Re: [Numpy-discussion] Getting the indexes of the myarray.min()

[Robert K. <rk...@uc...>]

Perry Greenfield wrote:

> =C1lvaro Tejero Cantero wrote:
>=20
>>It works great... but what about efficiency? If I do times.min() and
>>then numarray.nd_image.minimum_positioan(times) I am running twice
>>essentially the same extremum-finding routine, which is prohibitibe for
>>large N..., am I right?
>>
>=20
> Well, yes. But when you ask to find all the things that equal
> the minimum, you pretty much must look twice (if you want to know
> where they all are if more than one). Once to determine the
> minimum, the next time to locate all of them.

Nah, you can accumulate indices corresponding to the current minimum=20
value as you go. Discard the list of indices and start again if you get=20
a new minimum value.

minval =3D data[0]  # let's suppose data is a vector for demonstration
minindices =3D [0]

for i in xrange(1, len(data)):
     x =3D data[i]
     if x < minval:
         minindices =3D [i]
         minval =3D x
     elif x =3D=3D minval:
         minindices.append(i)

Whether or not this is faster (when implemented in C) than going over it=20
twice using numarray functions is another question. My guess: not enough.

[snip]

>>Yes... although for the problem at hand that motivated my query, my
>>times matrix is symmetric... I don't really need all the minima, but
>>does numarray have any special datatype for symmetric matrixes, that
>>prevents storage of unneded (e.g. supradiagonal) elements?.
>>
>=20
> Not for special cases like this. One could probably write a special
> subclass to do this, but for a savings of a factor of 2 in memory,
> it usually would not be worth the trouble (unlike sparse matrices)

OTOH, having subclasses that follow LAPACK's symmetric packed storage=20
scheme would be very useful not because of the space factor but the time=20
saved by being able to use the symmetric algorithms in LAPACK. I think.

> Perry

--=20
Robert Kern
rk...@uc...

"In the fields of hell where the grass grows high
  Are the graves of dreams allowed to die."
   -- Richard Harter