Sort Numpy Array With Custom Predicate

I'd like to sort my numpy array of shape [n,4], along first dimension (size:n) using a custom predicate operating on the 2nd dimension vector (size:4). The C++ version of what I'd

Solution 1:

In numpy you would apply the (vectorized) order defining function to the array, then use np.argsort to sort by the result.

This is less space efficient than the C++ version, but that is how you usually achieve performance with numpy.

import numpy as np    

defmyfn(x):
    return np.sin(x[:, 1])  # example: sort by the sine of the second column

a = np.random.randn(10, 4)

predicate = myfn(a)  # not sure if predicate is the best name for this variable
order = np.argsort(predicate)

a_sorted = a[order]