iterating in reverse

[Bengt Richter]

On Sat, 18 Jan 2003 20:25:46 GMT, "Neil Hodgson" <nhodgson at bigpond.net.au> wrote:

>Steve Holden:
>
>> Here's my own non-reversing solution. Again, it would need tweaking for
>> negative numbers to avoid results like "-,123".
>>
>>     def commafy(val):
>> ...
>
>   Well we may as well try a one liner (almost):
>
>def commafy(val):
> u = str(val)
> return ''.join([u[i]+(['',',',''][(len(u)-i)%3]) for i in
>range(len(u))])[:-1]
>
>for i in [1, 12, 123, 1234, 12345, 123456, 1234567, 12345678]:
> print i, ":", commafy(i)
>
>   Mmm, Perl envy ...
>

 >>> def commafy(val):
 ...     return reduce(lambda x,c:[x[0]+c+','[:x[1]%3==1],x[1]-1],str(val),['',len(str(val))])[0][:-1]
 ...
 >>> for x in [1,12,123,1234,12345,123456,1234567,12345678]: print commafy(x)
 ...
 1
 12
 123
 1,234
 12,345
 123,456
 1,234,567
 12,345,678

Well, it's a one line expression, but it _is_ ugly.
Couldn't leave it at that. This one's an actual one-liner on my screen ;-)

 >>> def commafy(val): return val<1000 and str(val) or commafy(val/1000)+','+commafy(val%1000)
 ...
 >>> for x in [1,12,123,1234,12345,123456,1234567,12345678]: print commafy(x)
 ...
 1
 12
 123
 1,234
 12,345
 123,456
 1,234,567
 12,345,678

To handle negative numbers is a little longer:

 >>> def cfy(val):
 ...   return val<0 and '-'+cfy(abs(val)) or val<1000 and str(val) or cfy(val/1000)+','+cfy(val%1000)
 ...
 >>> for x in [1,12,123,1234,12345,123456,1234567,12345678]: print '%12s %12s' % (cfy(x),cfy(-x))
 ...
            1           -1
           12          -12
          123         -123
        1,234       -1,234
       12,345      -12,345
      123,456     -123,456
    1,234,567   -1,234,567
   12,345,678  -12,345,678

Regards,
Bengt Richter