Replacing only specific column values in an array in Matlab

Question

I have an array that has about 70,000 rows that look like the following:

A = [ 1 2 3 4 5 6 0 723 1 22

      1 2 3 4 5 7 0 NaN 2 10 

      etc..                 ]

I would like to replace all NaN values in my array with the preceding value in the same column as the NaN value so that I would have something like this:

B = [ 1 2 3 4 5 6 0 723 1 22

      1 2 3 4 5 7 0 723 2 10 

      etc..                 ]

Since the array has about 70,000 rows, I am guessing some sort of loop would be the best way to achieve this. Using the following:

for ii = 1:size(A,2)
        I = A(1,ii);
        for jj = 2:size(A,1)
            if isnan(A(jj,ii))
                A(jj,ii) = I;
            else
                I  = A(jj,ii);
            end
        end
        end

I have been able to make a loop that replaces the whole row with the preceding row, but I am unsure how to modify it to target only NaN values. Any help would be greatly appreciated!

Answer 1

As long as there are now NaN 's in the first row, you can just replace the values directly:

for ii = 1:size(A,2)
    for jj = 2:size(A,1)
        if isnan(A(jj,ii))
            A(jj,ii) = A(jj-1,ii);
        end
    end
end

Which avoids creating a second matrix B .

I think you're right, an you do need to use for -loops since your matrix is so large, but here is a non- for loop version anyway:

while(any(isnan(A(:))))
    A(find(isnan(A)))=A(find(isnan(A))-1)
end

Answer 2

Not sure why the I = A(1, ii); or the else branch is needed. If a NaN is found, just replace it with the value from the previous row:

for ii = 1:size(A,2)
    for jj = 2:size(A,1)
        if (isnan(A(jj,ii)))
            A(jj,ii) = A(jj-1, ii); %jj-1 is the previous row
        end
    end
end