How do I construct a sort with conditions in python? Let's say if I have a list
a: [-1, 3, 4, 2, -1, -1, 1, 0] How do I sort only the elements that is not -1? (In return:
[-1, 0, 1, 2, -1, -1, 3, 4] ) How do I sort every other element? (In return a: [-1, 3, -1, 2, 1, -1, 4, 0] ) The syntax of the code is wrong, but would it be something similar to this?
result=sorted(a, key=lambda x: x!=-1,reverse=True) result=sorted(a, key=lambda x: [::2],reverse=True) If a vectorised approach interests you, this is possible via 3rd party library numpy:
import numpy as np a = np.array([-1, 3, 4, 2, -1, -1, 1, 0]) a[a!=-1] = np.sort(a[a!=-1]) # array([-1, 0, 1, 2, -1, -1, 3, 4]) Sorting every other element is equally trivial:
a[::2] = np.sort(a[::2]) # array([-1, 3, -1, 2, 1, -1, 4, 0]) Related: Why NumPy instead of Python lists?
You can use next and iter after sorting all contents from s that are positive:
def sort_by(s, avoid = -1): sorted_vals = iter(sorted(filter(lambda x:x >= 0, s))) return [i if i == avoid else next(sorted_vals) for i in s] print(sort_by([-1, 3, 4, 2, -1, -1, 1, 0])) Output:
[-1, 0, 1, 2, -1, -1, 3, 4] 
