| Re: Interesting WHERE function gotcha [message #33935 is a reply to message #33934] |
Thu, 06 February 2003 18:35   |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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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. One element, because LETTER only has one element:
IDL> help, veggie_letter EQ letter
<Expression> BYTE = Array[1]
There are many times that I wish that IDL has an easier way to get the
ASCII value of a character.
By the way, why not do this instead?
index = where( strmid(array,0,1) EQ 'a')
This avoids the whole issue of converting to a different
representation, and it just looks less gobbledygooky.
Happy guacamole,
Craig
--
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Craig B. Markwardt, Ph.D. EMAIL: craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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