numpy-discussion

[Numpy-discussion] suggestions for Matrix-related changes

[JJ <jos...@ya...>]

Hello.  For what its worth, as a newly ex-matlab user I would like to make a few
suggestions on use of matrices in numpy.  As per earlier discussions, I like the
idea of being able to choose matrices as the default (vs arrays).  But if
possible, it would be nice if all functions etc that took matrices also returned
matrices.  I know effort has been made on this.  Here are my suggestions:

1) is it possible to get the function unique() to work with matrices, perhaps
with a unique_rows() function to work with matrices of more than one column?

2) It would be very convienient to have some simple way to delete selected
columns of a matrix.  For example, in matlab the command is X[:,[3,5,7]]=[] to
delete the three selected columns.  It would be nice if such a command would
also work with selections, as in X[:,A[0,:]<4] = [], where X and A are matrices.
 For me, the single most frustrating and time-consuming aspect of switching to
and using numpy is determing how to select columns/rows of arrays/matrices in
different situations.  In addition to being time consuming to figure out
(relative to Matlab), often the code is quite verbose.  I have made a few
suggestions on this topic in an earlier post (under the title "Whats wrong with
matrices?").

3) It would be nice if matrices could be used for iterations.  For example, if M
was a 1 x n matrix, it would be nice to be able to use:  for i in M:  and
iterate over the individual items in M.  

4) It would be nice if the linear algebra package and other packages returned
matrices if given matrices.  For example, if M is a matrix, svd(M) now returns
arrays.

Just some suggestions.  I wish I knew more so I could help implement them. 
Maybe one day.
JJ

Re: [Numpy-discussion] suggestions for Matrix-related changes

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

JJ wrote:

>Hello.  For what its worth, as a newly ex-matlab user I would like to make a few
>suggestions on use of matrices in numpy.  As per earlier discussions, I like the
>idea of being able to choose matrices as the default (vs arrays).  But if
>possible, it would be nice if all functions etc that took matrices also returned
>matrices.  I know effort has been made on this.  Here are my suggestions:
>
>1) is it possible to get the function unique() to work with matrices, perhaps
>with a unique_rows() function to work with matrices of more than one column?
>
>2) It would be very convienient to have some simple way to delete selected
>columns of a matrix.  
>
This is a good idea.  It would be nice to address it at some point.  
There is a Python syntax for it, but we are not using it yet:

del X[...]

Of course one of the problems with this syntax (as opposed to a function 
that returns a new array) is that because X can share it's data with 
other arrays, you can't just re-size it's memory or other arrays 
depending on that chunk of memory will be in deep trouble.

So, we are probably not going to be able to have a "syntax-style" delete. 

But, some kind of function that returns an array with specific entries 
deleted would be nice.

>3) It would be nice if matrices could be used for iterations.  For example, if M
>was a 1 x n matrix, it would be nice to be able to use:  for i in M:  and
>iterate over the individual items in M.  
>  
>
They can be used as iterators.  The problem here is simply convention 
(rows are iterated over first).  We could over-ride the iterator 
behavior of matrices, though to handle 1xn and nx1 matrices identically 
if that is desirable.

>4) It would be nice if the linear algebra package and other packages returned
>matrices if given matrices.  For example, if M is a matrix, svd(M) now returns
>  
>
Svd returns matrices now.  Except for the list of singular values which 
is still an array.  Do you want a 1xn matrix instead of an array?

-Travis

Re: [Numpy-discussion] suggestions for Matrix-related changes

[Keith G. <kwg...@gm...>]

On 7/11/06, Travis Oliphant <oli...@ee...> wrote:
> JJ wrote:
> >4) It would be nice if the linear algebra package and other packages returned
> >matrices if given matrices.  For example, if M is a matrix, svd(M) now returns
> >
> >
> Svd returns matrices now.  Except for the list of singular values which
> is still an array.  Do you want a 1xn matrix instead of an array?

That sounds good to me.

The same goes for eig and eigh:

>> eigval,eigvec = linalg.eig(rand(2,2))

>> eigval
array([-0.06035002,  0.14320639])

>> eigvec

matrix([[ 0.54799954, -0.83647863],
        [-0.83647863, -0.54799954]])

Re: [Numpy-discussion] suggestions for Matrix-related changes

[JJ <jos...@ya...>]

Travis Oliphant <oliphant <at> ee.byu.edu> writes:
 
> But, some kind of function that returns an array with specific 
> entries deleted would be nice.

I agree.  This would be just fine.


> We could over-ride the iterator 
> behavior of matrices, though to handle 1xn and nx1 matrices 
> identically if that is desirable.

I had tried this iteration on a month-old version of numpy and it did 
not work. I guess this now has been changed.  I just updated my copy but 
have not yet tried it.  An over-ride might be nice.  But just off the 
topic, could you get a matrix of real numbers such as A= [[1.0 2.0,3.0]]
to be used to select rows/colums as in B[:,A]?  I guess this would 
require a hidden conversion to integers, as well as code to handle 
selection using a matrix.


> Svd returns matrices now.  Except for the list of singular values 
> which is still an array.  Do you want a 1xn matrix instead of an 
> array?

