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pythonarraysnumpyfloating-pointint
Basj
Basj

Reputation: 46381

Numpy Cannot cast ufunc multiply output from dtype

I'd like to multiply an int16 array but a float array, with auto rounding, but this fails :

import numpy

A = numpy.array([1, 2, 3, 4], dtype=numpy.int16)
B = numpy.array([0.5, 2.1, 3, 4], dtype=numpy.float64)

A *= B

I get:

TypeError: Cannot cast ufunc multiply output from dtype('float64') to dtype('int16') with casting rule 'same_kind'

Upvotes: 38

Views: 55887

Answers (3)

seralouk
seralouk

Reputation: 33137

2 ways to solve this:

You can solve this by replacing

A *= B

with 

A = (A * B)

or with

numpy.multiply(A, B, out=A, casting='unsafe')

Upvotes: 50

Ahmet İlgin
Ahmet İlgin

Reputation: 55

import numpy as np

A = np.float_(A)
A *= B

try this. I think are different array type you get fail.

Cast

Upvotes: 4

Nickil Maveli
Nickil Maveli

Reputation: 29711

You could use broadcasting to multiply the two arrays and take only the integer part as follows:

In [2]: (A*B).astype(int)
Out[2]: array([ 0,  4,  9, 16])

Timing Constraints:

In [8]: %timeit (A*B).astype(int)
1000000 loops, best of 3: 1.65 µs per loop

In [9]: %timeit np.multiply(A, B, out=A, casting='unsafe')
100000 loops, best of 3: 2.01 µs per loop

Upvotes: 6

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