comparing a string 1-d numpy array elementwise

Question

I have two 1-d arrays (a and b) containing strings, which I want to compare element wise to get output c like shown below. I tried converting it to set and comparing, however that does not give the correct solution. Also logical_xor does not work for string. I can write a loop to do this but then it defeats the purpose of using arrays, What can be the best way to do this without a loop?

  >>  a
      array(['S', 'S', 'D', 'S', 'N', 'S', 'A', 'S', 'M'], 
          dtype='|S1')
  >>  b
      array(['T', 'I', 'D', 'N', 'G', 'B', 'A', 'J', 'M'], 
          dtype='|S1')

  >> c 
     array([False, False, True, False, False, False, True, False, True], 
      dtype=bool)

timgeb · Accepted Answer · 2016-02-06 22:05:09Z

4

Just use the ndarray's __eq__ method, i.e. ==

>>> a = array(['S', 'S', 'D', 'S', 'N', 'S', 'A', 'S', 'M'], dtype='|S1')
>>> b = array(['T', 'I', 'D', 'N', 'G', 'B', 'A', 'J', 'M'], dtype='|S1')
>>> a == b
array([False, False,  True, False, False, False,  True, False,  True], dtype=bool)

answered Feb 6, 2016 at 22:05

timgeb

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1 Comment

Common man Over a year ago

Aah! missed the simplest option ... Thank you :)

cromod · Accepted Answer · 2017-07-01 16:36:14Z

1

You can use numpy.equal :

import numpy as np
c = np.equal(a,b)

Or numpy.core.defchararray.equal :

c = np.core.defchararray.equal(a, b)

EDIT

np.equal has been deprecated in the last numpy's releases and now raises a FutureWarning:

>>> c = np.equal(a,b)
__main__:1: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
>>> c
NotImplemented

The equality operator == will suffer the same fate as np.equal. So I suggest using:

c = np.array([a == b], dtype=bool)

edited Jul 1, 2017 at 16:36

answered Feb 6, 2016 at 22:20

cromod

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3 Comments

luxon Over a year ago

This does not work. You get the error:

FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison NotImplemented

cromod Over a year ago

Ok this is a deprecation of numpy (see this stuff). I edit my answer :)

Newbielp Over a year ago

This post should be the selected response.

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