How to obtain total sum by group
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Hi, Could someone explain how to compute the total sum by group (without using for loop)?
A example is: groupid = [1; 1; 1; 2; 2; 3; 3;3]; value=[1;2;3;4;5;6;7;8]. the desired result is: runsum = [6;6;6;9;9; 21;21;21].
I am trying to get the result without using loop. Can "accumarray" do this?
Thanks a lot
Siying
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Accepted Answer
Geoff Hayes
on 18 Dec 2015
Siying - yes, accumarray can be used to sum elements of the same group. For example, we can do
groupSums = accumarray(groupid,value);
which gives us
groupSums =
6
9
21
with your runsum as
runsum = groupSums(groupid);
3 Comments
Geoff Hayes
on 22 Dec 2015
Which is the matrix: the group ids or values, or both? Please provide an example.
More Answers (2)
jgg
on 18 Dec 2015
Yes. This should work; it's probably not the shortest way to do it, but it avoids looping.
key = unique(groupid);
tsum = accumarray(groupid,value);
[~,idx] = ismember(groupid,key);
runsum = tsum(idx);
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Walter Roberson
on 18 Dec 2015
group_sum = accumarray(groupid, value);
runsum = group_sum(A);
Your question reminds me of the similar http://uk.mathworks.com/matlabcentral/answers/260501-unique-and-min-by-index
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