Re: Undocumented array indexing feature?

[Craig Markwardt <craigmnet(at)astrog.physics.wisc.edu>]

Liam Gumley <Liam.Gumley@ssec.wisc.edu> writes:
> 
> Here's a way to index an array I hadn't seen before:
> 
> a = indgen(10,10)
> x = [3,5,8,9]
> y = [2,7]
> print, (a[x,*])[*,y]
>       23      25      28      29
>       73      75      78      79
> 
> Does anyone know where this feature (i.e. enclosing an array with
> parentheses and appending an index) is documented? I couldn't find it in
> my printed IDL 5.0 documentation.

It's not really an array indexing feature per se.  In your example, a
temporary expression is first created from a[x,*], and then *that*
temporary expression is indexed according to [*,y].  Internally, IDL
must do some more copying than straight array indexing, so your
operation is not as efficient.  On the other hand it probably does
what you want.  I like it!

By the way, indexing of expressions is allow, as long as you throw a
pair of parentheses around it.  The following sometimes appears in my
code:

n = (size(x))[0]

The result of size(x) is an array; this statement takes the first
element of that array.

Craig


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Craig B. Markwardt, Ph.D.         EMAIL: craigmnet@astrog.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
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