How to select a group of minimum values out of a numpy array?

Question

fitVec = np.zeros((100, 2)) #Initializing the fitVec, where first column` will be the indices and second column will contain the values

After Initialization, fitVec gets assigned some values by running a function. Final fitVec values:

fitVec [[  2.00000000e+01   2.42733444e+10]
 [  2.10000000e+01   2.53836270e+10]
 [  2.20000000e+01   2.65580909e+10]
 [  2.30000000e+01   2.76674886e+10]
 [  2.40000000e+01   2.88334239e+10]
 [  2.50000000e+01   3.00078878e+10]
 [  2.60000000e+01   3.11823517e+10]
 [  2.70000000e+01   3.22917494e+10]
 [  2.80000000e+01   3.34011471e+10]
 [  2.90000000e+01   3.45756109e+10]
 [  3.00000000e+01   3.57500745e+10]
 [  3.10000000e+01   3.68594722e+10]
 [  3.20000000e+01   3.79688699e+10]
 [  3.30000000e+01   3.90782676e+10]
 [  3.40000000e+01   4.02527315e+10]
 [  3.50000000e+01   4.14271953e+10]
 [  3.60000000e+01   4.25365930e+10]
 [  3.70000000e+01   4.36476395e+10]]

**I haven't shown all of the 100*4 matrix to make it look less messy. Now I want to select the twenty (20*4) minimum values out of it. I'm trying winner = np.argmin(fitVec[100,1]) but it gives me only one minimum value whereas I want 20 min values. How should I go about it?

use argsort instead of argmin, and then take the first 20 elements. — Julien
– Julien, Commented Aug 8, 2016 at 11:37
Check out the functions numpy.partition and numpy.argpartition. — Warren Weckesser
– Warren Weckesser, Commented Aug 8, 2016 at 12:36

Nico Schlömer · Accepted Answer · 2016-10-02 12:22:13Z

First off, I'd separate indices and values; no need to store them both as float. After that, numpy.argsort is your friend:

import numpy
idx = numpy.arange(20, 38, dtype=int)
vals = numpy.random.rand(len(idx))

i = numpy.argsort(vals)

sorted_idx = idx[i]
sorted_vals = vals[i]

print(idx)
print(vals)
print
print(sorted_idx)
print(sorted_vals)

Output:

[20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37]
[ 0.00560689  0.73380138  0.53490514  0.1221538   0.45490855  0.39076217
  0.39906252  0.59933451  0.7163099   0.393409    0.15854323  0.4631854
  0.92469362  0.69999709  0.67664291  0.73184184  0.52893679  0.60365631]

[20 23 30 25 29 26 24 31 36 22 27 37 34 33 28 35 21 32]
[ 0.00560689  0.1221538   0.15854323  0.39076217  0.393409    0.39906252
  0.45490855  0.4631854   0.52893679  0.53490514  0.59933451  0.60365631
  0.67664291  0.69999709  0.7163099   0.73184184  0.73380138  0.92469362]

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