Mailman 3 Bug in ndarray.argmax - NumPy-Discussion

1 May 2006

      Hello,

I notices a bug in ndarray.argmax which prevent from getting the argmax
from any axis but the last one.
I join a patch to correct this.
Also, here is a small python code to test the behaviour of argmax I
implemented :

==8<====8<====8<====8<====8<====8<====8<====8<===8<===

from numpy import array, random, all

a = random.normal( 0, 1, ( 4,5,6,7,8 ) )
for i in xrange( a.ndim ):
  amax = a.max( i )
  aargmax = a.argmax( i )
  axes = range( a.ndim )
  axes.remove( i )
  assert all( amax == aargmax.choose( *a.transpose( i, *axes ) ) )

==8<====8<====8<====8<====8<====8<====8<====8<===8<===

Pierre

diff numpy-0.9.6/numpy/core/src/multiarraymodule.c numpy-0.9.6.mod/numpy/core/src/multiarraymodule.c
1952a1953,1955
...
If orign > ap->nd, then we cannot "swap it back" 
   as the dimension does not exist anymore. It means 
   the axis must be put back at the end of the array.
1956c1959,1979
< 			(op) = (PyAO *)PyArray_SwapAxes((ap), axis, orign); \

...
int nb_dims = (ap)->nd;				\
      	if (orign > nb_dims-1 ) {			\
      		PyArray_Dims dims;			\
      		int i;					\
      		dims.ptr = ( intp* )malloc( sizeof( intp )*nb_dims );\
      		dims.len = nb_dims;			\
      		for(i = 0 ; i < axis ; ++i)		\
      		{					\
      			dims.ptr[i] = i;		\
      		}					\
      		for(i = axis ; i < nb_dims-1 ; ++i)	\
      		{					\
      			dims.ptr[i] = i+1;		\
      		}					\
      		dims.ptr[nb_dims-1] = axis;		\
      		(op) = (PyAO *)PyArray_Transpose((ap), &dims );	\
      	}						\
      	else						\
      	{						\
      		(op) = (PyAO *)PyArray_SwapAxes((ap), axis, orign); \
      	}						\

Bug in ndarray.argmax

Pierre Barbier de Reuille

Pau Gargallo

tags

participants (2)