I have the rather simple function func1()
defined below, mainly composed of two for
blocks.
It works perfectly for small N
values, but being that the for
blocks are combined it grows very quickly to the point of taking minutes when N>1000
.
How can I use numpy
broadcasting to improve the performance of this function?
import numpy as np
import time as t
def func1(A_triang, B_triang):
aa = []
for i, A_tr in enumerate(A_triang):
for j, B_tr in enumerate(B_triang):
# Absolute value of differences.
abs_diff = abs(np.array(A_tr) - np.array(B_tr))
# Store the sum of the differences and the indexes
aa.append([sum(abs_diff), i, j])
return aa
# Generate random data with the proper format
N = 500
A_triang = np.random.uniform(0., 20., (N, 3))
A_triang[:, 0] = np.ones(N)
B_triang = np.random.uniform(0., 20., (N, 3))
B_triang[:, 0] = np.ones(N)
# Call function.
s = t.clock()
aa = func1(A_triang, B_triang)
print(t.clock() - s)
Here's one with NumPy broadcasting
, leveraging a modified version of indices_merged_arr_generic_using_cp
for index assignment part -
import functools
# Based on https://stackoverflow.com/a/46135435/ @unutbu
def indices_merged_arr_generic_using_cp(arr):
"""
Based on cartesian_product
http://stackoverflow.com/a/11146645/190597 (senderle)
"""
shape = arr.shape
arrays = [np.arange(s, dtype='int') for s in shape]
broadcastable = np.ix_(*arrays)
broadcasted = np.broadcast_arrays(*broadcastable)
rows, cols = functools.reduce(np.multiply, broadcasted[0].shape),\
len(broadcasted)+1
out = np.empty(rows * cols, dtype=arr.dtype)
start, end = rows, 2*rows
for a in broadcasted:
out[start:end] = a.reshape(-1)
start, end = end, end + rows
out[0:rows] = arr.flatten()
return out.reshape(cols, rows).T
def func1_numpy_broadcasting(a,b):
val = np.abs(a[:,None,:] - b).sum(-1)
return indices_merged_arr_generic_using_cp(val)
If the first column of the inputs are always 1s
, then, we won't need to compute their differences as their sum of differences would be zero. So, alternatively to get val
, we can simply use the last two columns -
val = np.abs(a[:,1,None] - b[:,1]) + np.abs(a[:,2,None] - b[:,2])
This would save memory as we are not going 3D
this way.
Using numexpr
module -
import numexpr as ne
def func1_numexpr_broadcasting(a,b):
a3D = a[:,None,:]
val = ne.evaluate('sum(abs(a3D - b),2)')
return indices_merged_arr_generic_using_cp(val)
Leveraging the fact that the first columns are 1s
, we would have -
def func1_numexpr_broadcasting_v2(a,b):
a1 = a[:,1,None]
b1 = b[:,1]
a2 = a[:,2,None]
b2 = b[:,2]
val = ne.evaluate('abs(a1-b1) + abs(a2-b2)')
return indices_merged_arr_generic_using_cp(val)
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