2 104. ]
[ 51.25 136. ]
[ 73.73 116. ]
[ 66.05 147. ]
[ 40.84 15. ]
[ 57.85 58. ]
[ 39.2 110. ]
[ 67.13 75. ]
[ 46.98 145. ]
[ 55.78 77. ]
[ 62.08 76. ]
[ 46.28 65. ]
[ 46.21 140. ]
[ 48.8 86. ]
[ 60.84 112. ]
[ 62.6 144. ]
[ 76.85 124. ]
[ 48.8 143. ]
[ 47.53 139. ]
[ 43.97 141. ]
[ 68.29 52. ]
[ 51.25 120. ]
[ 50.57 50. ]
[ 45. 102. ]
[ 57.22 129. ]
[ 54.5 107. ]
[ 57.22 125. ]
[ 40.93 130. ]
[ 56.48 109. ]
[ 55.78 118. ]
[ 53.89 105. ]
[ 45.94 122. ]
[ 51.25 135. ]
[ 50. 117. ]
[ 43.81 131. ]]
推荐答案
distance.argsort()返回索引数组. ith索引不会不告诉您distance中ith元素的排名.相反，ith索引告诉您排序数组中的ith元素是distance[i].
distance.argsort() returns an array of indices. The ith index does not tell you the rank of the ith element in distance. Rather, the ith index tells you that the ith element in the sorted array is distance[i].
换句话说，
idx = distance.argsort()
assert (distance[idx] == np.sort(distance)).all()
考虑以下小示例来自文档:
In [236]: x = np.array([3, 1, 2])
In [237]: np.argsort(x)
Out[237]: array([1, 2, 0])
In [238]: x[np.argsort(x)]
Out[238]: array([1, 2, 3])
In [239]: x[1], x[2], x[0]
Out[239]: (1, 2, 3)
调用两次argsort确实会给您distance中ith元素的等级:
Calling argsort twice does gives you the rank of the ith element in distance:
In [240]: np.argsort(np.argsort(x))
Out[240]: array([2, 0, 1])
了解这是如何工作的，可以很好地检验您对argsort的理解.但是，两次调用argsort来找到排名是无效的.尤其是对于较大的数组还有其他更快的方法来查找排名.
Understanding how this works is a good test of your understanding of argsort. However, calling argsort twice to find the rank is inefficient. Especially for larger arrays there are other, faster, ways to find the rank.
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