[ncl-talk] Advice on extracting point locations from array

[Will Hobbs]

Hi Hoffman

Thanks for your suggestion – I actually just wrote something similar, except looping through the vertical coords and using ndtooned(). Your method of using reshape() is much more elegant though (albeit with a larger memory burden).

Thanks again

Will


From: Hoffman Cheung <hoffmancheung at gmail.com>
Date: Wednesday, 17 October 2018 at 4:52 PM
To: Will Hobbs <will.hobbs at utas.edu.au>
Cc: ncl-talk NCL <ncl-talk at ucar.edu>
Subject: Re: [ncl-talk] Advice on extracting point locations from array

Hi Will,

Given that your 4-D ocean data is T4D(time|nTime,depth|nDepth,lat|nLat,lon|nLon) and you are trying to extract the profile pInd(nProfile,3),
where pInd(:,0), pInd(:,1) and pInd(:,2) corresponds to the index of time, lat and lon dimension, respectively.

I would try to do the following way:

; rearrange the 4-D ocean data
T4DR = T4D(depth|:,time|:,lat|:,lon|:) ; make sure all dimensions are named

; convert the 4-D data into 2-D data
T2D = reshape(T4DR, (/nDepth, nTime*nLat*nLon/))

; convert pInd
pInd1D = pInd(:,0)*(nLat*nLon) + pInd(:,1)*nLon + pInd(:,2)

; extract the profile from 2-D data
TProfile = T2D(:,pInd1D)

If the above method runs out of memory, you might perform a do loop over the dimension depth, instead of every profile (if you have a huge amount of profiles).

Regards,
Hoffman




Will Hobbs <will.hobbs at utas.edu.au<mailto:will.hobbs at utas.edu.au>> 於 2018年10月16日 週二 下午1:53寫道：
Hi all

This one is for the NCL ‘Brains Trust’. I’m extracting a large number of ocean profiles from a 4-d array (dimensioned time,depth,lat,lon), using coordinate triplets based on time,lat and lon, (i.e. a 2-d array of indices dimensioned nprofile, 3),  but it’s a slow process and I wonder if anyone has any smart ideas on speeding it up.

At the moment (partly to manage memory), I’m reading each profile in a loop, viz:


Tprof = new((/nlev,nprofile/),float)
do i = 0, nprofile-1
     Tprof(:,i) = fi->T(pInd(i,0),:,pInd(i,1),pInd(i,2))    ;pInd is an array with coord triplets
end do

Obviously one obvious way of speeding things up would be to read the entire array of input data and extract the locations, to avoid multiple i/o calls. What I really want to do though is somehow get rid of the loop entirely. Is there any way of extracting coordinate pairs (or in this case triplets) without looping through each profile?

The only way I can think of is to turn the input array (T in my example above) into a 1-d array, and somehow convert ‘pInd’ into the elements of that 1-d array – this seems fraught with opportunity for error though. Catastrophic error I can deal with – it’s the sneaky, inobvious error that scares me….

Hoping someone has a great idea….

Will


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