inserting period after every 3 chars in a string

Question

I have this:

from __future__ import print_function

def f_comma(p_string):
   v_string = p_string
   if (type(v_string) == type(int()) or type(v_string) == type(long()) or  
       type(v_string) == type(float())):
      v_string = str(v_string)
   else:   
      l_string = list(v_string)
      for v_index in range(3, len(l_string), 4):
         l_string.insert(v_index, ',')
      v_result = ''.join(l_string)
   return (v_result)

print (f_comma('qwertyuiopaq'))

It seems that i can't figure it out why if i use a string longer than 11 chars the period stops inserting, but with only 11 chars it works fine. What i'm doing wrong in this piece?

Answer 1

You can insert a comma after every nth character like this:

>>> my_str = 'qwertyuiopaq'
>>> ','.join(my_str[i:i+3] for i in range(0, len(my_str), 3))
'qwe,rty,uio,paq'

This should work for any arbitrary length of strings too.

Edit: Written as a function in a similar style to @mhawke's answer, with an option to change the grouping/characters.

>>> def f_comma(my_str, group=3, char=','):
...     my_str = str(my_str)
...     return char.join(my_str[i:i+group] for i in range(0, len(my_str), group))
... 
>>> f_comma('qwertyuiopaq')
'qwe,rty,uio,paq'
>>> f_comma('qwertyuiopaq', group=2)
'qw,er,ty,ui,op,aq'
>>> f_comma('qwertyuiopaq', group=2, char='.')
'qw.er.ty.ui.op.aq'

Answer 2

Here is an alternative way to do it using slicing:

def f_comma(p_string, n=3):
    return ','.join(p_string[i:i+n] for i in range(0, len(p_string), n))

I don't think that the type checking in your version is necessary. Your code checks for instances of an int, long or float and then converts any of these to a string. You can just convert to a string without checking the type:

def f_comma(p_string, n=3):
    p_string = str(p_string)
    return ','.join(p_string[i:i+n] for i in range(0, len(p_string), n))

>>> f_comma('abcdefghijklmnop')
'abc,def,ghi,jkl,mno,p'
>>> f_comma(1234567890)
'123,456,789,0'
>>> import math
>>> f_comma(math.pi)
'3.1,415,926,535,9'

Now this won't handle all unicode strings:

>>> f_comma(u'abcdefg\u3030\u3031\u3032\u3033')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in f_comma
UnicodeEncodeError: 'ascii' codec can't encode characters in position 7-10: ordinal not in range(128)

Here you can use isinstance() (preferable to type() ==) to aid conversion of non-string types:

def f_comma(p_string, n=3):
    if not isinstance(p_string, basestring):    # str or unicode
        p_string = str(p_string)                # convert only non-strings
    return ','.join(p_string[i:i+n] for i in range(0, len(p_string), n))

>>> f_comma(u'abcdefg\u3030\u3031\u3032\u3033')    # returns unicode
u'abc,def,g\u3030\u3031,\u3032\u3033'
>>> f_comma('wowwowwowwow')                        # returns str
'wow,wow,wow,wow'
>>> f_comma(math.pi)                               # returns str
'3.1,415,926,535,9'

Also notice the use of a default argument to specify the segment length:

>>> f_comma('abcdefghijklmnop')
u'abc,def,ghi,jkl,mno,p'
>>> f_comma('abcdefghijklmnop', 6)
u'abcdef,ghijkl,mnop'

Answer 3

Here's why it doesn't work. (Rather than solving your method which is a pretty inefficient one, as others have shown.)

When you .insert() something into your list, every element gets shifted forward a position to make room.

The indexes you calculated earlier with range(3, len(l_string), 4) are then no longer what you want them to be.