Create a numpy array of coordinates from a list of points

Question

Given a list of points I would like to create a numpy array with coordinates. Related questions are here and here. Does anyone know the correct or a more efficient way of doing this?

import numpy as np

# Please note that we have no control over the point class.
# This code just to generate the example.
class Point:
    x = 0.0
    y = 0.0


points = list()
for i in range(10):
    p = Point()
    p.x = 10.0
    p.y = 5.0
    points.append(p)

# The question starts here.
# The best I could come up with so far:
x = np.fromiter((p.x for p in points), float)
y = np.fromiter((p.y for p in points), float)
arr = np.vstack((x,y)).transpose()

First of all, x and y are static properties of Point. What you want is to define a __init__(self, x, y) function in Point and do self.x = x; self.y = y, which makes x and y belong to a Point, not the Point class. — hyperneutrino
– hyperneutrino, Commented Oct 27, 2017 at 2:11
That is just to set up the sample. I am given a list of objects that have proper x and y properties, The question is how to get to a numpy array from the python list. — Robert Altena
– Robert Altena, Commented Oct 27, 2017 at 2:13
Okay. You could just replace the last three lines with this: arr = np.vstack(zip((p.x for p in points), (p.y for p in points))) — hyperneutrino
– hyperneutrino, Commented Oct 27, 2017 at 2:18
Honestly you could set up the entire thing like this: tio.run/##bY/BagMxDETP8VfoaIMxG0Ivhf5D7yUYk3gT011ZWE5Yf/… — hyperneutrino
– hyperneutrino, Commented Oct 27, 2017 at 2:20
Thank you. That is very kind of you. I should have clarified in the question that I have no control over the Point class. — Robert Altena
– Robert Altena, Commented Oct 27, 2017 at 2:22

f5r5e5d · Accepted Answer · 2017-10-27 03:15:23Z

2

grab both x, y at once, sitck in list in a listcomp and it has the structure you want

np.array([[e.x, e.y] for e in points])  

Out[203]: 
array([[ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.],
       [ 10.,   5.]])

answered Oct 27, 2017 at 3:15

f5r5e5d

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1 Comment

Robert Altena Over a year ago

Thank you. That is a very elegant looking line of code. Appreciated.

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