| Re: Search single column of array - removing nasty loop [message #78511 is a reply to message #78510] |
Thu, 01 December 2011 04:00   |
Yngvar Larsen
Messages: 134
Registered: January 2010
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Senior Member |
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On Dec 1, 11:37 am, Rob <rj...@le.ac.uk> wrote:
> On Nov 30, 8:15 pm, Yngvar Larsen <larsen.yng...@gmail.com> wrote:
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>> On Nov 29, 6:53 pm, Heinz Stege <public.215....@arcor.de> wrote:
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>>> Hi Rob,
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>>> no loop necessary:
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>>> array=(randomu(seed,2,6,360,42)-.1)>0. ; sample array
>>> array=reform(array,n_elements(array)/42,42,/overwrite)
>>> ii=where(min(array,dim=2) eq 0.,count)
>>> if count ge 1 then array[ii,*]=0.
>>> array=reform(array,2,6,360,42,/overwrite)
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>> Hm. The /OVERWRITE keyword to REFORM was new to me. Thanks!
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>> Silly me. I have somehow always imagined that the compiler was smart
>> enough to do this (i.e. not copy any data, only alter the internal IDL
>> descriptor of the ARRAY variable) automatically when input and output
>> to REFORM is the same variable. But a bit of profiling shows this is
>> not at all the case. This will be _very_ useful many places in my
>> operational code...
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>> A small comment to the code above: "where(min(array,dim=2) eq 0.)"
>> obviously only works if array contains only non-negative data. If not,
>> "where(total(array eq 0, 2) gt 0)" will do the trick also for floating
>> point data containing negative numbers, with more or less the same
>> performance.
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>> --
>> Yngvar
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> Thanks, that explains why a few results were coming out slightly
> differently as there are a few negative values.
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> Also, the code fails when the final column only has 1 element in it.
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> IDL> help, array
> ARRAY DOUBLE = Array[4320, 1]
> IDL> help, total(array eq 0, 2)
> % TOTAL: For input argument <BYTE Array[4320]>, Dimension must be
> 1.
If the final column has only 1 element, the operation is not necessary
at all since all elements are already 0 :)
IDL sometimes behaves rather idiotic with singleton dimensions:
IDL> help, fltarr(4320, 1)
<Expression> FLOAT = Array[4320]
This is a problem when arrays are expected to be 2D, and suddenly are
automatically 1D. You can avoid it by adding an explicit REFORM
statement at the appropriate place in the code:
;; Force ARRAY to be 2D always
if (size(array, /n_dimensions) eq 1) then $
array = reform(array, n_elements(array), 1, /overwrite)
--
Yngvar
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