[Numpy-discussion] Slicing/selection in multiple dimensions simultaneously

Jonathan Taylor jonathan.taylor@utoronto...
Thu Feb 26 21:00:24 CST 2009


Am I right to assume that there is no way elegant way to interact with
slices.  i.e. Is there anyway to get

a[ix_([2,3,6],:,[3,2])]

to work?  So that the dimension is completely specified?  Or perhaps
the only way to do this is via

a[ix_([2,3,6],range(a.shape[1]),[3,2])]

If anyone knows a better way?

Thanks,
Jonathan.

On Tue, Sep 11, 2007 at 6:13 PM, Travis E. Oliphant
<oliphant@enthought.com> wrote:
> Timothy Hochberg wrote:
>>
>>
>> On 9/11/07, *Robert Kern* <robert.kern@gmail.com
>> <mailto:robert.kern@gmail.com>> wrote:
>>
>>     Mike Ressler wrote:
>>     > The following seems to be a wart: is it expected?
>>     >
>>     > Set up a 10x10 array and some indexing arrays:
>>     >
>>     > a=arange(100)
>>     > a.shape=(10,10)
>>     > q=array([0,2,4,6,8])
>>     > r=array([0,5])
>>     >
>>     > Suppose I want to extract only the "even" numbered rows from a -
>>     then
>>     >
>>     > print a[q,:]
>>     >
>>     > <works - output deleted>
>>     >
>>     > Every fifth column:
>>     >
>>     > print a[:,r]
>>     >
>>     > <works - output deleted>
>>     >
>>     > Only the even rows of every fifth column:
>>     >
>>     > print a[q,r]
>>     >
>>     >
>>     ---------------------------------------------------------------------------
>>
>>     > <type 'exceptions.ValueError'>            Traceback (most recent
>>     call last)
>>     >
>>     > /.../.../.../<ipython console> in <module>()
>>     >
>>     > <type 'exceptions.ValueError '>: shape mismatch: objects cannot be
>>     > broadcast to a single shape
>>     >
>>     > But, this works:
>>     >
>>     > print a[q,:][:,r]
>>     >
>>     > [[ 0  5]
>>     >  [20 25]
>>     >  [40 45]
>>     >  [60 65]
>>     >  [80 85]]
>>     >
>>     > So why does the a[q,r] form have problems? Thanks for your insights.
>>
>>     It is intended that the form a[q,r] be the general case: q and r
>>     are broadcasted
>>     against each other to a single shape. The result of the indexing
>>     is an array of
>>     that broadcasted shape with elements found by using each pair of
>>     elements in the
>>     broadcasted q and r arrays as indices.
>>
>>     There are operations you can express with this form that you
>>     couldn't if the
>>     behavior that you expected were the case whereas you can get the
>>     result you want
>>     relatively straightforwardly.
>>
>>     In [6]: a[q[:,newaxis], r]
>>     Out[6]:
>>     array([[ 0,  5],
>>            [20, 25],
>>            [40, 45],
>>            [60, 65],
>>            [80, 85]])
>>
>>
>>
>> At the risk of making Robert grumpy: while it is true the form we
>> ended up with is more general I've come to the conclusion that it was
>> a bit of a mistake. In the spirit of making simple things simple and
>> complex things possible, I suspect that having fancy-indexing do the
>> obvious thing here[1] and delegating the more powerful but also more
>> difficult to understand case to a function or method would have been
>> overall more useful. Cases where the multidimensional features of
>> fancy-indexing get used are messy enough that they don't benefit much
>> from the conciseness of the indexing notation, at least in my experience.
> This is a reasonable argument.    It is reasonable enough that I
> intentionally made an ix_ function to do what you want.
>
> a[ix_(q,r)]
>
> does as originally expected if a bit more line-noise.
>
> -Travis
>
>
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