| Re: Array indexing: what is IDL doing? [message #60696] |
Tue, 10 June 2008 07:16 |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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On Jun 9, 11:34 am, Heinz Stege <public.215....@arcor.de> wrote:
> On Fri, 6 Jun 2008 09:50:33 -0700 (PDT), Conor wrote:
>> So I want to pull out a small subsection from within a larger idl
>> array. I want to do something like this:
>
>> bigarr = findgen(500,100000)
>> bigx = [indgen(50)+100,indgen(25)+200]
>> bigy = indgen(10000)+50000
>
>> res = bigarr[bigx,bigy]
>
>> However IDL doesn't like that and says:
>
>> % All array subscripts must be same size. Var = BIGARR
>> % Execution halted at: $MAIN$
>
> You may want to try this command:
> res = bigarr[bigx,bigy,0]
>
> IDL accepts arrays of different size as subscripts, if at least one of
> the subsripts is a scalar. Here the "0" addresses an "extra"
> dimension. Since this extra dimension is of size 1, it is degenerated
> in the result. Array res has two dimensions, 75x10000.
>
> Heinz
Hmm... this does work! It seems very strange though. Seems like a
rather arbitrary rule...
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| Re: Array indexing: what is IDL doing? [message #60704 is a reply to message #60696] |
Mon, 09 June 2008 10:12  |
Jean H.
Messages: 472
Registered: July 2006
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Senior Member |
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Conor wrote:
> On Jun 6, 5:15 pm, Jean H <jghas...@DELTHIS.ucalgary.ANDTHIS.ca>
> wrote:
>>> cx = n_elements(wx)
>>> cy = n_elements(wy)
>>> nx = n_elements(bigarr[*,0])
>>> ind = transpose(rebin(wy,cy,cx))*nx + rebin(wx,cx,cy)
>>> res = bigarr[ind]
>> You don't have to use nx.
>> res = bigarr[rebin(bigx,75,10000),rebin(bigy,10000,75)]
>>
>> Note that you must have the same number of elements in every subscripted
>> dimension.
>>
>> Readhttp://www.dfanning.com/code_tips/asterisk.html
>>
>> Jean
>
> On second thought, I'm now thoroughly confused. I don't understand
> this bit:
>
> res = bigarr[rebin(bigx,75,10000),rebin(bigy,10000,75)]
>
> obviously rebin(bigx,75,10000) and rebin(bigy,10000,75) give you two
> arrays with exactly opposite dimensions, so I don't understand how you
> can index bigarr with these two arrays. Also, I'm not even sure what
> IDL is doing when you index an array like that. My first thought was
> that this was the equivelent of:
>
> res = bigarr[ [rebin(bigx,75,10000),rebin(bigy,10000,75)] ]
>
> But that obviously can't be the case because since they have different
> dimensions, you can't concatenate the two arrays. So how does IDL
> pull out an array index in your above example?
Ah, I guess you are right... I was typing too fast (and the no-error
didn't warn me..)
it should have been:
IDL> help, bigarr[rebin(bigx,75,10000),transpose(rebin(bigy,10000,75))]
So, rebin(bigx,75,10000) replicates your 75 entries 10 000 times.
rebin(bigy,10000,75)) replicates the 10 000 entries 75 times. Now,
transpose(rebin(bigy,10000,75)), well, transposes it so the two
subscript now have the same dimensions. At last, for each corresponding
index in both subscripts, the value of bigArr is returned.
Was i posted was working because there are the same number of elements
in both subscripts... though the combination was wrong and therefore it
returns the wrong result..
Sorry for the confusion!
Jean
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| Re: Array indexing: what is IDL doing? [message #60706 is a reply to message #60704] |
Mon, 09 June 2008 08:34 |
Heinz Stege
Messages: 189
Registered: January 2003
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Senior Member |
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On Fri, 6 Jun 2008 09:50:33 -0700 (PDT), Conor wrote:
> So I want to pull out a small subsection from within a larger idl
> array. I want to do something like this:
>
> bigarr = findgen(500,100000)
> bigx = [indgen(50)+100,indgen(25)+200]
> bigy = indgen(10000)+50000
>
> res = bigarr[bigx,bigy]
>
> However IDL doesn't like that and says:
>
> % All array subscripts must be same size. Var = BIGARR
> % Execution halted at: $MAIN$
>
You may want to try this command:
res = bigarr[bigx,bigy,0]
IDL accepts arrays of different size as subscripts, if at least one of
the subsripts is a scalar. Here the "0" addresses an "extra"
dimension. Since this extra dimension is of size 1, it is degenerated
in the result. Array res has two dimensions, 75x10000.