I had just tried this with my new version of numpy, but I had used svd 
as follows:
import scipy.linalg as la
res = la.svd(M)
That returned arrays, but I see that using:
res = linalg.svd(M)
returns matrices.  Apparently, both numpy and scipy have linalg 
packages, which differ.  I did not know that.  Whoops.

Re: [Numpy-discussion] suggestions for Matrix-related changes

[Sven S. <sve...@gm...>]

JJ schrieb:
> Travis Oliphant <oliphant <at> ee.byu.edu> writes:

> 
>> Svd returns matrices now.  Except for the list of singular values 
>> which is still an array.  Do you want a 1xn matrix instead of an 
>> array?

Although I'm a matrix supporter, I'm not sure here. Afaics the pro
argument is to have *everything* a matrix when you're in that camp. Fair
enough. But then it's already not clear if you want a row or a column,
and you carry an extra dimension around, which is sometimes annoying
e.g. for cumulation of the values, which I do a lot (for eigenvalues,
that is). So for my personal use I came to the conclusion that the
status quo of numpy (array for the value list, matrix for the decomp) is
just fine.

So maybe the people in favor of values-in-1xn-matrices can tell why they
need to matrix-multiply the value array afterwards, because that's the
only benefit I can see here.

> 
> I had just tried this with my new version of numpy, but I had used svd 
> as follows:
> import scipy.linalg as la
> res = la.svd(M)
> That returned arrays, but I see that using:
> res = linalg.svd(M)
> returns matrices.  Apparently, both numpy and scipy have linalg 
> packages, which differ.  I did not know that.  Whoops.
> 

I'm trying to get by with numpy (good that kron was brought over!), but
eventually I will need scipy -- I was hoping that all the matrix
discussion in the numpy list implicitly applied to scipy as well. Is
that not true?

Cheers,
Sven

Re: [Numpy-discussion] suggestions for Matrix-related changes

[Michael S. <mic...@gm...>]

On 7/12/06, JJ <jos...@ya...> wrote:

> 2) It would be very convienient to have some simple way to delete selected
> columns of a matrix.  For example, in matlab the command is X[:,[3,5,7]]=[] to
> delete the three selected columns.  It would be nice if such a command would
> also work with selections, as in X[:,A[0,:]<4] = [], where X and A are matrices.

+1.
In R negative integers are used for this purpose and a copy of the
array is returned. e.g.
> x = 1:10
> x
 [1]  1  2  3  4  5  6  7  8  9 10
> x[-1]
[1]  2  3  4  5  6  7  8  9 10
> x[c(-1,-2)]
[1]  3  4  5  6  7  8  9 10
> x[-c(1,2)]
[1]  3  4  5  6  7  8  9 10

I like the current use of negative indices in numpy, but I do find
myself missing the ability to easily make a copy of the array without
certain indices.

Mike

Re: [Numpy-discussion] suggestions for Matrix-related changes

[Johannes L. <a.u...@gm...>]

On Wednesday 12 July 2006 02:07, Michael Sorich wrote:
> On 7/12/06, JJ <jos...@ya...> wrote:
> > 2) It would be very convienient to have some simple way to delete
> > selected columns of a matrix.  For example, in matlab the command is
> > X[:,[3,5,7]]=[] to delete the three selected columns.  It would be nice
> > if such a command would also work with selections, as in X[:,A[0,:]<4] =
> > [], where X and A are matrices.
> (...)
> I like the current use of negative indices in numpy, but I do find
> myself missing the ability to easily make a copy of the array without
> certain indices.

Maybe use complex numbers?

draft:
>>> a = arange(10)
>>> print a[ 3j ]
 [ 0 1 2 4 5 6 7 8 9 ]

Johannes

Re: [Numpy-discussion] suggestions for Matrix-related changes

[David H. <dav...@gm...>]

2006/7/11, JJ <jos...@ya...>:
>
> 1) is it possible to get the function unique() to work with matrices,
> perhaps
> with a unique_rows() function to work with matrices of more than one
> column?


The problem is that applying unique to different rows will return vectors
with different lengths. So you could not return an array, much less a
matrix. You'd have to return a list of arrays or 1D matrices.

David

Re: [Numpy-discussion] suggestions for Matrix-related changes

[Keith G. <kwg...@gm...>]

On 7/11/06, David Huard <dav...@gm...> wrote:
>
>
> 2006/7/11, JJ <jos...@ya...>:
> > 1) is it possible to get the function unique() to work with matrices,
> perhaps
> > with a unique_rows() function to work with matrices of more than one
> column?
>
>
> The problem is that applying unique to different rows will return vectors
> with different lengths. So you could not return an array, much less a
> matrix.

...unless it returned the unique rows instead of the unique elements
in each row. So if the matrix is

1 2 2
3 4 5
1 2 2

then the unique rows would be

1 2 2
3 4 5

Re: [Numpy-discussion] suggestions for Matrix-related changes

[JJ <jos...@ya...>]

 
> ...unless it returned the unique rows instead of the
> unique elements
> in each row. So if the matrix is
> 
> 1 2 2
> 3 4 5
> 1 2 2
> 
> then the unique rows would be
> 
> 1 2 2
> 3 4 5
> 
Hello Keith.  Yes, that is what I mean also.
JJ

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