| Re: Interesting WHERE function gotcha [message #33931] |
Fri, 07 February 2003 08:04  |
JD Smith
Messages: 850
Registered: December 1999
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Senior Member |
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On Thu, 06 Feb 2003 19:35:31 -0700, Craig Markwardt wrote:
> David Fanning <david@dfanning.com> writes:
>>
>> How about this:
>>
>> Help, veggie_letter, letter
>> VEGGIE_LETTER BYTE = Array[4]
>> LETTER BYTE = Array[1]
>>
>> "LETTER, a byte *array*!? You don't suppose..." Try this:
>>
>> index = WHERE( (Byte(array))[0,*] EQ (Byte('a'))[0], count) Print,
>> count
>> 2
>>
>> Hummm. V-e-r-y interesting...
>>
>> Now I know how to fix the problem, but I don't know exactly what the
>> problem is. (Although this is not so different from most computer
>> problems, when you come to think of it.) Is the problem that the BYTE
>> function always makes a byte *array* when extracting string arguments?
>> Or is it that the WHERE function acts in a, uh, non-intuitive way when
>> there are two vectors in a boolean expression?
>
> Hi David!
>
> You are being bitten by the "feature" that I love so much. Namely that
> in IDL, when you do "X OPERATION Y", and X and Y are both arrays, then
> the expression is trimmed to the smaller of the two arrays.
>
> So it's not anything special regarding WHERE, or boolean expressions,
> but rather that VEGGIE_LETTER EQ LETTER evaluates to a 1-element array.
Yes, the tension between single element arrays and scalars is felt
deeply, even within the core IDL development group. It's a
regrettable legacy which must, unfortunately, be preserved for
compatibitlity. There are, however, minor changes and improvements:
e.g., with v5.6, KEYWORD_SET now considers a single element array TRUE
only if its single element is non-zero; i.e. it's treated just like a
scalar for this (and only this) function.
JD
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