numpy-discussion

[Numpy-discussion] how to use argsort result?

[Emanuele O. <oli...@it...>]

Hi,
I don't understand how to use argsort results. I have a 2D matrix and
I want to sort values in each row and obtain the index array of that
sorting. Argsort(1) is what I need, but the problem is how to use its
result in order to obtain a sorted matrix. Here is the simple example:

A = array([[2,3,1],[5,4,6]])
indexes = a.argsort(1)

now indexes is:
array([[2, 0, 1],
        [1, 0, 2]])

I'd like to apply indexes to A and obtain:
array([[1, 2, 3],
        [4, 5, 6]])

or better, I'm interested both in a subset of indexes, i.e. indexes[:,1:], and
the related values of A matrix.

How can I do this? If I simpy say: A[indexes] I get an IndexError.

Thanks in advance,

Emanuele

P.S. numpy.__version__ is '0.9.8'.

Re: [Numpy-discussion] how to use argsort result?

[Pau G. <pau...@gm...>]

here goes a first try:

A[arange(2)[:,newaxis],indexes]


pau

On 7/11/06, Emanuele Olivetti <oli...@it...> wrote:
> Hi,
> I don't understand how to use argsort results. I have a 2D matrix and
> I want to sort values in each row and obtain the index array of that
> sorting. Argsort(1) is what I need, but the problem is how to use its
> result in order to obtain a sorted matrix. Here is the simple example:
>
> A = array([[2,3,1],[5,4,6]])
> indexes = a.argsort(1)
>
> now indexes is:
> array([[2, 0, 1],
>         [1, 0, 2]])
>
> I'd like to apply indexes to A and obtain:
> array([[1, 2, 3],
>         [4, 5, 6]])
>
> or better, I'm interested both in a subset of indexes, i.e. indexes[:,1:], and
> the related values of A matrix.
>
> How can I do this? If I simpy say: A[indexes] I get an IndexError.
>
> Thanks in advance,
>
> Emanuele
>
> P.S. numpy.__version__ is '0.9.8'.
>
>
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Re: [Numpy-discussion] how to use argsort result?

[Emanuele O. <oli...@it...>]

Wow. I have to study much more indexing. It works pretty well.

Just to help indexing newbie like on using your advice:
A[arange(A.shape[0])[:,newaxis],indexes]

Thanks a lot!

Emanuele

Pau Gargallo wrote:
> here goes a first try:
> 
> A[arange(2)[:,newaxis],indexes]

Re: [Numpy-discussion] how to use argsort result?

[Pau G. <pau...@gm...>]

On 7/12/06, Victoria G. Laidler <la...@st...> wrote:
> Hi,
>
> Pardon me if I'm reprising an earlier discussion, as I'm new to the list.
>
> But is there a reason that this obscure syntax
>
> A[arange(2)[:,newaxis],indexes]
>
> A[arange(A.shape[0])[:,newaxis],indexes]
>
> is preferable to the intuitively reasonable thing that the Original
> Poster did?
>
> A[indexes]
>

i don't think so.
The obscure syntax is just a way you can solve the problem with the
current state of NumPy. Of course, a more clearer syntax would be
better, but for this, something in NumPy should be changed.

This other syntax is longer but clearer:
  ind = indices(A.shape)
  ind[ax] = A.argsort(axis=ax)
  A[ind]

Which brings me to the question:

Would it be reasonable if argsort returned the complete tuple of
indices, so that
A[A.argsort(ax)] would work ?

pau

Re: [Numpy-discussion] how to use argsort result?

[Stefan v. d. W. <st...@su...>]

On Tue, Jul 11, 2006 at 11:32:48AM +0200, Emanuele Olivetti wrote:
> Hi,
> I don't understand how to use argsort results. I have a 2D matrix and
> I want to sort values in each row and obtain the index array of that
> sorting. Argsort(1) is what I need, but the problem is how to use its
> result in order to obtain a sorted matrix. Here is the simple example:
>=20
> A =3D array([[2,3,1],[5,4,6]])
> indexes =3D a.argsort(1)
>=20
> now indexes is:
> array([[2, 0, 1],
>         [1, 0, 2]])
>=20
> I'd like to apply indexes to A and obtain:
> array([[1, 2, 3],
>         [4, 5, 6]])
>=20
> or better, I'm interested both in a subset of indexes, i.e. indexes[:,1=
:], and
> the related values of A matrix.
>=20
> How can I do this? If I simpy say: A[indexes] I get an IndexError.

