# Add an element to a vector after taking difference

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Christoph Meier on 3 Sep 2015
Commented: Star Strider on 3 Sep 2015
Dear Matlab community,
I am currently running some stationarity test, for which I take log-differences. However, if stationatiry tests still suggest non-stationarity, I would have to take the second difference.
The problem obviously is, that taking another difference changes the vector size, as now some of my variables are by 1 element shorter (those with the second difference).
I would like to add a zero as the first element to those column vectors, in order to restore equal dimensions.
It would basically look like this:
A =
X
X
X
B=
0
X
X
X
Thank you very much in advance!
Chris

Star Strider on 3 Sep 2015
That is easy enough to do:
B = [0; A];
However another way might be to use interp1 to artificially ‘stretch’ ‘A’:
A = randi(9, 14, 1); % Create Data: ‘A’
Ax = [1:14]'; % Independent Variable Vector For ‘A’
Bx = [1:15]'; % Independent Variable Vector For ‘B’
B = interp1(Ax, A, Bx, 'linear', 'extrap'); % Interpolate/Extrapolate To Calculate ‘B’
I offer it as an alternative.