[Numpy-tickets] [NumPy] #387: Small polynomial bug (with patch suggestion)

[NumPy]

#387: Small polynomial bug (with patch suggestion)
----------------------+-----------------------------------------------------
 Reporter:  clovisgo  |       Owner:  somebody   
     Type:  defect    |      Status:  new        
 Priority:  normal    |   Milestone:  1.0 Release
Component:  Other     |     Version:             
 Severity:  normal    |    Keywords:             
----------------------+-----------------------------------------------------
 {{{
 Subject: Small bug in polynomial
 There seems to exist a small bug in the numpy.polynomial functions:
 Consider the following script:

 ################################
 import numpy
 poly1 = numpy.poly1d([1.0])
 poly2 = numpy.poly1d([1.0, 1.0])
 poly3 = numpy.poly1d([2.0,1.0])
 print "Poly1 is"
 print poly1
 print type(poly1)
 print "Poly2 is"
 print poly2
 print type(poly2)
 print "Poly3 is"
 print poly3
 print type(poly3)
 print "Poly1+Poly2 = "
 auxvar = numpy.polyadd(poly1,poly2)
 print auxvar
 print type(auxvar)
 print "Poly2+Poly3 = "
 auxvar = numpy.polyadd(poly2,poly3)
 print auxvar
 print type(auxvar)
 ################################

 Raw output is given below:

 Poly1 is
 1
 <class 'numpy.lib.polynomial.poly1d'>
 Poly2 is
 1 x + 1
 <class 'numpy.lib.polynomial.poly1d '>
 Poly3 is
 2 x + 1
 <class 'numpy.lib.polynomial.poly1d'>
 Poly1+Poly2 =
 1 x + 2
 <class 'numpy.lib.polynomial.poly1d'>
 Poly2+Poly3 =
 [ 3.  2.]
 <type 'numpy.ndarray'>

 ################################
 Comments. The last test (Poly2+Poly3) is bad!
 The result shoud be:
 3 x + 2
 and the type should be
 <class 'numpy.lib.polynomial.poly1d'>
 and not:
 <type ' numpy.ndarray'>

 The same problem exists with polysub.
 The solution seems to be simple. The original
 code for the polyadd function (with line numbers) is:

 1   def polyadd(a1, a2):
 2       """Adds two polynomials represented as sequences
 3       """
 4       truepoly = (isinstance(a1, poly1d) or isinstance(a2, poly1d))
 5       a1 = atleast_1d(a1)
 6       a2 = atleast_1d(a2)
 7       diff = len(a2) - len(a1)
 8       if diff == 0:
 9           return a1 + a2
 10       elif diff > 0:
 11          zr = NX.zeros(diff, a1.dtype)
 12          val = NX.concatenate((zr, a1)) + a2
 13       else:
 14          zr = NX.zeros(abs(diff), a2.dtype)
 15          val = a1 + NX.concatenate((zr, a2))
 16      if truepoly:
 17          val = poly1d(val)
 18      return val

 In order to correct the problem is sufficient
 to replace the original line #9 by:

 9        val = a1 + a2

 If this change is made, the return value
 will be an array (if input variables are
 arrays) or a poly1d (if one of the input
 variables are polynomials).

 A similar change should be made for the polysub function.
 Clovis
 }}}

-- 
Ticket URL: <http://projects.scipy.org/scipy/numpy/ticket/387>
NumPy <http://projects.scipy.org/scipy/numpy>
The fundamental package needed for scientific computing with Python.