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max - matlab: sum values of different sized subarrays without loop

问题描述:

I wonder if it is possible to get min/max values of different sized subarrays

without using a loop in matlab.

% create a 1D vector with arbitory floating point values

A = rand(100,1,'double');

% get start indexes of sections of A (eg. A>0.9)

pos01 = A>0.9;

posIdx= [1;find(pos01);size(A,1)];

% if 1 or size(A,1) where not required, remove them

posIdx = unique(posIdx);

% min/max all sections:

for ix=1:size(posIdx,1)-1

Amin(ix) = min(A(posIdx(ix):posIdx(ix+1)));

Amax(ix) = max(A(posIdx(ix):posIdx(ix+1)));

end

This last row can be very slow if you have a very large vector A and lots of sections.

I wonder how to vectorize this loop in matlab.

I tried to come up with a solution using arrayfun, remap, bsxfun and others.

But all the solutions I can think of, require the sections to have equal size - which is not the case :(

Any ideas ?

best,

Jens Henrik

网友答案:

Use cumsum and accumarray

A = rand(100,1,'double');
pos01 = A > 0.9;
subs = ( pos01(1) == 0 ) + cumsum( pos01(:) ); % convert posIdx to subs. note the special treatment of the first entry. 
Amin = accumarray( subs, A, [], @min );
Amax = accumarray( subs, A, [], @max );
网友答案:

bsxfun approach to replace your for-loop part -

t1 = bsxfun(@le,1:size(A,1),posIdx(2:end)) & bsxfun(@ge,1:size(A,1),posIdx(1:end-1));
t1 = t1';
t2 = A(:,ones(1,size(t1,2))).*t1;
t2(~t1)=nan;
Amin = nanmin(t2);
Amax = nanmax(t2);
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