How to perform a "find" in Matlab using multiple parameters?

Question

I'm looking for a way to "vectorize" the following code. That is, I want to get rid of the for loop, which is taking a long time (this for loop is nested in another for loop that repeats more than 40,000 times).

for k=1:length
    if coords(k,1)<=4 && coords(k,2) <=8
        upperLeft(countUL,:) = coords(k,:);
        countUL=countUL+1;
    end
    if coords(k,1)>4 && coords(k,2) <=8
        upperRight(countUR,:) = coords(k,:);
        countUR=countUR+1;
    end
    if coords(k,1)>4 && coords(k,2) >8
        lowerRight(countLR,:) = coords(k,:);
        countLR=countLR+1;
    end
    if coords(k,1)<=4 && coords(k,2) >8
        lowerLeft(countLL,:) = coords(k,:);
        countLL=countLL+1;
    end
end

I tried at first to use the Matlab find function (e.g. find(coords(k,1)<=4)), but in my case I have two parameters that I need to "find". I tried something like find(coords(:,1)<=4 && coords(:,2)<=8), but since the operands of && are not scalar, this doesn't work. Any ideas on how to do this would be most appreciated!

I suggest you don't use length as a variable name (or maybe that was a typo) — Jonas
– Jonas, Commented Oct 9, 2012 at 12:07
use Matlab's FEX Consolidator? mathworks.com/matlabcentral/fileexchange/8354 — bla
– bla, Commented Oct 9, 2012 at 14:42

Jonas · Accepted Answer · 2012-11-02 15:55:36Z

7

&& and || work only for scalar comparisons, as you've noticed. & and | work on vectors, though. Note that you don't even need find:

idxUL = coords(:,1) <= 4 & coords(:,2) <=8;
idxUR = coords(:,1) > 4 & coords(:,2) <=8;

upperLeft = coords(idxUL,:);
upperRight = coords(idxUR,:); %# etc

edited Nov 2, 2012 at 15:55

answered Oct 9, 2012 at 12:06

Jonas

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Jonas Over a year ago

@robguinness: I implemented your proposed edit that was (IMO wrongly) rejected. Thanks for spotting this!

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