Simple List division problem

[thebjorn]

On Jan 13, 2:02 pm, Fredrik Lundh <fred... at pythonware.com> wrote:
> thebjorn wrote:
>
>  > Eh...
>
> oh, forgot that it was "pulling requirements out of thin air" week on
> c.l.python.

Well, the OP requirements were to control the number of chunks, not
the size of them, so I guess we both got it wrong initially.

> > def chop(lst, length):
> >     n = len(lst) / length
> >     z = [lst[i:i+n] for i in xrange(0, len(lst), n)]
> >     if len(z[-1]) < n and len(z) > 1:
> >          z[-2].extend(z.pop(-1))
> >     return z
>
> > gives
> >>>> chop([1], 3)
> > Traceback (most recent call last):
> >   File "<stdin>", line 1, in <module>
> >   File "beforemeth.py", line 9, in chop
> >     if len(z[-1]) < n and len(z) > 1:
> > ValueError: xrange() arg 3 must not be zero
>
> well, it doesn't.  there's no xrange on that line.

It's from this line

    z = [lst[i:i+n] for i in xrange(0, len(lst), n)]

(I executed the file with python -i beforemeth.py to get to an
interactive prompt, I'm sure you're familiar with the technique. You
could have just debugged your own program to find it though, or just
looked at the code -- not that many xrange calls in there, eh?)

>  > Perhaps something like this?
>
>  > from itertools import islice
>
> or just use an if-statement, or the max function.  but I guess those
> tools are too old and boring for c.l.python these days...

I didn't realize correct code was too new-school. Perhaps you should
publish a list of modules you don't like, or perhaps I should just use
a sufficiently venerable version of Python? Ok, here you go:

C:\Python22>python
'import site' failed; use -v for traceback
Python 2.2.3 (#42, May 30 2003, 18:12:08) [MSC 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> def chop2(lst, nchunks):
...     chunksize = len(lst) // nchunks
...     if not chunksize:
...         raise ValueError('More chunks than elements in list.')
...     res = []
...     begin, end = 0, chunksize
...     for i in range(nchunks-1):
...         res.append(lst[begin:end])
...         begin, end = end, end+chunksize
...     res.append(lst[begin:])
...     return res
...
>>> chop2(range(1,6), 2)
[[1, 2], [3, 4, 5]]
>>> chop2(range(1,6), 3)
[[1], [2], [3, 4, 5]]
>>> chop2(range(1,11), 3)
[[1, 2, 3], [4, 5, 6], [7, 8, 9, 10]]
>>>

Sufficiently old-school (or do I need to take out the // division
also?)

> </F>

Shall we perhaps drop some of the attitude? (you used to be so much
nicer before you wrote sre ;-)

-- bjorn