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pythonnumpy
Håkon Hægland
Håkon Hægland

Reputation: 40778

Convert Numpy array of ASCII codes to string

I would like to convert a NumPy array of integers representing ASCII codes to the corresponding string. For example ASCII code 97 is equal to character "a". I tried:

from numpy import *
a=array([97, 98, 99])
c = a.astype('string')
print c

which gives:

['9' '9' '9']

but I would like to get the string "abc".

Upvotes: 10

Views: 19410

Answers (6)

ivanbgd
ivanbgd

Reputation: 191

import numpy as np
np.array([97, 98, 99], dtype='b').tobytes().decode("ascii")

Output:

'abc'

Data type objects (dtype)

tostring() is deprecated since version 1.19.0. Use tobytes() instead.

Upvotes: 6

nth
nth

Reputation: 1522

Solutions that rely on Python loops or string formatting will be slow for large datasets. If you know that all of your data are ASCII, a faster approach could be to use fancy indexing:

import numpy as np
a = np.array([97, 98, 99])
np.array([chr(x) for x in range(127)])[a]
# array(['a', 'b', 'c'], dtype='<U1')

An advantage is that it works for arbitrarily shaped arrays.

Upvotes: 1

nouseforname
nouseforname

Reputation: 748

from numpy import array

a = array([97, 98, 99])
print("{0:c}{1:c}{2:c}".format(a[0], a[1], a[2]))

Of course, join and a list comprehension can be used here as well.

Upvotes: 1

coderforlife
coderforlife

Reputation: 1537

Another solution that does not involve leaving the NumPy world is to view the data as strings:

arr = np.array([97, 98, 99], dtype=np.uint8).view('S3').squeeze()

or if your numpy array is not 8-bit integers:

arr = np.array([97, 98, 99]).astype(np.uint8).view('S3').squeeze()

In these cases however you do have to append the right length to the data type (e.g. 'S3' for 3 character strings).

Upvotes: 11

jtaylor
jtaylor

Reputation: 2434

create an array of bytes and decode the the byte representation using the ascii codec:

np.array([98,97,99], dtype=np.int8).tostring().decode("ascii")

note that tostring is badly named, it actually returns bytes which happens to be a string in python2, in python3 you will get the bytes type back which need to be decoded.

Upvotes: 7

Ashoka Lella
Ashoka Lella

Reputation: 6729

print "".join([chr(item) for item in a])

output

abc

Upvotes: 11

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