| Variable stride in array indices [message #15249] |
Fri, 07 May 1999 00:00  |
bowman
Messages: 121
Registered: September 1991
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Senior Member |
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We haven't had a good argument about IDL syntax lately, so I thought I
would ask why IDL does not allow variable stride in array indices. For
example, if
zz = z[0:5:2]
then zz would contain [z[0], z[2], z[4]].
I realize that this kind of thing can be done with array subscript lists
such as z[[0, 2, 4]], but this can be awkward for multidimensional
arrays. It is also necessary to *store* the lists of subscripts, and I
believe that implementation through indirect subscripts must be slower
than implementing strides through direct iteration (i.e., internal
compiled DO loops).
Any comments?
Ken Bowman
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| Re: Variable stride in array indices [message #15451 is a reply to message #15249] |
Tue, 18 May 1999 00:00  |
DBorland
Messages: 10
Registered: March 1999
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Junior Member |
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----------
In article <bowman-1805990954070001@bowman.tamu.edu>, bowman@null.edu
(Kenneth P. Bowman) wrote:
> 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
What you need to do it then sent the results here as the index for the
array. ie:
IDL> a[(a[*,2*LINDGEN(3)])[2*LINDGEN(3),*]] = -1
When you do this, the values from above are set to -1
IDL> print,a
-1 1 -1 3 -1 5
6 7 8 9 10 11
-1 13 -1 15 -1 17
18 19 20 21 22 23
-1 25 -1 27 -1 29
30 31 32 33 34 35
David Borland
Software Engineer
Electrical Geodesics, Inc.
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| Re: Variable stride in array indices [message #15454 is a reply to message #15249] |
Tue, 18 May 1999 00:00 |
bowman
Messages: 121
Registered: September 1991
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Senior Member |
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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
--
Dr. Kenneth P. Bowman, Professor 409-862-4060
Department of Meteorology 409-862-4466 fax
Texas A&M University bowmanATcsrp.tamu.edu
College Station, TX 77843-3150 Replace AT with @
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| Re: Variable stride in array indices [message #15460 is a reply to message #15249] |
Tue, 18 May 1999 00:00 |
bowman
Messages: 121
Registered: September 1991
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Senior Member |
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In article <ySg03.87161$A6.43176220@news1.teleport.com>, "DBorland"
<dborland@egi.com> wrote:
> IDL> a[(a[*,2*LINDGEN(3)])[2*LINDGEN(3),*]] = -1
>
> When you do this, the values from above are set to -1
> IDL> print,a
> -1 1 -1 3 -1 5
> 6 7 8 9 10 11
> -1 13 -1 15 -1 17
> 18 19 20 21 22 23
> -1 25 -1 27 -1 29
> 30 31 32 33 34 35
This only works because the original array was created with LINDGEN. It
won't work in the general case.
I still like
a[0:*:2,0:*:2] = -1
for aesthetic reasons alone.
Ken
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