Re: Array subscript question

[davidf(at)dfanning.com (David Fanning)]

Kenneth P. Bowman (bowman@null.edu) writes:

> Can someone explain this behavior to me?  I can't find anything in the
> documentation that states that repeated subscripts are handled
> differently.
> 
> IDL> a = FINDGEN(5)
> IDL> i = [1, 2, 3]
> IDL> a[i] = a[i] + 10.0
> IDL> PRINT, a
>       0.00000      11.0000      12.0000      13.0000      4.00000
> 
> This is the behavior I expect.
> 
> 
> IDL> a = FINDGEN(5)
> IDL> i = [2, 2, 2]
> IDL> a[i] = a[i] + 10.0
> IDL> PRINT, a
>       0.00000      1.00000      12.0000      3.00000      4.00000
> 
> Why does it only do the operation *once* when
> IDL> HELP, a[i]
> <Expression>    FLOAT     = Array[3]

Uh, I'm sure it's doing the operation three times, Ken.
But always to the same value. :-)

Sounds what you want is a loop. I'm not too fond
of them in IDL, but certainly it's appropriate here.
I think the answer to your question really is this:
array subscripting operations are NOT loops!

  FOR I=0,2 DO a[2] = a[2] + 10.0

Cheers,

David
-- 
David Fanning, Ph.D.
Fanning Software Consulting
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