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Re: Filling the gaps in data arrays

From: Przemek Klosowski
Subject: Re: Filling the gaps in data arrays
Date: Thu, 26 Jul 2018 11:37:15 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.8.0

On 07/26/2018 10:47 AM, Mat086 wrote:
I don't need to interpolate the data, actually the best thing would be to
have NaN where the y-data doesn't match the X vector.
this script is doing what I needed to do:

dx = x(2)-x(1);
Y = zeros(1,length([min(x):dx:max(x)]));
X = [min(x):dx:max(x)];
for k=1:length(X)
   if isempty(y(x==X(k)))
     Y(k) = 0;
     Y(k) = y(st);
You still have the problem with accuracy; I think you will zero out some valid numbers because your calculated X values do not have to fall exactly on your x values.
For a contrived example, x=[0 0.3333333 0.666667 1]
x =

   0.00000   0.33333   0.66667   1.00000

octave:14> dx = x(2)-x(1);
octave:15> Y = zeros(1,length([min(x):dx:max(x)]));
octave:16> X = [min(x):dx:max(x)]
X =

   0.00000   0.33333   0.66667   1.00000

octave:17> x==X
ans =

  1  1  0  0

As you see, x and X are so very close that they print as identical, but they do not match. You can search for closeness:

match = ( abs(x-X)<my_eps )

and then exploit the match vector to just transfer the matching y values:

dx = x(2)-x(1);
X = [min(x):dx:max(x)];
Y = zeros(1,length(X));
Y(find(match))= y

Octave/Matlab language has those 'vector' expressions that make it quite fast 
and concise. The cost is maybe the extra storage (e.g. the match vector, and 
the temporary vectors used by the find() operation) but in practice it is a big 
win and it's good to learn to write code this way.

BTW, if you want to really use NaNs, you could do this:
Y = repmat(NaN,1,length(X));

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