MATLAB: Taking mean of certain range ignorning NaN in matrix

matrix indexingmeannanmeanskipping values

I have a matrix Phi of size 500×359 with NaNs interspersed throughout. I need to take the column wise mean of the first 25 rows of non-NaN values for each column, but unlike nonmean() I dont want to consider rows with NaNs as rows at all. I want them to be skipped over completely. So I want to kinda of do the following nanmean(Phi(1:25)) where 1 is the first nonmean value regardless of its actual location in the matrix whether its the 1, 100, 200 etc and take the mean of the following 25 non-Nan values regardless of the location.
For example:
X=magic(4); X([1 3 7:9 14]) = repmat(NaN,1)
X =
   NaN     2   NaN    13
     5    11    10   NaN
   NaN   NaN     6    12
     4   NaN    15     1
nanmean(X(1:2,3)) ans =
    10
but what I want is (10+6)/2 = 8
and… so my end result would look like this…
phi_mean = [4.5000 6.5000 8 12.5000]
thank you

Best Answer

nn = ~isnan(X);
ii = cumsum(nn).*nn;
out = mean(reshape(X(ii >= 1 & ii <= 25),25,[]));
other way
X = phidp;
k = 25;
ll = ~isnan(X);
[~,j0] = find(ll);
out = accumarray(j0,X(ll),[1 size(X,2)], @(x)mean(x(1:min(k,numel(x)))) );
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