[Numpy-discussion] genfromtxt converter question

Olivier Delalleau shish@keba...
Fri Jun 17 08:51:02 CDT 2011


2011/6/17 Bruce Southey <bsouthey@gmail.com>

> On 06/17/2011 08:22 AM, gary ruben wrote:
> > Thanks Olivier,
> > Your suggestion gets me a little closer to what I want, but doesn't
> > quite work. Replacing the conversion with
> >
> > c = lambda x:np.cast[np.complex64](complex(*eval(x)))
> > b = np.genfromtxt(a,converters={0:c, 1:c, 2:c,
> > 3:c},dtype=None,delimiter=18,usecols=range(4))
> >
> > produces
> >
> > [[(-3.97000002861-5.03999996185j) (-1.1318000555-2.56929993629j)
> >   (-4.60270023346-0.142599999905j) (-1.42490005493+1.73300004005j)]
> >   [(-5.4797000885+0j) (1.85850000381-1.5501999855j)
> >   (4.41450023651-0.763800024986j) (-0.480500012636-1.19760000706j)]
> >   [0j (6.26730012894+0j) (-0.45039999485-0.0289999991655j)
> >   (-1.34669995308+1.65789997578j)]
> >   [0j 0j (-3.5+0j) (2.56189990044-3.37080001831j)]]
> >
> > which is not yet an array of complex numbers. It seems close to the
> > solution though.
> >
> > Gary
> >
> > On Fri, Jun 17, 2011 at 8:40 PM, Olivier Delalleau<shish@keba.be>
>  wrote:
> >> If I understand correctly, your error is that you convert only the
> second
> >> column, because your converters dictionary contains a single key (1).
> >> If you have it contain keys from 0 to 3 associated to the same function,
> it
> >> should work.
> >>
> >> -=- Olivier
> >>
> >> 2011/6/17 gary ruben<gruben@bigpond.net.au>
> >>> I'm trying to read a file containing data formatted as in the
> >>> following example using genfromtxt and I'm doing something wrong. It
> >>> almost works. Can someone point out my error, or suggest a simpler
> >>> solution to the ugly converter function? I thought I'd leave in the
> >>> commented-out line for future reference, which I thought was a neat
> >>> way to get genfromtxt to show what it is trying to pass to the
> >>> converter.
> >>>
> >>> import numpy as np
> >>> from StringIO import StringIO
> >>>
> >>> a = StringIO('''\
> >>>   (-3.9700,-5.0400) (-1.1318,-2.5693) (-4.6027,-0.1426) (-1.4249,
> 1.7330)
> >>>   (-5.4797, 0.0000) ( 1.8585,-1.5502) ( 4.4145,-0.7638)
> (-0.4805,-1.1976)
> >>>   ( 0.0000, 0.0000) ( 6.2673, 0.0000) (-0.4504,-0.0290) (-1.3467,
> 1.6579)
> >>>   ( 0.0000, 0.0000) ( 0.0000, 0.0000) (-3.5000, 0.0000) (
> 2.5619,-3.3708)
> >>> ''')
> >>>
> >>> #~ b = np.genfromtxt(a,converters={1:lambda
> >>> x:str(x)},dtype=object,delimiter=18)
> >>> b = np.genfromtxt(a,converters={1:lambda
> >>> x:complex(*eval(x))},dtype=None,delimiter=18,usecols=range(4))
> >>>
> >>> print b
> >>>
> >>> --
> >>>
> >>> This produces
> >>> [ (' (-3.9700,-5.0400)', (-1.1318-2.5693j), ' (-4.6027,-0.1426)', '
> >>> (-1.4249, 1.7330)')
> >>>   (' (-5.4797, 0.0000)', (1.8585-1.5502j), ' ( 4.4145,-0.7638)', '
> >>> (-0.4805,-1.1976)')
> >>>   (' ( 0.0000, 0.0000)', (6.2673+0j), ' (-0.4504,-0.0290)', ' (-1.3467,
> >>> 1.6579)')
> >>>   (' ( 0.0000, 0.0000)', 0j, ' (-3.5000, 0.0000)', ' (
> 2.5619,-3.3708)')]
> >>>
> >>> which I just need to unpack into a 4x4 array, but I get an error if I
> >>> try to apply a different view.
> >>>
> >>> thanks,
> >>> Gary
> >>> _______________________________________________
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> >>> http://mail.scipy.org/mailman/listinfo/numpy-discussion
> >>
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> >>
> >>
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> Just an observation for the StringIO object, you have multiple spaces
> within the parentheses but, by default, you are using whitespace
> delimiters in genfromtxt. So, yes, genfromtxt is going have issues.
>
> If you can rewrite the input, then you need a non-space and non-comma
> delimiter then specify that delimiter to genfromtxt. Otherwise you are
> probably going to have to write you own parser - for each line, split on
> ' (' etc.
>
> Bruce


It's funny though because that part (the parsing) actually seems to work.

However I've been playing a bit with his example and indeed I can't get
numpy to return a complex array. It keeps resulting in an "object" dtype.
The only way I found was to convert it temporarily into a list to recast it
in complex64, adding the line:
  b = np.array(map(list, b), dtype=np.complex64)

-=- Olivier
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