Index of NaN Values

Question

Tiago Dias 2018-7-23

编辑： Walter Roberson 2018-7-23

Hello,

My objective is to know the index of NaN values per column.

y = [1,2;NaN,4;5,NaN;7,8;9,NaN]
idx = isnan(y)
for j = 1:m
  for i = 1:n
      if idx(i,j) == 1
          idx_all(i,j) = i
      end
  end
end

I get a result of

idx_all =    0     0
               2     0
               0     3
               0     0
               0     5

I would like the result to be something like

idx = [2 3
       0 5]

Because the 1st column has just one NaN on row 2, and the 2nd column has NaN for row 3 and 5.

Thanks.

jonas 2018-7-23

Is it really necessary to save the indices in that form? You should read about the find function.

Tiago Dias 2018-7-23

yeah, but i would like to eliminate the zeros as well.

Answer 1

Walter Roberson 2018-7-23

A loop is easiest.

nc = size(y,2);
idx = zeros(0, nc);
for C = 1 : nc
   nidx  = find(isnan(y(:,C)));
   idx(1:length(nidx), C) = nidx;
end

Tiago Dias 2018-7-23

Thanks !! :)