Sort randomly generated Numpy array according to a different array

Question

I have created 2 variables. One to hold 200 randomly generated ages, the other to hold 200 randomly generated marks.

from numpy import *
age = random.random_integers(18,40, size=(1,200))
marks = random.random_integers(0,100, size=(1,200))

I'd like to use NumPy to sort the marks array by the age array. eg:

#random student ages
[32 37 53 48 39 44 33 40 56 47]
#random student marks
[167 160 176 163 209 178 201 164 190 156]

#sorted marked according to ages
[32 33 37 39 40 44 47 48 53 56]
[167 201 160 209 164 178 156 163 176 190]

It is a similar question to this one. I am just unsure if a similar solution applies due to the elements being randomly generated.

yes your linked answer does apply to your situation.. in your case you would sort marks according to age with: sorted_marks = marks[age.argsort()]. For this to work smoothly however you should eliminate the extraneous axis in your array by using size=200 instead of size=(1,200) — Aaron
– Aaron, Commented May 29, 2018 at 13:35
Possible duplicate of How can I "zip sort" parallel numpy arrays? — Aaron
– Aaron, Commented May 29, 2018 at 13:39

jpp · Accepted Answer · 2019-05-22 09:19:25Z

2

One way is to first calculate an ordering via argsort, then use this to index your input arrays::

import numpy as np

np.random.seed(0)

ages = np.random.randint(18, 40, size=10)   # [30 33 39 18 21 21 25 27 37 39]
marks = np.random.randint(0, 100, size=10)  # [36 87 70 88 88 12 58 65 39 87]

order = ages.argsort()                      # [3 4 5 6 7 0 1 8 2 9]

print(ages[order])                          # [18 21 21 25 27 30 33 37 39 39]
print(marks[order])                         # [88 88 12 58 65 36 87 39 70 87]

edited May 22, 2019 at 9:19

answered May 29, 2018 at 13:37

jpp

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2 Comments

Christoph90 Over a year ago

Correct me if I'm wrong, but this does not do, what OP intended since it sorts the rows of the stacked array independently.

jpp Over a year ago

@Christoph90, Good spot, I've corrected the answer now.

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