| Re: More For Loops [message #19680 is a reply to message #19676] |
Thu, 13 April 2000 00:00  |
Liam E. Gumley
Messages: 378
Registered: January 2000
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Senior Member |
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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.
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