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From: Thomas Clark on 23 Apr 2010 17:01
Wazza,

Disclaimer: It's 10pm on friday night - totally not thinking straight, so there may yet be a better alternative. Still, this might get you some way there.

I think sortrows() might help, although depending on the size of your data it may not be numerically very efficient. I think sortrows will be quicker than a direct for-loop, but know that there is room for improvement in the underlying sorting algorithm it uses.

Don't have matlab running here, so errors and ommissions excepted - but code should probably go like this:

% Define a vector with a 1 wherever a change is made
dmask = [0 abs(diff(mask))]
% data = [ 1 2 3 4 5 6 7 1 2 3 4 5 6 1 2];
% mask = [ 0 1 1 0 1 1 1 1 0 0 0 0 1 1 1];
% dmask = [ 0 1 0 1 1 0 0 0 1 0 0 0 1 0 0];

% Accumulate that to give ascending, unique values:
cumdmask = cumsum(dmask);
% cumdmask = [ 0 1 1 2 3 3 3 3 4 4 4 4 5 5 5];

% Use sortrows to sort in descending order
% within the groups defined by cumdmask:
inds = 1:numel(data);
sorting_array = [cumdmask', data', inds];
sorted = sortrows(sorting_array, [1 -2]);

% Select the data you want
max_vals = sorted(:, 2);

% And index back into a results array using cumdmask. Note the addition +1 to give one-based indexing.
results = max_vals(cumdmask(:)+1);

In the sortrows command, I included an 'inds' array, which can be used to determine indices of the maximum locations into the original dataset if required, with:
orig_inds = sorted(cumdmask(:)+1,3).
If not required, sortrows can be sped up by ommitting this from the input matrix.

Hope this helps

Tom Clark








Wazza <the.duckman(a)gmail.com> wrote in message <11757c5e-891a-4a02-af53-0199f6836e1f(a)g1g2000pre.googlegroups.com>...
> G'Day All,
>
> Does anyone know if is possible to perfom max calculation for
> alternating values of a mask without using a for loop?
>
> e.g.
> data = [ 1 2 3 4 5 6 7 1 2 3 4 5 6 1 2];
> mask = [ 0 1 1 0 1 1 1 1 0 0 0 0 1 1 1];
> mask_max = [ 1 3 3 4 7 7 7 7 5 5 5 5 6 6 6];
>
> I have a feeling this should be possible, I just cant think of the
> right functions.
>
> -Wazza
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