Create matrix by repeatedly overlapping a vector

Question

I'm having great difficulty coding the following in MATLAB: Suppose you have the following vector:

a   
b
c
d
e
f
g
h
...

Specifying an (even) window size, create the following matrix of dimensions L rows by n columns (example, L = 4):

a c e ...
b d f ...
c e g ...
d f h ...

Even more difficult is taking a vector of arbitrary length, specifying the number of windows, and optimizing (maximizing) the window size so less values at the end of the vector are dumped.

Answer 1

Create the matrix of indices into your vector. For L=4 (I assume you are overlapping by L/2), the indices are [1,2,3,4;3,4,5,6;5,6,7,8] etc. Let x = 1:L, y = L/2, the vector of indices is x+0y,x+1y,x+2y, and so on.

% let your initial data be in vector "data"
L = 4
N = floor(length(data)/(L/2))-1 % number of windows, or you specify this
mi = repmat(1:L,[N,1]) + repmat((L/2) * (0:(N-1))',[1,L]) % x + y * 0,1,2...
out = data(mi) % out is N-by-L, transpose to L-by-N if you like