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numpy.interp bug (Trac #1064) #1662
Description
Original ticket http://projects.scipy.org/numpy/ticket/1064 on 2009-03-23 by trac user changimeno, assigned to @charris.
interp(x, xp, fp, left=None, right=None)
the "right"--parameter option does not work properly.
Example:
In [2]: x = arange(4)*2 + 2
In [3]: y=x**2
In [4]: xx = arange(11)
In [5]: x
Out[5]: array([2, 4, 6, 8])
In [6]: y
Out[6]: array([ 4, 16, 36, 64])
In [7]: xx
Out[7]: array([ 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
In [14]: numpy.interp(xx,x,y)
Out[14]: array([ 4., 4., 4., 10., 16., 26., 36., 50., 64., 64., 64.]) #---->>>> RIGHT!!!!
In [15]: numpy.interp(xx,x,y,left=0.)
Out[15]: array([ 0., 0., 4., 10., 16., 26., 36., 50., 64., 64., 64.]) #---->>>> RIGHT!!!!
In [16]: numpy.interp(xx,x,y,left=0.,right=0.)
Out[16]: array([ 0., 0., 4., 10., 16., 26., 36., 50., 0., 0., 0.]) #---->>>> WRONG!!!!
numpy.interp set the vales of the interpolated array (yy) for "xx[i] >= x[len(x)-1]" to the given RIGHT value. So, the last GOOD value (xx[i] == x[len(x)-1]) is wrongly set to the RIGHT value instead of the proper value yy[i] = y[len(x)-1].
I edited the file numpy/lib/src/_compiled_base.c and fixed this bug (see attachment). Now
In [19]: numpy.interp(xx,x,y,0.,0.)
Out[19]: array([ 0., 0., 4., 10., 16., 26., 36., 50., 64., 0., 0.]) #---->>>> RIGHT!!!!
My only worry is that I modified "binary_search" together with "arr_interp" and I do not know if this function is called by other routines and whether this modification can alter the results.
diff
> /* sgg */
> else if (dval > dlist [len-1])
> result = len ;
> /* sgg */
448c454,455
< else if (indx >= lenxp - 1)
---
> /*sgg else if (indx >= lenxp - 1) */
> else if (indx > lenxp - 1)
Cheers,
Chan
Activity
thouis commentedon Oct 19, 2012
Attachment added by trac user changimeno on 2009-03-23: _compiled_base.c
thouis commentedon Oct 19, 2012
@charris wrote on 2009-04-11
Attach a diff, please.
Note that the whole d*mn file could use a cleanup and contains duplicate functionality with some functions in core/src. I think it should eventually be merged with the rest of the c stuff.
thouis commentedon Oct 19, 2012
Attachment added by @charris on 2009-04-11: _compiled_base.patch
thouis commentedon Oct 19, 2012
@charris wrote on 2010-05-24
Should be fixed in r8433.