Something's not quite right here.  The argsort docstring states that:

    argsort(a,axis=3D-1) return the indices into a of the sorted array
    along the given axis, so that take(a,result,axis) is the sorted array=
.

But

N.take(A,A.argsort()) breaks.  Either this is a bug, or the docstring
needs to be updated.

Cheers
St=E9fan

Re: [Numpy-discussion] how to use argsort result?

[Pau G. <pau...@gm...>]

On 7/11/06, Stefan van der Walt <st...@su...> wrote:
> On Tue, Jul 11, 2006 at 11:32:48AM +0200, Emanuele Olivetti wrote:
> > Hi,
> > I don't understand how to use argsort results. I have a 2D matrix and
> > I want to sort values in each row and obtain the index array of that
> > sorting. Argsort(1) is what I need, but the problem is how to use its
> > result in order to obtain a sorted matrix. Here is the simple example:
> >
> > A =3D array([[2,3,1],[5,4,6]])
> > indexes =3D a.argsort(1)
> >
> > now indexes is:
> > array([[2, 0, 1],
> >         [1, 0, 2]])
> >
> > I'd like to apply indexes to A and obtain:
> > array([[1, 2, 3],
> >         [4, 5, 6]])
> >
> > or better, I'm interested both in a subset of indexes, i.e. indexes[:,1=
:], and
> > the related values of A matrix.
> >
> > How can I do this? If I simpy say: A[indexes] I get an IndexError.
>
> Something's not quite right here.  The argsort docstring states that:
>
>     argsort(a,axis=3D-1) return the indices into a of the sorted array
>     along the given axis, so that take(a,result,axis) is the sorted array=
.
>
> But
>
> N.take(A,A.argsort()) breaks.  Either this is a bug, or the docstring
> needs to be updated.
>
> Cheers
> St=E9fan
>

I think the docstring is wrong, because take doesn't do that.
if you N.take(A,A.argsort(1), 1), it doesn't break but it doesn't sort
A neither.

Take seems to peek entire columns, but take's docstring is missing.

For the argsort docstring, it may be usefull to indicate that if one do
>>> ind =3D indices(A.shape)
>>> ind[ax] =3D A.argsort(axis=3Dax)
then A[ind] is the sorted array.


pau

Re: [Numpy-discussion] how to use argsort result?

[Stefan v. d. W. <st...@su...>]

On Tue, Jul 11, 2006 at 12:37:23PM +0200, Pau Gargallo wrote:
> > Something's not quite right here.  The argsort docstring states that:
> >
> >     argsort(a,axis=3D-1) return the indices into a of the sorted arra=
y
> >     along the given axis, so that take(a,result,axis) is the sorted a=
rray.
> >
> > But
> >
> > N.take(A,A.argsort()) breaks.  Either this is a bug, or the docstring
> > needs to be updated.
> >
> > Cheers
> > St=E9fan
> >
>=20
> I think the docstring is wrong, because take doesn't do that.
> if you N.take(A,A.argsort(1), 1), it doesn't break but it doesn't sort
> A neither.
>=20
> Take seems to peek entire columns, but take's docstring is missing.
>=20
> For the argsort docstring, it may be usefull to indicate that if one do
> >>> ind =3D indices(A.shape)
> >>> ind[ax] =3D A.argsort(axis=3Dax)
> then A[ind] is the sorted array.

We can always adapt the documentation at

http://numeric.scipy.org/numpydoc/numpy-9.html#pgfId-36425

into a docstring.  I'll file a ticket.

Cheers
St=E9fan

Re: [Numpy-discussion] how to use argsort result?

[Victoria G. L. <la...@st...>]

Hi,

Pardon me if I'm reprising an earlier discussion, as I'm new to the list.

But is there a reason that this obscure syntax

A[arange(2)[:,newaxis],indexes]   

A[arange(A.shape[0])[:,newaxis],indexes]

is preferable to the intuitively reasonable thing that the Original 
Poster did?

A[indexes]

Doesn't it violate the pythonic "principle of least surprise" for the 
simpler syntax not to work?

