MATLAB: Sum nX2 matrix

sum elements

Hi
I'm new to matlab. I want to sum elements in a matrix nx2
1   132
1   8989
2   898
2   9898
2   89898
3   90909
3   90909
In my matrix first column is Var1 and second Var2 I want to sum all elements with the same Var1 and create a matrix 12*1 where all elements Var1=x are summed togehter.

Best Answer

Hi Gudmund,
Let's call the matrix, M. Let's find all the entries in column 2, corresponding to the value of 1 in column 1. You can use logical indexing ( to do that as illustrated below:
>> M(M(:,1)==1,2)
ans =
         132
        8989
The above command gives you all the entries in the second column corresponding to an entry of 1 in the first. Hence, you can pass the output as an argument to the "sum" command,
>> sum(M(M(:,1)==1,2))
ans =
          9121
If you want to repeat this for all the values in Var1(i.e. 1,2 and 3) then you can make use of MATLAB for-loop like this:
for i = 1:3
sum1(i,1) = sum(M(M(:,1)==i,2));
end
Refer to the following documentation for logical indexing:
http://www.mathworks.com/help/matlab/math/matrix-indexing.html#bq7eg38
I hope this helps!
Thanks,
Tushar
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