| Re: Object programming with data... [message #30856 is a reply to message #30855] |
Sun, 19 May 2002 14:22   |
Randall Skelton
Messages: 169
Registered: October 2000
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Senior Member |
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In response to some of my own questions I found a post from Ken Knighton
in 1996 that demonstrates one way of emulating operator overloading using
IDL functions... I'm open to other suggestions.
Cheers,
Randall
-- From way, way back in 1996 --
> The answer to this will have to be on the installment plan. :-)
>
> 1) Polymorphism
>
> a. Functions/procedures can be called with a variable number of
> formal parameters.
>
> b. Since identifiers are dynamically typed, a single func/pro
> can be devised that performs an operation on a variety of
> input argument types.
>
> The following tiny function shows how, by virtue of the fact that
> IDL is dynamically typed, functions can be designed with varying
> types and numbers of parameters. Note that type checking could
> be added to this function to produce errors if incompatible data
> types were used. Or, one could use the CATCH statement to react
> to any errors that may occur (such as failure to convert a string
> to a number if mixed strings and numbers were being used).
>
> ;Trivial, contrived, and useless example of "polymorphism" in IDL.
> FUNCTION Add, p1, p2, p3, p4, p5, p6, p7, p9, p10
>
> lParams = N_PARAMS()
>
> CASE lParams OF
>
> 2L: xSum = p1+p2
> 3L: xSum = p1+p2+p3
> 4L: xSum = p1+p2+p3+p4
> 5L: xSum = p1+p2+p3+p4+p5
> 6L: xSum = p1+p2+p3+p4+p5+p6
> 7L: xSum = p1+p2+p3+p4+p5+p6+p7
> 8L: xSum = p1+p2+p3+p4+p5+p6+p7+p8
> 9L: xSum = p1+p2+p3+p4+p5+p6+p7+p8+p9
> 10L: xSum = p1+p2+p3+p4+p5+p6+p7+p8+p9+p10
>
> ELSE: MESSAGE, 'Must use 2 through 10 parameters.'
> ENDCASE
>
> RETURN, xSum
> END
>
> There are also ways of doing the above without using a CASE statement.
> One of these is to use the EXECUTE command and a FOR loop:
>
> xSum = p1+p2
> FOR i=3, lParams DO BEGIN
> aExec = 'xSum = xSum + p'+STRTRIM(i,2)
> lErr = EXECUTE(aExec)
> ENDFOR
>
> Of course, the case statement runs much more quickly and is more
> obvious in its logic. However, the EXECUTE statement has its place
> and provides on-the-fly compilation and execution of statements.
>
> If you call the above function using a variety of input types, you will
> soon notice that the actual parameters can be of any numeric or string
> type and can be either scalars or arrays. If strings and numerics are
> mixed, then the strings must be able to convert to numeric type. One
> can not use structures in the above example, but one could modify this
> code to check for structures using the SIZE function and then take
> action accordingly.
>
> As you can see, it is fairly easy to write one function that takes
> care of a wide variety of possibilities for input arguments.
>
> I'll try to continue this discussion later. Any feedback is welcome.
> If someone has a better example, please post.
>
> Ken Knighton knighton@gav.gat.com knighton@cts.com
> General Atomics
> San Diego, CA
>
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