howto remove the thousand separator

[Rotwang]

On 15/04/2013 02:14, Steven D'Aprano wrote:
> On Sun, 14 Apr 2013 17:44:28 -0700, Mark Janssen wrote:
>
>> On Sun, Apr 14, 2013 at 5:29 PM, Steven D'Aprano
>> <steve+comp.lang.python at pearwood.info> wrote:
>>> On Sun, 14 Apr 2013 12:06:12 -0700, Mark Janssen wrote:
>>>
>>>> cleaned=''
>>>> for c in myStringNumber:
>>>>     if c != ',':
>>>>       cleaned+=c
>>>> int(cleaned)
>>>
>>> ....due to being an O(N**2)  algorithm.
>>
>> What on earth makes you think that is an O(n**2) algorithm and not O(n)?
>
> Strings are immutable. Consider building up a single string from four
> substrings:
>
> s = ''
> s += 'fe'
> s += 'fi'
> s += 'fo'
> s += 'fum'
>
> Python *might* optimize the first concatenation, '' + 'fe', to just reuse
> 'fe', (but it might not). Let's assume it does, so that no copying is
> needed. Then it gets to the second concatenation, and now it has to copy
> characters, because strings are immutable and cannot be modified in
> place.

Actually, I believe that CPython is optimised to modify strings in place 
where possible, so that the above would surprisingly turn out to be 
O(n). See the following thread where I asked about this:

http://groups.google.com/group/comp.lang.python/browse_thread/thread/990a695fe2d85c52

(Sorry for linking to Google Groups. Does anyone know of a better c.l.p. 
web archive?)