concatenate numpy string array along an axis?

Question

I have a 2-d numpy array of strings. Is there a way to concatenate the strings in each row and then join the resulting strings with a separator string, e.g. a newline?

Example:

pic = np.array([ 'H','e','l','l','o','W','o','r','l','d']).reshape(2,5)

I want to get:

"Hello\nWorld\n"

Answer 1

You had the right ideas there. Here's a vectorized NumPythonic implementation trying to go along those ideas -

# Create a separator string of the same rows as input array
separator_str = np.repeat(['\n'], pic.shape[0])[:,None]

# Concatenate these two and convert to string for final output
out = np.concatenate((pic,separator_str),axis=1).tostring()

Or a one-liner with np.column_stack -

np.column_stack((pic,np.repeat(['\n'], pic.shape[0])[:,None])).tostring()

Sample run -

In [123]: pic
Out[123]: 
array([['H', 'e', 'l', 'l', 'o'],
       ['W', 'o', 'r', 'l', 'd']], 
      dtype='|S1')

In [124]: np.column_stack((pic,np.repeat(['\n'], pic.shape[0])[:,None])).tostring()
Out[124]: 'Hello\nWorld\n'

Answer 2

It's not hard to do outside of numpy:

>>> import numpy as np
>>> pic = np.array([ 'H','e','l','l','o','W','o','r','l','d']).reshape(2,5)
>>> pic
array([['H', 'e', 'l', 'l', 'o'],
       ['W', 'o', 'r', 'l', 'd']], 
      dtype='|S1')
>>> '\n'.join([''.join(row) for row in pic])
'Hello\nWorld'

There is also the np.core.defchararray module which has "goodies" for working with character arrays -- However, it states that these are merely wrappers around the python builtin and standard library functions so you'll probably not get any real speedup by using them.