Heinz
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| Re: Array indexing: what is IDL doing? [message #60710 is a reply to message #60706] |
Mon, 09 June 2008 06:02 |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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On Jun 6, 5:15 pm, Jean H <jghas...@DELTHIS.ucalgary.ANDTHIS.ca>
wrote:
>> cx = n_elements(wx)
>> cy = n_elements(wy)
>> nx = n_elements(bigarr[*,0])
>> ind = transpose(rebin(wy,cy,cx))*nx + rebin(wx,cx,cy)
>> res = bigarr[ind]
>
> You don't have to use nx.
> res = bigarr[rebin(bigx,75,10000),rebin(bigy,10000,75)]
>
> Note that you must have the same number of elements in every subscripted
> dimension.
>
> Readhttp://www.dfanning.com/code_tips/asterisk.html
>
> Jean
On second thought, I'm now thoroughly confused. I don't understand
this bit:
res = bigarr[rebin(bigx,75,10000),rebin(bigy,10000,75)]
obviously rebin(bigx,75,10000) and rebin(bigy,10000,75) give you two
arrays with exactly opposite dimensions, so I don't understand how you
can index bigarr with these two arrays. Also, I'm not even sure what
IDL is doing when you index an array like that. My first thought was
that this was the equivelent of:
res = bigarr[ [rebin(bigx,75,10000),rebin(bigy,10000,75)] ]
But that obviously can't be the case because since they have different
dimensions, you can't concatenate the two arrays. So how does IDL
pull out an array index in your above example?
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| Re: Array indexing: what is IDL doing? [message #60711 is a reply to message #60710] |
Mon, 09 June 2008 05:56 |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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On Jun 7, 9:12 am, Jeremy Bailin <astroco...@gmail.com> wrote:
>> So I want to pull out a small subsection from within a larger idl
>> array. I want to do something like this:
>
>> bigarr = findgen(500,100000)
>> bigx = [indgen(50)+100,indgen(25)+200]
>> bigy = indgen(10000)+50000
>
>> res = bigarr[bigx,bigy]
>
> In the particular case above, I would expect that this would be most
> efficient:
>
> res = [bigarr[100:149,50000:59999], bigarr[200:224,50000:59999]]
>
> Of course, that's only going to work when you know a priori that
> everything will be in a relatively small number of chunks.
>
> -Jeremy.
Interesting, thanks!
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| Re: Array indexing: what is IDL doing? [message #60722 is a reply to message #60711] |
Sat, 07 June 2008 06:12 |
Jeremy Bailin
Messages: 618
Registered: April 2008
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Senior Member |
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> So I want to pull out a small subsection from within a larger idl
> array. I want to do something like this:
>
> bigarr = findgen(500,100000)
> bigx = [indgen(50)+100,indgen(25)+200]
> bigy = indgen(10000)+50000
>
> res = bigarr[bigx,bigy]
In the particular case above, I would expect that this would be most
efficient:
res = [bigarr[100:149,50000:59999], bigarr[200:224,50000:59999]]
Of course, that's only going to work when you know a priori that
everything will be in a relatively small number of chunks.
-Jeremy.
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| Re: Array indexing: what is IDL doing? [message #60729 is a reply to message #60722] |
Fri, 06 June 2008 14:15 |
Jean H.
Messages: 472
Registered: July 2006
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Senior Member |
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> cx = n_elements(wx)
> cy = n_elements(wy)
> nx = n_elements(bigarr[*,0])
> ind = transpose(rebin(wy,cy,cx))*nx + rebin(wx,cx,cy)
> res = bigarr[ind]
You don't have to use nx.
res = bigarr[rebin(bigx,75,10000),rebin(bigy,10000,75)]
Note that you must have the same number of elements in every subscripted
dimension.
Read http://www.dfanning.com/code_tips/asterisk.html
Jean
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