numpy array of chars to string

Question

I have a 2D numpy char array (from a NetCDF4 file) which actually represents a list of strings. I want to convert it into a list of strings.

I know I can use join() to concatenate the chars into a string, but I can only find a way to do this one string at a time:

data = np.array([['a','b'],['c','d']])
for row in data[:]:
    print ''.join(row)

But it's very slow. How can I return an array of strings in a single command? Thanks

Why are you copying data in your for loop?

Joel Cornett
– Joel Cornett

2012-06-11 17:07:32 +00:00
Commented Jun 11, 2012 at 17:07 — Joel Cornett
– Joel Cornett, Commented Jun 11, 2012 at 17:07

Phil Cooper · Accepted Answer · 2012-06-11 18:27:33Z

14

The list comprehension is the most "pythonic" way.

The most "numpythonic" way would be:

>>> data = np.array([['a','b'],['c','d']])
# a 2D view
>>> data.view('S2')
array([['ab'],
       ['cd']], 
      dtype='|S2')
# or maybe a 1D view ...fastest solution:
>>> data.view('S2').ravel()
array(['ab', 'cd'], 
      dtype='|S2')

No looping, no list comprehension, not even a copy. The buffer just sits there unchanged with a different "view" so this is the fastest solution available.

edited Jun 11, 2012 at 18:27

answered Jun 11, 2012 at 17:37

Phil Cooper

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1 Comment

shoyer Over a year ago

An important caveat is that the array must be contiguous in memory -- otherwise the view fails. You can ensure this by using data = np.ascontiguousarray(data).

Chris · Accepted Answer · 2012-06-11 17:10:11Z

5

Try a list comprehension:

>> s = [''.join(row) for row in data]
>> s
['ab', 'cd']

which is just your for loop rewritten.

answered Jun 11, 2012 at 17:10

Chris

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2 Comments

Chris Over a year ago

@DavidRobinson Hadn't thought of that - very nice.

David Robinson Over a year ago

@AdrianR- don't forget to accept his answer (by clicking on the green checkmark) if it answered your question.

user545424 · Accepted Answer · 2012-06-11 19:00:09Z

2

[row.tostring() for row in data]

answered Jun 11, 2012 at 19:00

user545424

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