| Re: newbie question on array manipulation [message #66283] |
Tue, 05 May 2009 23:54 |
jpardila
Messages: 2
Registered: May 2009
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Junior Member |
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Thank you so much for your help Jean and Ben. The link on histogram is
very good. I follow the tutorial and yes it can be quite handy. For
now I just used the where statement as I only have to change a few
values. I am happy to finally find out this here as I always used so
far the Raster Calculator in ArcGis when doing raster operations. Now
to completely close the gap I wonder if those operations can also be
done effectively to model relationships between different layers which
are georreferenced and overlap each other (they come from ENVI) but
might cover different extents. Something like if in arrayA[a] = 5 then
arrayB= 100. This is based purely on the geographic location of pixels
(x,y coordinates). I hope it is clear what I mean.
Next cup of coffee is on me!! Cheers,
JP
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| Re: newbie question on array manipulation [message #66285 is a reply to message #66283] |
Tue, 05 May 2009 06:24  |
ben.bighair
Messages: 221
Registered: April 2007
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Senior Member |
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On May 5, 7:53 am, "ben.bighair" <ben.bigh...@gmail.com> wrote:
> On May 5, 7:40 am, jpardila <blu...@gmail.com> wrote:
>
>> What is the best way to manipulate one array and remap its values
>> according to conditions?
>> Let's say I have a 2-d array, for all the elements equal to 5 I want
>> to change their values to 0. For values equal to 10 I want to convert
>> to 1 and so on. I am really confused and don't know if to use IF,
>> where, for or while. Thanks for answering this basic one.
>> JP
>
> Hi,
>
> The simplest is to use where - don't forget to use the output COUNT
> argument shown here as "na".
>
> a = WHERE(array EQ 10, na)
> if (na GT 0) then array[a] = 1
>
> If you are doing a multiple replacements (of different values) in a
> single array then you'll want to strap your sneakers on and use
> HISTOGRAM with it's REVERSE_INDICES output keyword. Seehttp://dfanning.com/tips/histogram_tutorial.htmlfor more on that - it
> is the swiss knife of IDL.
>
Hi again,
Another thought after a cup of coffee... if your application uses
floating point values instead of integers then you should search this
news group for info on how to test for equality with floats (single or
double precision).
Cheers (again),
Ben
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| Re: newbie question on array manipulation [message #66287 is a reply to message #66285] |
Tue, 05 May 2009 06:06 |
Jean H.
Messages: 472
Registered: July 2006
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Senior Member |
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ben.bighair wrote:
> On May 5, 7:40 am, jpardila <blu...@gmail.com> wrote:
>> What is the best way to manipulate one array and remap its values
>> according to conditions?
>> Let's say I have a 2-d array, for all the elements equal to 5 I want
>> to change their values to 0. For values equal to 10 I want to convert
>> to 1 and so on. I am really confused and don't know if to use IF,
>> where, for or while. Thanks for answering this basic one.
>> JP
>
> Hi,
>
> The simplest is to use where - don't forget to use the output COUNT
> argument shown here as "na".
>
> a = WHERE(array EQ 10, na)
> if (na GT 0) then array[a] = 1
Also, if you have several values to changes, you can create a new array.
For example, if you want to change 5 -> 0 and 0 -> 10, but you don't
want to change the old 5 to 10!
copy = array ;copy the array
a = WHERE(array EQ 10, na)
if (na GT 0) then copy[a] = 1
Jean
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| Re: newbie question on array manipulation [message #66290 is a reply to message #66287] |
Tue, 05 May 2009 04:53 |
ben.bighair
Messages: 221
Registered: April 2007
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Senior Member |
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On May 5, 7:40 am, jpardila <blu...@gmail.com> wrote:
> What is the best way to manipulate one array and remap its values
> according to conditions?
> Let's say I have a 2-d array, for all the elements equal to 5 I want
> to change their values to 0. For values equal to 10 I want to convert
> to 1 and so on. I am really confused and don't know if to use IF,
> where, for or while. Thanks for answering this basic one.
> JP
Hi,
The simplest is to use where - don't forget to use the output COUNT
argument shown here as "na".
a = WHERE(array EQ 10, na)
if (na GT 0) then array[a] = 1
If you are doing a multiple replacements (of different values) in a
single array then you'll want to strap your sneakers on and use
HISTOGRAM with it's REVERSE_INDICES output keyword. See
http://dfanning.com/tips/histogram_tutorial.html for more on that - it
is the swiss knife of IDL.
CHeers,
Ben
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