# How to replace a NaN element with an preceeding element in a matrix using logical indexing?

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Ganeshkumar M on 28 Feb 2017
Answered: Jos (10584) on 2 Mar 2017
Hi I have a 300x200 matrix with numbers ranging from 0 to 3. I also have NaN interspersed within the matrix like
[2 1 1 1 1 1 1 2 2 2 2 2;
2 2 3 3 3 N 2 3 3 3 0 0;
2 2 0 N 1 2 1 N 1 1 2 3;] NaN represented as N
is there a way to use logical indexing to replace the elements NaN to the number that precedes it? and perhaps to replace Nan with the element that occurs in the highest frequency out of the preceding 3 elements? Thank you!
I can use if and for loop but would prefer to keep the time short in favour of speed. Any use if I could use isnan to create the matrix to identify NaN first?
Jos (10584) on 28 Feb 2017
• What do you mean by preceding (same row, column before)?
• What if there are two or more NaNs in a row?
• What if the row starts with a NaN?
• What if there is no mode value (mode = value that occurs the most)?

Jos (10584) on 28 Feb 2017
To replace NaNs by the preceding value in the row, assuming no NaN's in the first column:
A = [ 2 1 1 1 1 1 1 2 2 2 2 2
2 2 3 3 111 NaN 2 3 3 3 0 0
2 2 222 NaN 1 2 333 NaN 1 1 2 3]
S2Ifun =@(x,y) sub2ind(size(A), x, y) % an anonymous helper function
[r,c] = find(isnan(A)) % where are the NanN's
A(S2Ifun(r,c)) = A(S2Ifun(r,c-1))
Jos (10584) on 2 Mar 2017
And what about a NaN in the first column?

Chris Turnes on 1 Mar 2017
As far as replacing with the previous non-NaN element, you could check out the fillmissing function that was introduced in R2016b. The fill method 'previous' will replace NaNs with the previous non-NaN element.
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Ganeshkumar M on 2 Mar 2017
This code will be the best. Came across fillts and inpaint_nans but they use interpolation instead of taking the previous.

Jos (10584) on 2 Mar 2017
If you can replace the NaNs with a zeros, you can use my function FILLZERO. If there are zeros in your matrix you can replace them first with a novel value, and later replace them as well.
NovelValue = max(A(:)) + 1 % find a value not in A
A(A==0) = NovelValue % replace zeros
A(isnan(A)) = 0
A = fillzero(A,2)
A(A==NovelValue) = 0 % restore zeros
You can find my FILLZERO function here: