# How to: sum up elements of a row vector into bins of a matrix

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Tero on 15 Jan 2021
Commented: Cris LaPierre on 18 Jan 2021
Hello,
so I've got a matrix and bin index vectors from histcounts2; I've binned some data over a sphere.
[N,~,~,bx,by]=histcounts2(dataX, dataY, (0:5:180)', (0:5:355)' );
I have another row vector, power, that is equal in length with dataX, dataY, so the bx and by can be used to collect data from this power vector.
Question is, how do I use the bx and by, to generate a matrix out of the power vector, which equals N in size and in terms of the number of elements within bins, but the bin value should be the sum of the elements dedicated for this bin and collected from the power vector?
Thanks,
Tero

Cris LaPierre on 15 Jan 2021
Edited: Cris LaPierre on 15 Jan 2021
With a little creativity, you can use groupsummary. Since you haven't shared enough of your code to use that for an example, here's one modified from an example on the histcount2 documentation page.
% Set up the data
x = randn(100,1);
y = randn(100,1);
[N,~,~,bx,by] = histcounts2(x,y);
% create a 3rd vector, pwr
pwr = randn(100,1);
% Sum the data by grouping pwr by bin numbers by and bx.
% Sum data in the same group.
% Include data for all possible bins
[P,G,C] = groupsummary(pwr,[by,bx],'sum',"IncludeEmptyGroups",true,"IncludeMissingGroups",true);
% Reshape the data to be the same size an N
p = reshape(P,size(N))
p = 5×6
0 1.9431 -0.8699 0.8640 -1.8837 0.0999 0 -1.1247 2.2520 2.9794 2.7351 0 0.1459 -3.4099 2.6683 -1.2993 1.0805 0.9534 2.2462 0.1371 5.2773 0.1546 0 0 0 0 0.1829 2.2118 0 0
N % compare to verify that when N==0, P==0.
N = 5×6
0 2 4 6 4 1 0 7 6 12 6 0 1 6 7 12 6 1 1 2 10 4 0 0 0 0 1 1 0 0
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Cris LaPierre on 18 Jan 2021
You may be able to achieve something similar using grpstats. You could also try findgroups and splitapply.