[Numpy-discussion] Simple problem. Is it possible without a loop?

[Bruce Southey]

/
> >/  Given a certain value delta, I would like to get a subset of x, named
> />/  y,
> />/  where (y[i+1] - y[i])>= delta
> /
> So in fact the problem is to find y such that
>
> (y[i(k)+n] - y[i(k)])>= delta
> for n<= len(x) - 1 - i
> and i(0) = 0, i(k+1) = i(k) + n
>
> ? Well a loop or list comparison seems like a good choice to me. It is
> much more obvious at the expense of two LOCs. Did you profile the two
> possibilities and are they actually performance-critical?
>
> cheers
>
> Am Mittwoch, den 09.06.2010, 10:14 +0200 schrieb "V. Armando Solé":
> >/  That was my first thought, but that only warrants me to skip one point
> />/  in x but not more than one.
> />/
> />/   >>>  x= numpy.arange(10.)
> />/   >>>  delta = 3
> />/   >>>  print x[(x[1:] - x[:-1])>= delta]
> />/  []
> />/
> />/  instead of the requested [0, 4, 8]
> />/
> />/  Armando
> />/
> />/  Francesc Alted wrote:
> />/  >  A Wednesday 09 June 2010 10:00:50 V. Armando Solé escrigué:
> />/  >
> />/  >>  Well, this seems to be quite close to what I need
> />/  >>
> />/  >>  y = numpy.cumsum((x[1:]-x[:-1])/delta).astype(numpy.int)
> />/  >>  i1 = numpy.nonzero(y[1:]>  y[:-1])
> />/  >>  y = numpy.take(x, i1)
> />/  >>
> />/  >
> />/  >  Perhaps this is a bit shorter:
> />/  >
> />/  >  y = x[(x[1:] - x[:-1])>= delta]
> />/  >
> />/  >
> />/
> />/
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/

Playing around with range/arange can be misleading as
 >> x[x%4==0]
array([ 0.,  4.,  8.])

I don't know you really want because your first code
 >>> x= numpy.arange(10.)
 >>> delta=3
 >>> y=[x[0]]
 >>> for value in x:
...     if (y[-1] -value) < delta:
...        y.append(value)
...
 >>> y
[0.0, 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0]

Which is not [0, 4, 8].

Bruce


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