As a casual numpy user, I can't imagine being able to remember the 
obscure syntax in order to use the result of an argsort.

curiously,
Vicki Laidler
(numarray user who came from IDL)


Emanuele Olivetti wrote:

>Wow. I have to study much more indexing. It works pretty well.
>
>Just to help indexing newbie like on using your advice:
>A[arange(A.shape[0])[:,newaxis],indexes]
>
>Thanks a lot!
>
>Emanuele
>
>Pau Gargallo wrote:
>  
>
>>here goes a first try:
>>
>>A[arange(2)[:,newaxis],indexes]
>>    
>>
>
>
>-------------------------------------------------------------------------
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>Get stuff done quickly with pre-integrated technology to make your job easier
>Download IBM WebSphere Application Server v.1.0.1 based on Apache Geronimo
>http://sel.as-us.falkag.net/sel?cmd=lnk&kid=120709&bid=263057&dat=121642
>_______________________________________________
>Numpy-discussion mailing list
>Num...@li...
>https://lists.sourceforge.net/lists/listinfo/numpy-discussion
>  
>

Re: [Numpy-discussion] how to use argsort result?

[Travis O. <oli...@ie...>]

Pau Gargallo wrote:
> On 7/12/06, Victoria G. Laidler <la...@st...> wrote:
>   
>> Hi,
>>
>> Pardon me if I'm reprising an earlier discussion, as I'm new to the list.
>>
>> But is there a reason that this obscure syntax
>>
>> A[arange(2)[:,newaxis],indexes]
>>
>> A[arange(A.shape[0])[:,newaxis],indexes]
>>
>> is preferable to the intuitively reasonable thing that the Original
>> Poster did?
>>
>> A[indexes]
>>
>>     
>
> i don't think so.
> The obscure syntax is just a way you can solve the problem with the
> current state of NumPy. Of course, a more clearer syntax would be
> better, but for this, something in NumPy should be changed.
>
> This other syntax is longer but clearer:
>   ind = indices(A.shape)
>   ind[ax] = A.argsort(axis=ax)
>   A[ind]
>
> Which brings me to the question:
>
> Would it be reasonable if argsort returned the complete tuple of
> indices, so that
> A[A.argsort(ax)] would work ?
>
>   
I think this is reasonable.  We would need a way for the argsort() 
function to work as it does now.  I'm not sure if anybody actually uses 
the multidimensional behavior of argsort now, but it's been in Numeric 
for a long time.

-Travis

Re: [Numpy-discussion] how to use argsort result?

[Pau G. <pau...@gm...>]

On 7/13/06, Travis Oliphant <oli...@ie...> wrote:
> Pau Gargallo wrote:
> > On 7/12/06, Victoria G. Laidler <la...@st...> wrote:
> >
> >> Hi,
> >>
> >> Pardon me if I'm reprising an earlier discussion, as I'm new to the list.
> >>
> >> But is there a reason that this obscure syntax
> >>
> >> A[arange(2)[:,newaxis],indexes]
> >>
> >> A[arange(A.shape[0])[:,newaxis],indexes]
> >>
> >> is preferable to the intuitively reasonable thing that the Original
> >> Poster did?
> >>
> >> A[indexes]
> >>
> >>
> >
> > i don't think so.
> > The obscure syntax is just a way you can solve the problem with the
> > current state of NumPy. Of course, a more clearer syntax would be
> > better, but for this, something in NumPy should be changed.
> >
> > This other syntax is longer but clearer:
> >   ind = indices(A.shape)
> >   ind[ax] = A.argsort(axis=ax)
> >   A[ind]
> >
> > Which brings me to the question:
> >
> > Would it be reasonable if argsort returned the complete tuple of
> > indices, so that
> > A[A.argsort(ax)] would work ?
> >
> >
> I think this is reasonable.  We would need a way for the argsort()
> function to work as it does now.  I'm not sure if anybody actually uses
> the multidimensional behavior of argsort now, but it's been in Numeric
> for a long time.
>

actually I use the multidimensional behavior of argmin and argmax in
its current state, and found it useful as it is, even if
A[A.argmax(ax)] doesn't work.

May be a keyword could be added, so that A.argxxx( axis=ax,
indices=True ) returns the tuple of indices. (The keyword name and
default should be discussed)

I don't know if that's *the* way, but ...

pau

Re: [Numpy-discussion] how to use argsort result?

[Eric F. <ef...@ha...>]

> 
> Would it be reasonable if argsort returned the complete tuple of
> indices, so that
> A[A.argsort(ax)] would work ?

+1

This is the behavior one would naturally expect.

Eric