Re: Variable stride in array indices

[bowman(at)null.edu (Kenneth P. Bowman)]

In article <3732EFA0.1F6C49C0@ssec.wisc.edu>, Liam Gumley
<Liam.Gumley@ssec.wisc.edu> wrote:

> Or I can select the columns and rows at the same time:
> 
> IDL> print,(a[x,*])[*,x]
>        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

I had a chance to look at my code to see if Liam's suggestion would work,
and I realized that it won't, because I need to use the variable stride on
the *left* side of the equal sign, i.e.

IDL> a = LINDGEN(6,6)
IDL> print, a
           0           1           2           3           4           5
           6           7           8           9          10          11
          12          13          14          15          16          17
          18          19          20          21          22          23
          24          25          26          27          28          29
          30          31          32          33          34          35
IDL> print, (a[*,2*LINDGEN(3)])[2*LINDGEN(3),*]
           0           2           4
          12          14          16
          24          26          28

works fine *extracting* data, but

IDL> (a[*,2*LINDGEN(3)])[2*LINDGEN(3),*] = -1
% Temporary variables are still checked out - cleaning up...
IDL> PRINT, A
           0           1           2           3           4           5
           6           7           8           9          10          11
          12          13          14          15          16          17
          18          19          20          21          22          23
          24          25          26          27          28          29
          30          31          32          33          34          35

fails.

So it is possible to extract values (gather) but not to insert values
(scatter) with a variable stride using this method.

I repeat my call for a simple stride syntax in IDL

a[0:5:2,0:5:2] = -1

Ken

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Dr. Kenneth P. Bowman, Professor                409-862-4060
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