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Re: Weird MIN behavior [message #42743 is a reply to message #42742]

Sun, 20 February 2005 17:12

Michael Wallace
Messages: 409
Registered: December 2003

Senior Member

Mark Hadfield wrote:
> Michael Wallace wrote:
>
>> Can someone explain this?
>>
>>
>> Normal case; everything works as you expect.
>>
>> IDL> a = [1.0, 2.0, 3.0, 4.0, 5.0]
>> IDL> b = min(a, MAX = c)
>> IDL> print, b, c
>> 1.00000 5.00000
>>
>>
>> Abnormal case; why doesn't this work?
>>
>> IDL> a = [1.0, 2.0, 3.0, 4.0, 5.0]
>> IDL> r = dblarr(2)
>> IDL> r[0] = min(a, MAX = r[1])
>> % MIN: Expression must be named variable in this context: <DOUBLE ((
>> 0.00000, 0.00000))>.
>
>
> It's complaining about the argument passed to the MAX keyword. As it
> says, this must be a named variable, and a subscripted variable does not
> qualify because it is passed by value. You're lucky that IDL tells you
> that it doesn't accept a subscripted variable here--in other situations
> it quitely leaves the value unmodified and let's you puzzle it out later.

Thanks. I had no clue what the message meant by "named variable." You
wouldn't believe how long I stared at that error message trying to
figure out what it meant. I knew things would work when I used
non-subscripted variables, but the message still didn't make sense.
Now, the message could have said "Look you idiot, a subscripted variable
is sent by value, so the function can't modify the value. This probably
isn't what you want." Now that makes sense. ;-)

I guess I had gotten it into my head that since you can change primitive
values within a function by using KEYWORD = value, you must also be able
to change array elements as well. It shouldn't surprise me that IDL is
backwards. In every other language, arrays and array elements are
passed by reference and primitives are passed by value.

This will get added to my ever growing list of gotchas.

-Mike

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[Message index]

		Re: Weird MIN behavior By: David Fanning on Mon, 21 February 2005 16:56
		Re: Weird MIN behavior By: Michael Wallace on Mon, 21 February 2005 16:49
		Re: Weird MIN behavior By: James Kuyper on Mon, 21 February 2005 16:13
		Re: Weird MIN behavior By: Michael Wallace on Sun, 20 February 2005 17:56
		Re: Weird MIN behavior By: David Fanning on Sun, 20 February 2005 17:31
		Re: Weird MIN behavior By: Michael Wallace on Sun, 20 February 2005 17:12
		Re: Weird MIN behavior By: Mark Hadfield on Sun, 20 February 2005 14:42
		Re: Weird MIN behavior By: David G. Grier on Sun, 20 February 2005 14:28

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