# How to use threads to retrieve sub-array

363
December 23, 2016, at 3:31 PM

Assume that I have a numpy array with size of (10,3,4). Now I want to do some operations at subarray among first dimension like the following code:

``````a = np.arange(120).reshape([10,3,4])
for batch in range(10):
result = do_something_func( a[batch,:,:] )
# other operations in for
# other operations
``````

My question is I want to use multi-threads to finish this task without using index to iterate and retrieve the subarray by a for loop. Does anyone know how to do that?

Possibly with `gevent`:

``````import gevent
from gevent.pool import Pool
import numpy as np
pool = Pool(8)
def map_subarray(sub_array):
return sub_array
a = np.arange(120).reshape([10,3,4])
for batch in range(10):
result = pool.map(map_subarray, a[batch,:,:])
print(batch, list(result))
``````

Output:

``````(0, [array([0, 1, 2, 3]), array([4, 5, 6, 7]), array([ 8,  9, 10, 11])])
(1, [array([12, 13, 14, 15]), array([16, 17, 18, 19]), array([20, 21, 22, 23])])
(2, [array([24, 25, 26, 27]), array([28, 29, 30, 31]), array([32, 33, 34, 35])])
(3, [array([36, 37, 38, 39]), array([40, 41, 42, 43]), array([44, 45, 46, 47])])
(4, [array([48, 49, 50, 51]), array([52, 53, 54, 55]), array([56, 57, 58, 59])])
(5, [array([60, 61, 62, 63]), array([64, 65, 66, 67]), array([68, 69, 70, 71])])
(6, [array([72, 73, 74, 75]), array([76, 77, 78, 79]), array([80, 81, 82, 83])])
(7, [array([84, 85, 86, 87]), array([88, 89, 90, 91]), array([92, 93, 94, 95])])
(8, [array([96, 97, 98, 99]), array([100, 101, 102, 103]), array([104, 105, 106, 107])])
(9, [array([108, 109, 110, 111]), array([112, 113, 114, 115]), array([116, 117, 118, 119])])
``````

Reference:

• http://sdiehl.github.io/gevent-tutorial/
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