[Numpy-discussion] Extract subset from an array

Nicola Creati ncreati@inogs...
Tue Feb 16 06:42:37 CST 2010


Hello,
I need to extract a subset from a Nx3 array. Each row has x, y, and z 
coordinates.
The subset is just a portion of the array in which the following 
condition realizes

x_min < x < x_max and y_min < y < y_max

The problem reduce to the extraction of points inside a rectangular box 
defined by
x_min, x_max, y_min, y_max.

I work with large arrays, the number or rows is always larger than 5x1e7.
I'm looking for a fast way to extract the subset.

At the moment I found a solution that seems the best. This is a small 
example:

import numpy as np

# Create a large 1e7x3 array of random numbers
array = np.random.random((10000000, 3))

# Define rectangular box
x_min = 0.3
x_max = 0.5
y_min = 0.4
y_max = 0.7

# Create bool array that indicates the elemnts of array to extract
condition = (array[:,0]>x_min) & (array[:,0]<x_max) & (array[:,1]>y_min) 
& (array[:,1]<y_max)

# Extract the subset
subset = array[condition]

Are there any faster solution?

Thanks,

Nicola

-- 
Nicola Creati
Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS www.inogs.it Dipartimento di Geofisica della Litosfera Geophysics of Lithosphere Department CARS (Cartography and Remote Sensing) Research Group http://www.inogs.it/Cars/ Borgo Grotta Gigante 42/c 34010 Sgonico - Trieste - ITALY ncreati@ogs.trieste.it
off.   +39 040 2140 213
fax.   +39 040 327307

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