Sum nX2 matrix

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Gudmund Hjelmqvist 2015-11-3

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回答： Stephen23 2015-11-6

采纳的回答： Tushar Sinha

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Hi

I'm new to matlab. I want to sum elements in a matrix nx2

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.

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Answer 1

Tushar Sinha 2015-11-5

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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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Gudmund Hjelmqvist 2015-11-5

Thank you! This was what I as looking for!

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Answer 2

Stephen23 2015-11-6

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Although there is already an accepted answer to this question, the accepted answer misses the much simpler and neater alternative accumarray, which does not require any loops:

>> M = [1,132;1,8989;2,898;2,9898;2,89898;3,90909;3,90909]
M =
     1     132
     1    8989
     2     898
     2    9898
     2   89898
     3   90909
     3   90909
>> S = accumarray(M(:,1),M(:,2))
S =
   9121
 100694
 181818