| Re: Using where() on slices of data cubes [message #68331 is a reply to message #68327] |
Tue, 20 October 2009 08:00   |
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Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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David Fanning wrote:
> Conor writes:
>
>> I feel like this should be an easy one, but I've never quite figured
>> it out. Let's say I got a data cube and I want to do something on
>> just a slice of it, say I want to turn certain values in a column into
>> something else:
>>
>> w = where( cube[1,*,*] lt 0 )
>>
>> It seems like you should be able to do something like this:
>>
>> cube[1,w] = 1e24
>>
>> But that doesn't work... Somehow I can't quite figure out the right
>> way to do this.
>
> It *seems* like there should be a simple way to do this,
> but if there is, I haven't found it. What would make
> sense to me is this:
>
> (cube[1,*,*])[w] = 1e24
>
> But this gives the error message that you can't store into
> a temporary variable. (The ol' pass by reference/pass by value
> thing, I suppose.)
Is it possible at all to use IDL pointer for aliasing? E.g. in Fortran I would do
something like the following
real, target :: cube(:,:,:)
real, pointer :: alias(:,:) => null()
alias => cube(1,:,:)
where ( alias < 0.0 )
alias = 1e24
end where
nullify(alias)
Is there an IDL equivalent to the "alias => cube(1,:,:)" statement in Fortran?
If there is, then that might get around the tired old "can't store into a temporary
variable" issue that plagues IDL due to it sticking to pass by reference only. (Man, I
wish they would fix that.)
The only way I can figure out to do the aliasing ends up copying the data.
Anyway...
cheers,
paulv
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