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| Re: Passing Arrays BY REFERENCE [message #91762 is a reply to message #91648] |
Sun, 23 August 2015 15:30  |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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On Monday, August 10, 2015 at 11:46:22 AM UTC-4, g.na...@gmail.com wrote:
> I was trying to find out how IDL can pass arrays BY REFERENCE instead of BY VALUE. I found out this but was not very helpful.
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> http://www.exelisvis.com/docs/PassingParameterArraysByRefere nce.html
>
> I wanted to return an array BY REFERENCE. Does anyone know how to do that?
Not exactly. All IDL variables are "by reference." If a parameter is passed by reference then it is returned by reference.
In this example,
a = [[1d,-0.01d],[.1d,1d]]
choldc, a, p
the variable A is passed "by reference" to CHOLDC. If you consult the documentation, CHOLDC modifies the array so that upon return, A is changed. So in that case it is returned by reference as well. The variable P does not need to be defined before calling CHOLDC, but upon return, P will contain new values. So in a sense the variable P was returned by reference.
The only case where "by value" would play a role is in this modified example,
a = [[1d,-0.01d],[.1d,1d]]
choldc, a[*,*], p
Here the changed part is A[*,*] instead of just A. The A[*,*] instructs IDL to create a new temporary array with all elements of the original array, and pass that to CHOLDC. When CHOLDC returns, that temporary array is discarded, so A does not change. We say that A[*,*] is passed by value. But P would still be passed (and returned) by reference.
That's because all IDL variables by themselves contain a reference to the actual data. They are a name, "A" and a reference to the data, in this case a square array of data. Passing a variable by name in a function call is always by reference.
Craig
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