CODEWITHSUNDEEP
pythonnumpy
sccds
sccds

Reputation: 11

numpy array multiply 10 got wrong result

I got a very wired test for python2.7 numpy array. Please look at this code.

import numpy as np
times = np.arange(5., 85, 0.1)
print times
times = np.array(times * 10, dtype=np.int)
print times

the original times should be [5.0 ~ 84.9]. After multiply 10, it should become [50 ~ 849], but result is like this:

[ 50 51 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 ... ]
There are two 51 between 50 and 52

Upvotes: 1

Views: 513

Answers (1)

Stephan
Stephan

Reputation: 2115

The problem is, that your third entry isn't exactly 52.0 but 51.999999999999993 (see Is floating point math broken?). Truncating that value therefore results in 51.

The correct way would be to first round the values. (As pointed out in Safest way to convert float to integer in python? all small enough integer numbers can be exactly expressed as a float.) You therefore have to calculate: times = np.array(np.round(times * 10), dtype=np.int)

Upvotes: 2

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