Sum nX2 matrix

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.

Answer 1

[Tushar Sinha]

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

[Gudmund Hjelmqvist]

Thank you! This was what I as looking for!

Answer 2

[Stephen23]

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