2D空间中的Python过滤点

Question

I have two arrays x=[1,2,3,4] and y=[1,0,0,1] describing 2D points (x,y). I want to know how many elements have x>2 and y==1 . What is the most simple way to do this (without any loops)? Is it possible to do something like x[x>2] , but for two conditions?

Answer 1

Assuming these are numpy arrays, since your x[x>2] is numpy syntax, you just need the and ( & ) operator:

meet_cond = (x > 2) & (y == 1)
how_many = meet_cond.sum()

which_x = x[meet_cond]
which_y = y[meet_cond]

Answer 2

If x and y belong together as points, you might want to pack them into a np 2D array:

>>> import numpy as np
>>> x = np.array([1, 2, 3, 4])
>>> y = np.array([1, 0, 0, 1])
>>> xy = np.array([x, y]).T
>>> xy[(x > 2) & (y == 1)]
array([[4, 1]])
>>> xy[(xy[:, 0] > 2) & (xy[:, 1] == 1)]
array([[4, 1]])
>>> np.count_nonzero((xy[:, 0] > 2) & (xy[:, 1] == 1))
1