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Re: Extract the middle N chars of a string

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Steven D'Aprano wrote:

> Extracting the first N or last N characters of a string is easy with
> slicing:
> 
> s[:N]  # first N
> s[-N:]  # last N
> 
> Getting the middle N seems like it ought to be easy:
> 
> s[N//2:-N//2]
> 
> but that is wrong. It's not even the right length!
> 
> py> s = 'aardvark'
> py> s[5//2:-5//2]
> 'rdv'
> 
> 
> So after spending a ridiculous amount of time on what seemed like it ought
> to be a trivial function, and an embarrassingly large number of off-by-one
> and off-by-I-don't-even errors, I eventually came up with this:
> 
> def mid(string, n):
>     """Return middle n chars of string."""
>     L = len(string)
>     if n <= 0:
>         return ''
>     elif n < L:
>         Lr = L % 2
>         a, ar = divmod(L-n, 2)
>         b, br = divmod(L+n, 2)
>         a += Lr*ar
>         b += Lr*br
>         string = string[a:b]
>     return string
> 
> 
> which works for me:
> 
> 
> # string with odd number of characters
> py> for i in range(1, 8):
> ...     print mid('abcdefg', i)
> ...
> d
> de
> cde
> cdef
> bcdef
> bcdefg
> abcdefg
> # string with even number of characters
> py> for i in range(1, 7):
> ...     print mid('abcdef', i)
> ...
> c
> cd
> bcd
> bcde
> abcde
> abcdef
> 
> 
> 
> Is this the simplest way to get the middle N characters?

>>> def mid(s, n):
...     shave = len(s) - n
...     if shave > 0:
...         shave //= 2
...         s = s[shave:shave+n]
...     return s
... 
>>> def show(s):
...     for i in range(len(s)+1):
...         print(i, repr(s), "-->", repr(mid(s, i)))
... 
>>> show("abcdefg")
0 'abcdefg' --> ''
1 'abcdefg' --> 'd'
2 'abcdefg' --> 'cd'
3 'abcdefg' --> 'cde'
4 'abcdefg' --> 'bcde'
5 'abcdefg' --> 'bcdef'
6 'abcdefg' --> 'abcdef'
7 'abcdefg' --> 'abcdefg'
>>> show("abcdef")
0 'abcdef' --> ''
1 'abcdef' --> 'c'
2 'abcdef' --> 'cd'
3 'abcdef' --> 'bcd'
4 'abcdef' --> 'bcde'
5 'abcdef' --> 'abcde'
6 'abcdef' --> 'abcdef'

Not exactly the same results as your implementation though.

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Thread

Extract the middle N chars of a string Steven D'Aprano <steve@pearwood.info> - 2016-05-19 01:47 +1000
  Re: Extract the middle N chars of a string Ian Kelly <ian.g.kelly@gmail.com> - 2016-05-18 10:34 -0600
  Re: Extract the middle N chars of a string Peter Otten <__peter__@web.de> - 2016-05-18 18:41 +0200
  Re: Extract the middle N chars of a string Peter Otten <__peter__@web.de> - 2016-05-18 18:54 +0200
    Re: Extract the middle N chars of a string Steven D'Aprano <steve@pearwood.info> - 2016-05-19 11:37 +1000
  Re: Extract the middle N chars of a string MRAB <python@mrabarnett.plus.com> - 2016-05-18 18:00 +0100
    Re: Extract the middle N chars of a string Steven D'Aprano <steve@pearwood.info> - 2016-05-19 11:34 +1000
  Re: Extract the middle N chars of a string Random832 <random832@fastmail.com> - 2016-05-18 17:28 -0400
    Re: Extract the middle N chars of a string Steven D'Aprano <steve@pearwood.info> - 2016-05-19 11:03 +1000
  Re: Extract the middle N chars of a string Grant Edwards <grant.b.edwards@gmail.com> - 2016-05-18 21:35 +0000
    Re: Extract the middle N chars of a string Steven D'Aprano <steve@pearwood.info> - 2016-05-19 10:48 +1000
  Re: Extract the middle N chars of a string Wildman <best_lay@yahoo.com> - 2016-05-18 23:17 -0500
  Re: Extract the middle N chars of a string boB Stepp <robertvstepp@gmail.com> - 2016-05-20 23:00 -0500

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