Vectorizing 'for' loops

This piece of code works as I want it to, but in the spirit of good MATLAB code, is there a way to vectorize this (previous is a k x 1 vector):

start = zeros(k,1);
for i = 2:length(previous)
    if (previous(i-1) == -1)
        start(previous(i))= start(previous(i))+1;
    end    
end

What, in general, is the intuitive way to go about vectorizing code in MATLAB?

You can do this without find, for maximum performance:

I = [false; previous(1:end-1) == -1];
idx = previous(I);
start(idx) = start(idx) + 1;

This also avoids the risk that previous(end) == -1, which would cause an index out-of-range error in the alternative.

Note that this doesn't work the same as your original if idx contains duplicated indices.

Use the find command in MATLAB, which returns the index (i) for which a vector is TRUE. So:

% precache indices i where previous(i-1) == 1
idx = find(previous==-1)+1;
start(previous(idx)) = start(previous(idx))+1;

The reason I precache idx is in case that previous is a big vector and doing the find takes a while. Otherwise you could just do

start( find(previous==-1)+1 ) = start( find(previous==-1) ) + 1;

I would do something like:

prev_is_minus1 = [false; previous(1:end-1)==-1]
start = accumarray(previous(prev_is_minus1), 1, k)

I believe this has the same effect as the code loop you posted.

来源：https://stackoverflow.com/questions/8462330/vectorizing-for-loops

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