[Numpy-discussion] fromiter shape argument -- was Re: For loop tips

Tim Hochberg tim.hochberg at ieee.org
Wed Aug 30 09:33:25 CDT 2006


Torgil Svensson wrote:
>>    return uL,asmatrix(fromiter((idx[x] for x in L),dtype=int))
>>     
>
> Is it possible for fromiter to take an optional shape (or count)
> argument in addition to the dtype argument? 
Yes. fromiter(iterable, dtype, count) works.

> If both is given it could
> preallocate memory and we only have to iterate over L once.
>   
Regardless, L is only iterated over once. In general you can't rewind 
iterators, so that's a requirement. This is accomplished by doing 
successive overallocation similar to the way appending to a list is 
handled. By specifying the count up front you save a bunch of reallocs, 
but no iteration.

-tim



> //Torgil
>
> On 8/29/06, Keith Goodman <kwgoodman at gmail.com> wrote:
>   
>> On 8/29/06, Torgil Svensson <torgil.svensson at gmail.com> wrote:
>>     
>>> something like this?
>>>
>>> def list2index(L):
>>>    uL=sorted(set(L))
>>>    idx=dict((y,x) for x,y in enumerate(uL))
>>>    return uL,asmatrix(fromiter((idx[x] for x in L),dtype=int))
>>>       
>> Wow. That's amazing. Thank you.
>>
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