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Math Forum / Math Software / MATLAB / July 2008Calculate mean within grouped rows
I have an N x 3 matrix and I would like to find the mean of values in column 3 grouped by unique rows formed from the values in columns 1 and 2. For example the following N x 3 matrix : 1 2 3 1 2 4 2 3 5 2 4 6 2 4 8 would give the following output : 1 2 3.5 2 3 5 2 4 7 Any suggestions would be greatly appreciated. Thanks, Curt Curt <97wideglide@gmail.com> wrote in message <g5g7kj$1i8$1@registered.motzarella.org>... > I have an N x 3 matrix and I would like to find the mean of values in > column 3 grouped by unique rows formed from the values in columns 1 and 2. [quoted text clipped - 14 lines] > > Any suggestions would be greatly appreciated. Find consolidator on the FEX. http://www.mathworks.com/matlabcentral/fileexchange/loadFile.do? objectId=8354&objectType=file HTH, John Curt <97wideglide@gmail.com> wrote in message <g5g7kj$1i8 $1@registered.motzarella.org>... > I have an N x 3 matrix and I would like to find the mean of values in > column 3 grouped by unique rows formed from the values in columns 1 and 2. [quoted text clipped - 17 lines] > Thanks, > Curt If you are a do-it-yourselfer, I can conceive of a method which uses the 'unique(-,rows)' function applied to the first two rows and uses the third returned index vector along with 'accumarray' to generate your sums (and means.) Is that enough of a hint? (I have the uncomfortable feeling this is very similar to what John's consolidator does. If so, my apologies to John.) Roger Stafford "Roger Stafford" <ellieandrogerxyzzy@mindspring.com.invalid> wrote in message <g5gajd$fh5$1@fred.mathworks.com>... > Curt <97wideglide@gmail.com> wrote in message <g5g7kj$1i8 > $1@registered.motzarella.org>... [quoted text clipped - 30 lines] > > Roger Stafford Consolidator does use unique(...,'rows') to do its work, with a trick. When the numbers are floating point, so a tolerance is applied, it scales the rows and then rounds them. This way unique will do its work properly. John Curt <97wideglide@gmail.com> wrote in message <g5g7kj$1i8 $1@registered.motzarella.org>... > I have an N x 3 matrix and I would like to find the mean of values in > column 3 grouped by unique rows formed from the values in columns 1 and 2. [quoted text clipped - 17 lines] > Thanks, > Curt temp = 1 2 3 1 2 4 2 3 5 2 4 6 2 4 8 >> uidx = unique(temp(:,1:2),'rows') uidx = 1 2 2 3 2 4 meanmat = accumarray(temp(:,1:2),temp(:,3),max(temp (:,1:2)),@mean) meanmat = 0 3.5000 0 0 0 0 5.0000 7.0000 >> FinalMeanMat = meanmat(sub2ind(size(meanmat),uidx (:,1),uidx(:,2))) FinalMeanMat = 3.5000 5.0000 7.0000 Hope that helps!!! Curt: <SNIP block-summing evergreen... one of the many solutions % the data m=-[ 2 4 8 1 2 3 1 2 4 2 3 5 2 4 6 ]; % the engine [mu,mx,mx]=unique(m(:,1:2),'rows'); r=accumarray(mx,m(:,3),[],@mean); % the result disp([mu,r]) %{ -2 -4 7 -2 -3 5 -1 -2 3.5 %} us |
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