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array indices again

Hi all,

I remember a few weeks back there was a lot of discussion about indexing
multiple dimensions of arrays, and doing things such as

IDL> print,(a[x,*])[*,x]

I just noticed something that may or may not have come up in those
discussions (but I wasn't paying attention).  It is most easily
explained by Liam's example of extracting every second row and column of
an array

IDL> a=indgen(10,10)
IDL> x=indgen(5)*2
IDL> print, a[x, x, 0]
       0       2       4       6       8
      20      22      24      26      28
      40      42      44      46      48
      60      62      64      66      68
      80      82      84      86      88

It seems that by extending the 2-d array a into another dimension, where
the extra dimension has no size (sounds like string theory!) the
indexing works the way I want it to.  I noticed this when I had a 3-d
array and was extracting a 2-d array from it, as in b[x, x, i] where x
is a 1-d array and i is a scalar.  This seems to work on arrays of any

I apologise if this was mentioned before,


Henry Chapman                          mailto:chapman9@llnl.gov
Information Science & Technology Program
Lawrence Livermore National Lab         
L-395, 7000 East Ave., Livermore CA 94550