How do I select n elements of a sequence in windows of m ? (matlab)

Question

Quick MATLAB question. What would be the best/most efficient way to select a certain number of elements, \'n\' in windows of \'m\'. In other words, I want to select the fir

Answer 1

Consider the following vectorized code:

x = 1:100;                                     %# an example sequence of numbers

nwind = 50;                                    %# window size
noverlap = 40;                                 %# number of overlapping elements
nx = length(x);                                %# length of sequence

ncol = fix((nx-noverlap)/(nwind-noverlap));    %# number of sliding windows
colindex = 1 + (0:(ncol-1))*(nwind-noverlap);  %# starting index of each

%# indices to put sequence into columns with the proper offset
idx = bsxfun(@plus, (1:nwind)', colindex)-1;   %'

%# apply the indices on the sequence
slidingWindows = x(idx)

The result (truncated for brevity):

slidingWindows =
     1    11    21    31    41    51
     2    12    22    32    42    52
     3    13    23    33    43    53
    ...
    48    58    68    78    88    98
    49    59    69    79    89    99
    50    60    70    80    90   100

In fact, the code was adapted from the now deprecated SPECGRAM function from the Signal Processing Toolbox (just do edit specgram.m to see the code).

I omitted parts that zero-pad the sequence in case the sliding windows do not evenly divide the entire sequence (for example x=1:105), but you can easily add them again if you need that functionality...