Python: String concatenation working differently inside for loop

Question

I have a NumPy array containing a list which contains strings of various lengths:

arr = np.array(["abcd", "abcdef", "ab"])

I'm trying to loop through the strings in order to pad them to a constant length. If i do it one at a time, like so:

new_len = 10

left_pad = divmod(new_len - len(arr[0]),2)[0]
right_pad = left_pad + divmod(new_len - len(arr[0]),2)[1]

abcd = arr[0].join(["_"*left_pad, "_"*right_pad])

I get my desired output of:

'___abcd___'

But if I try doing it in a loop, like so:

for i in range(arr.shape[0]):
    left_pad = divmod(new_len - len(arr[i]),2)[0]
    right_pad = left_pad + divmod(new_len - len(arr[i]),2)[1]
    arr[i] = arr[i].join(["_"*left_pad, "_"*right_pad])

I get this different output:

array(['___abc', '__abcd', '____ab'], dtype='<U6')

I'd like to understand why the behaviour is different in these two cases, and how I can get the desired output with a loop. Thanks in advance for any help or suggestions.

Please always include the numpy tag when asking NumPy questions. Python itself has an array module that is quite different from these arrays. — Chris
– Chris, Commented Feb 20, 2022 at 14:06

daniboy000 · Accepted Answer · 2022-02-20 14:34:10Z

3

Try to define your array as an array of objects like in the example bellow:

arr = np.array(["abcd", "abcdef", "ab"], dtype='object')

According to the output of your example you've created an array of char with length of 6 (dtype='<U6')

answered Feb 20, 2022 at 14:34

daniboy000

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rs_punia · Accepted Answer · 2022-03-02 09:29:34Z

0

To elaborate further as daniboy000 perfectly mentioned, <U6 means, the longest string of the np.arr declared here has the length of 6. So, for testing you can use dtype="<U11" also, but for a general use case dtype="object" is more appropriate.

Also, I wanted to add it just as a comment but currently, I do not match the required reputation.

edited Mar 2, 2022 at 9:29

answered Feb 20, 2022 at 15:09

rs_punia

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