Question

From the following vector:

depth = [0:0.1:20];

How could I create a new vector that had the same data as ‘depth’ but a value taken every 0.5, providing an outcome of:

d2 = depth(1:5:end);

Since my data is not actually spaced at 0.1 intervals but at random intervals I cannot apply the simple approach shown above. What would be the best methods of achieving this?

Answer 1

If you want to find the closest points in the array to a linearly spaced grid, you can do it like this:

% some example data
A = zeros(40,1);
for ii = 2:numel(A); A(ii)=A(ii-1)+rand; end

% your linear grid
Linds = A(1) : 0.5 : A(end);

% nearest indices
[~,inds] = min( abs(bsxfun(@minus, A, Linds)) );

Note that entries in inds might be repeated. Otherwise, if you want to do an interpolation in your vector, you need interp1. From the documentation:

YI = INTERP1(X,Y,XI) interpolates to find YI, the values of the
underlying function Y at the points in the array XI. X must be a
vector of length N.

Note that multiple different interpolation kernels are available, e.g., 'linear', 'spline', etc. Consult help interp1 for more info.