# [Numpy-discussion] Beginner's question

Laszlo Nagy gandalf at shopzeus.com
Sat Apr 16 15:08:55 EDT 2011

```   Hi All,

I have this example program:

import numpy as np
import numpy.random as rnd

def dim_weight(X):
weights = X[0]
volumes = X[1]*X[2]*X[3]
res = np.empty(len(volumes), dtype=np.double)
for i,v in enumerate(volumes):
if v>5184:
res[i] = v/194.0
else:
res[i] = weights[i]
return res

# TEST
N = 10
X = rnd.randint( 1,25, (4,N))
print dim_weight(X)

I want to implement the dim_weight() function effectively. But I'm not
sure how to construct a numpy expression that does the same think for me.

I got to this point:

def dim_weight(X):
weights = X[0]
volumes = X[1]*X[2]*X[3]
dweights = volumes/194.0
return ( weights[weights>5184] , dweights[weights<=5184] ) # ???

I don't know how to preserve the order of the elements and return the
result in one array. Maybe I need to do create a matrix multiply matrix?
But don't know how.

Thanks,

Laszlo

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