| Re: printing an array from pointers [message #48184 is a reply to message #48125] |
Wed, 29 March 2006 08:06  |
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Paul Van Delst[1]
Messages: 1157
Registered: April 2002
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Senior Member |
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bressert@gmail.com wrote:
> Hi Peter,
>
> Another question, since I have ran into a new bump. Is there a way to
> say
>
> arr = fltarr(A,8)
>
> where A is a number that fluctuates? So rather than stating that arr is
> A rows long, it is a number determined by the total output of the for
> loop? For example,
>
> arr = fltarr(150,8)
>
> will be sufficient in gathering all the 'for' outputs, but I will have
> trailing zeros that have not been assigned an output value. Using UNIQ
> or an 'if' to get rid of the zeros in the array does not work, since
> some of the output from the 'for' loop is zero. This was the original
> reason why I used the pointers, since there was no requirement of
> predetermination of the number of rows. Any suggestions or ideas would
> be greatly appreciated. Thanks again for the help.
Keep in mind that the first index is the one that increments in contiguous memory
(opposite to C) so maybe arr=fltarr(8,a) is required.
But, regardless, you must know the maximum limit of the for loop in advance, no? In other
examples posted in this thread, we';ve seen:
arr = fltarr(10,8)
for i = 0,9 do begin
...
arr[i,*] = (some 1 by 8 vector)
endfor
That can be re-written as:
loop_limit = func_to_compute_loop_limit()
arr = fltarr(8,loop_limit) ! More efficient that (loop_limit,8)
for i=0,loop_limit-1 do begin
....
arr[*,i] = (some 1 by 8 vector)
endfor
If you don;t know the loop limit in advance you can do two things:
1) Concatenation
i=-1
WHILE (some condition) DO BEGIN
i++
.....
if(i eq 0) then $
arr = (some 1 by 8 vector) $
else $
arr = [ [arr],[(some 1 by 8 vector)] ]
ENDWHILE
but this can be very slow if you concatenate a lot of stuff. For small arrays (low values
of i) it's great. For large values of i, not so good.
2) Truncation
arr=fltarr(8,big_enough)
FOR i=0,big_enough-1 DO BEGIN
...
arr[*,i] = (some 1 by 8 vector)
if ( some condition ) then begin
arr = arr[*,0:i]
BREAK
endif
ENDFOR
this is nearly always faster.
Note: all the above typed off top of head, so no guarantees. Test. Especially the
concatenate stuff. I can never remember how many groups of [[][]]'s to use for multi-D arrays.
paulv
--
Paul van Delst
CIMSS @ NOAA/NCEP/EMC
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