5

I want to write a data string to a NumPy array. Pseudocode:

d = numpy.zeros(10, dtype = numpy.character)
d[1:6] = 'hello'

Example result:

d=
  array(['', 'h', 'e', 'l', 'l', 'o', '', '', '', ''],
        dtype='|S1')

How can this be done most naturally and efficiently with NumPy?

I don't want for loops, generators, or anything iterative. Can it be done with one command as with the pseudocode?

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3 Answers 3

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3

Just explicitly make your text a list (rather than that it is iterable from Python) and NumPy will understand it automatically:

>>> text = 'hello'
>>> offset = 1
>>> d[offset:offset+len(text)] = list(text)
>>> d

array(['', 'h', 'e', 'l', 'l', 'o', '', '', '', ''],
      dtype='|S1')
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1 Comment

I tried using an iter to avoid the memory overhead for huge strings. You will not believe what numpy did --- d[1:6]=iter('hello')
2

There's little need to build a list when you have numpy.fromstring and numpy.fromiter.

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1 Comment

I'm writing many strings to different positions on the same array.
1

You might try Python's array type. It will not have as high an overhead as a list.

from array import array as pyarray
d[1:6] = pyarray('c', 'hello')
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1 Comment

Note that in Python 3 this will not work, as it is a TypeError to try to instantiate a character array from a string, and you should try to use bytes instead. See this for more detail and a workaround.

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