MATLAB remove NaN values from matrix and shift values left - matlab

Question

I am trying to compute column-wise differences in the following matrix:
A =
0 NaN NaN 0.3750 NaN
NaN 0.1250 0.2500 0.3750 NaN
I would like to obtain:
0.3750 NaN NaN
0.1250 0.1250 0.1250
Where I am essentially taking a columnwise difference, skipping NaN values and shifting values to the left.
A one-dimensional case would be:
A = [0 NaN 0.250 0.375 NaN 0.625];
NaN_diff(A) = [0.250 0.125 0.250];
Any way to do this efficiently in MATLAB without using inefficient find() queries per row?

Answer 1

Here's a solution that vectorizes most of the operations:
notNan = ~isnan(A);
numNN = sum(notNan,2);
shifted = NaN(size(A));
for r = 1:size(A,1)
myRow = A(r,:);
shifted(r,1:numNN(r)) = myRow(notNan(r,:));
end
nanDiff = diff(shifted,1,2);

Answer 2

Here is an alternative vectorized solution:
%// Convert to cell array without NaNs
[rows, cols] = size(A);
C = cellfun(#(x)x(~isnan(x)), mat2cell(A, ones(1, rows), cols), 'Uniform', 0);
%// Compute diff for each row and pad
N = max(sum(~isnan(A), 2));
C = cellfun(#(x)[diff(x) nan(1, N - length(x))], C, 'Uniform', 0);
%// Convert back to a matrix
nandiff = vertcat(C{:});
If you want to pad the result matrix with zeroes instead of NaN values, change the nan function call in nan(1, N - length(x)) to zeros.

Answer 3

Here is an alternative method that does require you to loop over each row, but should still have decent performance and feels very intuitive to me.
B = NaN(size(A,1),size(A,2)-1)
for i = 1:size(A,1)
idx = ~isnan(A(:,i))
B(i,1:sum(idx)) = diff(A(i,idx))
end

Answer 4

I'm aware that this is a rather old question, but for people like me who stumble into this page, here is a simpler (imho) solution to the question:
A = [0 NaN 0.250 0.375 NaN 0.625];
A(isnan(A))=[]; % identify index of NaN values and remove them from the array
B = diff(A);

Answer 5

Here is another simple solution without using a loop [but assuming all values are in ascending order]:
A=[0 NaN NaN 0.3750 NaN;NaN 0.1250 0.2500 0.3750 NaN]
A(isnan(A(:,1)))=0;
B=sort(A,2);
C=diff(B,1,2)