| Re: simple vectorizing problem [message #62502 is a reply to message #62427] |
Thu, 11 September 2008 11:56   |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Sep 10, 7:53 pm, dpm314 <meich...@gmail.com> wrote:
> Hi everyone and thanks a lot for your suggestions so far. That is a
> cute trick with the Total function, and I was just not aware that you
> could specify to total only rows or columns like that. This will
> work, and I have tried it but didn't get a chance to do the timing
> test.
> However, my first post must not be clear, because this doesn't really
> answer my question.
>
> I have these image arrays and need to perform several functions on
> them which return a scalar (or a 6 element array) for each image in a
> large stack. Is there *in general* a way to so something like:
>
> results = fltarr(num_images)
> results = myFunctThatDoesSomeAnalysis(image)
> ;now results holds an array of scalars for the result of
> myFunctThatDoesSomeAnalysis() on each image
>
> without either using a loop here or inside the analysis function
> iterating over each frame in the image?
>
> again, I've tried many things like:
>
> a = indgen(num_image)
> results(a) = myFunctThatDoesSomeAnalysis(image(*,*,a))
> which I thought would work but it does not.
>
> It is necessary to use a loop like this then in general :
> for i 0, num_images-1 do results(i) =
> myFunctThatDoesSomeAnalysis(image(*,*,i))
>
> ???
>
> I thought the trick with an indexing array ('a' in the example above)
> would work because if I say
> b = indgen(101)/(!pi/100)
> print, cos(b)
>
> I get not one number out but an array of cos() evaluated from 0 to pi
>
> I guess I don't understand how the cos() example gives me an array
> back, and
> myFunctThatDoesSomeAnalysis(image(*,*,a)) gives me just one number.
>
> If this question makes no sense I apologize, and I could provide more
> code to illustrate the problem if that would help. I am still trying
> to wrap my head around vectorizing code, which (I at least) find quite
> different from purely procedural languages like c and Fortran which
> I've written in for years.
There is no general mechanism to "thread" arbitrary code over
arbitrary dimensions within an array in IDL. Some languages have such
general operator threading (but they're not exactly fast). There
*are* a variety of internal IDL routines which can do such threading,
including TOTAL, and various others, which can be used together when
vectorizing many types of problems (certainly more than are
immediately obvious). Another way of saying that is that all IDL
vectorization, aside from basic array arithmetic, is explicit, in the
sense that it is specifically requested in the called procedure or
function (like TOTAL(array, dimension)).
There is such a thing as premature vectorization, however. In your
case, if the images in your stack are big enough (at least few hundred
or thousand pixels), simply looping through each one and accumulating
the result will likely perform just as well as any vectorized method
could. It's when you're looping over individual array elements that
you should be concerned.... "Embodiment of Pure Evil" notwithstanding.
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