[ncl-talk] Using functions

[Wei Huang]

Emma,

Your function is slow is because you did not take the advantage of NCL's
array operation, but looped through individuals.

You may try to change your code to something like:

undef("Qsave")
function Qsave(quality:numeric)
local ntime,nxtrack,nx,ii,ic,bit,bstr,nn,mm,xq,xq_tmp,xq_ref,xq_ref1
begin
  xq_ref1 = "1010101010011111" ; Never allow these flags to be set in the
filtering
  xq_ref = stringtocharacter(xq_ref1)
  nbits = dimsizes(xq_ref)
  q_save  = new(dimsizes(quality),integer)
  dims = dimsizes(quality)
  ntime   = dims(2)
  nxtrack = dims(1)
  ii = touint(quality)
  bstr = new((/ntime, nxtrack/), string)
  bstr = ""
  ic = ii

  do nn = 0, nbits-1
     bit  = (ic % 2) ; pick off the lowest bit
     ic   = ic/2 ; shift right one bit, drop lowest
     bstr = bit + bstr
  end do ; nn

  xq = toint64(bstr)
  q_save(nt,nx) = where(xq .eq. 0, 0, 1)

  return(q_save)
end

and let us know if this works for your case.

Regards,

Wei

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On Thu, Jan 15, 2015 at 3:35 AM, Emma Andersson <emma.andersson at chalmers.se>
wrote:

> Hi NCL-talk,
>
> I’m having some trouble with a my own written function that I call within
> a loop.
>
> The function I’ve written converts quality flags from a satellite data set
> into an array of integers with zeros and ones.
> The quality flags are given in ushort and corresponds to a 16-bit number.
> What I do is that I check each pixels
> bit number if certain flags has been set. I compare with a reference bit
> number which contains the flags that I don’t want to be set
> and then determines whether my new array value should be set to a one or a
> zero. I have basically written a kind of “bitand” function,
> where if the output from the comparison are all zeros then I set my new
> array value to zero. I hope this description was understandable.
>
> What happens when I call my function from a main script, is that it goes
> slower and slower for each step in the loop. My question is,
> why it goes slower? Is something accumulating inside the function for each
> time I call it? I’m quite sure it doesn’t accumulate outside
> in the main script, because I did some print statements inside the
> function and it was clearly there it was going slower. I also delete
> the output in the main script after it being used? Would it be better to
> use a procedure instead of a function?
>
> Kind regards,
> Emma Andersson
>
>
> -----------------------------------------------------------------------------------
> PhD student
> Global Environmental Measurement Techniques and Modelling
> Earth and Space Sciences
> Chalmers University of Technology
> Gothenburg, Sweden
> emma.andersson(at)chalmers.se
>
> -----------------------------------------------------------------------------------
>
>
>
>
>
>
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