Re: More For Loops

["Liam E.Gumley" <Liam.Gumley(at)ssec.wisc.edu>]

majewski@cygnus.uwa.edu.au_stralia wrote:
> I'm looking to get rid of the for loops below.
> They make two arrays;
>    one containing evey second column and evey second row
>    the other containing evey second column and evey alternate row
> 
> ;------------------------------------------------------------------------------------------------------------
>         x_data = 3072
>         y_data = 512
>         DATA_size = [x_data/2,y_data]
>         Data_sets_ev = bytarr(DATA_size[0],DATA_size[1])
>         Data_sets_od = bytarr(DATA_size[0],DATA_size[1])
> 
>         for i = 0, (DATA_size[0]/2)-1 do begin
>                 for j = 0, DATA_size[1]-1 do begin
>                         Data_sets_ev[i,j] = my_data[(2*i),(2*j)]
>                         Data_sets_od[i,j] = my_data[(2*i),(2*j+1)]
>                 endfor
>         endfor
> ;------------------------------------------------------------------------------------------------------------
> 
> This is to extract a couple of NOAA14  AVHRR Sea Surface Temperature
> measurements. The different spectral bands overlap in the data file,
> hence they need to be separated by the above.

The syntax required to sample a multi-dimensional array is not
immediately obvious. For example, consider the following two dimensional
array:

IDL> n = 5
IDL> arr = indgen(n, n)
IDL> print, arr
       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

To extract every second element along each dimension, you might try
indexing the array as follows:

IDL> index = lindgen((n + 1) / 2) * 2
IDL> print, index
           0           2           4
IDL> print, arr[index, index]
       0      12      24

However this only extracts every other element along the diagonal. To
extract the second element along each dimension, you must sample each
dimension in turn, e.g.

IDL> sub = arr[index, *]
IDL> print, sub
       0       2       4
       5       7       9
      10      12      14
      15      17      19
      20      22      24
IDL> sub = sub[*, index]
IDL> print, sub
       0       2       4
      10      12      14
      20      22      24

With array expression indexing, the same result is obtained with a more
compact syntax:

IDL> sub = (arr[index, *])[*, index]
IDL> print, sub
       0       2       4
      10      12      14
      20      22      24

Cheers,
Liam.
http://cimss.ssec.wisc.edu/~gumley
PS Say hi to MervL and NickB for me.