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How to shift certain arrays in a matrix downward

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SMA
SMA on 9 Feb 2016
Commented: SMA on 9 Feb 2016
I have a large matrix where data for each month of 100 years is stored for 12 variables. Some of the variables have data starting from later date. How can a shift those variables down. For example I have a matrix;
1 4 6 9 0 2
3 6 8 8 2 5
2 5 6 8 9 0
3 5 7 9 2 1
And I'd like the output to be (with NaNs replacing the empty places);
1 4 N N N 2
3 6 6 N N 5
2 5 8 9 0 0
3 5 6 8 2 1
Provided I have stored in a different matrix what the lag would be, in this case say lag=[0, 0, 1, 2, 2, 0].

Accepted Answer

Jos (10584)
Jos (10584) on 9 Feb 2016
This is a fast and easy-to-read solution:
M = [1 4 6 9 0 2
3 6 8 8 2 5
2 5 6 8 9 0
3 5 7 9 2 1] % matrix
S = [0 0 1 2 2 0] % shift for each column
M2 = nan(size(M)) ; % pre-allocation for speed
for k=1:size(M,2) % loop over columns
M2(1+S(k):end, k) = M(1:end-S(k), k) ; % copy column k with a shift
end
disp(M2)
  1 Comment
SMA
SMA on 9 Feb 2016
Great, I was doing something similar but messed up somewhere. It works with minor modification for my data.

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