numpy-discussion

[Numpy-discussion] What am I missing about concatenate?

[Christopher B. <Chr...@no...>]

I want to take two (2,) arrays and put them together into one (2,2) 
array. I thought one of these would work:

 >>> N.concatenate(((1,2),(3,4)),0)
array([1, 2, 3, 4])
 >>> N.concatenate(((1,2),(3,4)),1)
array([1, 2, 3, 4])

Is this the best I can do?

 >>> N.concatenate(((1,2),(3,4))).reshape(2,2)
array([[1, 2],
        [3, 4]])

Is it because the arrays I'm putting together are rank-1?

 >>> N.__version__
'0.9.6'

-Chris





-- 
Christopher Barker, Ph.D.
Oceanographer
                                     		
NOAA/OR&R/HAZMAT         (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chr...@no...

[Numpy-discussion] Re: What am I missing about concatenate?

[Robert K. <rob...@gm...>]

Christopher Barker wrote:
> I want to take two (2,) arrays and put them together into one (2,2)
> array. I thought one of these would work:
> 
>>>> N.concatenate(((1,2),(3,4)),0)
> array([1, 2, 3, 4])
>>>> N.concatenate(((1,2),(3,4)),1)
> array([1, 2, 3, 4])
> 
> Is this the best I can do?
> 
>>>> N.concatenate(((1,2),(3,4))).reshape(2,2)
> array([[1, 2],
>        [3, 4]])
> 
> Is it because the arrays I'm putting together are rank-1?

Yes.

Look at vstack() (and also its friends hstack(), dstack() and column_stack() for
completeness).

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco

Re: [Numpy-discussion] Re: What am I missing about concatenate?

[Christopher B. <Chr...@no...>]

Thanks all,


Robert Kern wrote:
> Look at vstack() (and also its friends hstack(), dstack() and column_stack() for
> completeness).

I like this, but need to keep Numeric/numarray compatibility for the 
moment -- I think, I've just sent out a query to my users.



Tim Hochberg wrote:
> If you are using real arrays, use newaxis:
> 
>     >>> a
>    array([0, 1, 2])
>     >>> b
>    array([3, 4, 5])
>     >>> concatenate([a[newaxis], b[newaxis]], 0)
>    array([[0, 1, 2],
>           [3, 4, 5]])

I like this, but again, not in Numeric -- I really need to dump that as 
soon as I can!

> hate newaxis, wrap the arrays in [] to give them an extra dimension. 
> This tends to look nicer, but I suspect has poorer performance than 
> above (haven't timed it though):
> 
>     >>> concatenate([[a], [b]], 0)
>    array([[0, 1, 2],
>           [3, 4, 5]])

Lovely. much cleaner.

By they way, wouldn't wrapping in a tuple, be slightly better, 
performance-wise (I know, probably negligible, but I always feel that I 
should use a tuple when I don't need mutability)

-thanks,

-chris




-- 
Christopher Barker, Ph.D.
Oceanographer
                                     		
NOAA/OR&R/HAZMAT         (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chr...@no...

Re: [Numpy-discussion] Re: What am I missing about concatenate?

[Tim H. <tim...@co...>]

Christopher Barker wrote:

> Thanks all,
>
>
> Robert Kern wrote:
>
>> Look at vstack() (and also its friends hstack(), dstack() and 
>> column_stack() for
>> completeness).
>
>
> I like this, but need to keep Numeric/numarray compatibility for the 
> moment -- I think, I've just sent out a query to my users.
>
>
>
> Tim Hochberg wrote:
>
>> If you are using real arrays, use newaxis:
>>
>>     >>> a
>>    array([0, 1, 2])
>>     >>> b
>>    array([3, 4, 5])
>>     >>> concatenate([a[newaxis], b[newaxis]], 0)
>>    array([[0, 1, 2],
>>           [3, 4, 5]])
>
>
> I like this, but again, not in Numeric -- I really need to dump that 
> as soon as I can!


In Numeric, you can use NewAxis instead for the same effect.

>
>> hate newaxis, wrap the arrays in [] to give them an extra dimension. 
>> This tends to look nicer, but I suspect has poorer performance than 
>> above (haven't timed it though):
>>
>>     >>> concatenate([[a], [b]], 0)
>>    array([[0, 1, 2],
>>           [3, 4, 5]])
>
>
> Lovely. much cleaner.
>
> By they way, wouldn't wrapping in a tuple, be slightly better, 
> performance-wise (I know, probably negligible, but I always feel that 
> I should use a tuple when I don't need mutability)

I doubt it would make a signifigant difference and the square brackets 
are much easier to read IMO. Your mileage may vary.

-tim

Re: [Numpy-discussion] What am I missing about concatenate?

[Alexandre F. <ale...@lo...>]

On Thu, Jun 01, 2006 at 11:32:06AM -0700, Christopher Barker wrote:
> I want to take two (2,) arrays and put them together into one (2,2)=20
> array. I thought one of these would work:
>=20
> >>> N.concatenate(((1,2),(3,4)),0)
> array([1, 2, 3, 4])
> >>> N.concatenate(((1,2),(3,4)),1)
> array([1, 2, 3, 4])
>=20
> Is this the best I can do?
>=20
> >>> N.concatenate(((1,2),(3,4))).reshape(2,2)
> array([[1, 2],
>        [3, 4]])
>=20
> Is it because the arrays I'm putting together are rank-1?

concatenate is not meant to do that. Try putting your arrays in a list
and building an array from that list.=20

a1 =3D array([1,2])
a2 =3D array([3,4])

print array([a1, a2])

/bin/bash: q: command not found

--=20
Alexandre Fayolle                              LOGILAB, Paris (France)
Formations Python, Zope, Plone, Debian:  http://www.logilab.fr/formations
D=E9veloppement logiciel sur mesure:       http://www.logilab.fr/services
Informatique scientifique:               http://www.logilab.fr/science

Re: [Numpy-discussion] What am I missing about concatenate?

[Alan G I. <ai...@am...>]

On Thu, 01 Jun 2006, Christopher Barker apparently wrote:=20
> Is this the best I can do?=20
>  >>> N.concatenate(((1,2),(3,4))).reshape(2,2)=20
> array([[1, 2],=20
>         [3, 4]])=20





>>> import numpy as N
>>> N.vstack([(1,2),(3,4)])
array([[1, 2],
       [3, 4]])

hth,
Alan Isaac

Re: [Numpy-discussion] What am I missing about concatenate?

[Tim H. <tim...@co...>]

Christopher Barker wrote:

> I want to take two (2,) arrays and put them together into one (2,2) 
> array. I thought one of these would work:
>
> >>> N.concatenate(((1,2),(3,4)),0)
> array([1, 2, 3, 4])
> >>> N.concatenate(((1,2),(3,4)),1)
> array([1, 2, 3, 4])
>
> Is this the best I can do?
>
> >>> N.concatenate(((1,2),(3,4))).reshape(2,2)
> array([[1, 2],
>        [3, 4]])
>
> Is it because the arrays I'm putting together are rank-1?

Yes.

You need to add a dimension somehow. There are (at least) two ways to do 
this. If you are using real arrays, use newaxis:

     >>> a
    array([0, 1, 2])
     >>> b
    array([3, 4, 5])
     >>> concatenate([a[newaxis], b[newaxis]], 0)
    array([[0, 1, 2],
           [3, 4, 5]])

Alternatively, if you don't know that 'a' and 'b' are arrays or you just 
hate newaxis, wrap the arrays in [] to give them an extra dimension. 
This tends to look nicer, but I suspect has poorer performance than 
above (haven't timed it though):

     >>> concatenate([[a], [b]], 0)
    array([[0, 1, 2],
           [3, 4, 5]])


-tim

>
> >>> N.__version__
> '0.9.6'
>
> -Chris
>
>
>
>
>