CODEWITHSUNDEEP
matlabvectorization
x1hgg1x
x1hgg1x

Reputation: 125

Vectorized programming with Matlab

I have a vector J which contains the row index of a vector. I would like to do the following code in a vectorized way : I want to sum the duplicate values to the variable 'Mode' from the variable 'M'. I have size(M)=size(J). This is my attempted code:

Mode=zeros(n,1);
for i=1:length(J)
    Mode(J(i))=Mode(J(i))+M(i);
end

I have tested with

Mode(J)=M

but the problem is that there are some duplicate index value in J. How can I implement it correctly ?

Upvotes: 1

Views: 79

Answers (1)

Stewie Griffin
Stewie Griffin

Reputation: 14939

You're probably looking for accumarray.

Have a look at this example:

J = [1 2 3 5 2 3].';
M = [1 1 1 1 1 1].';
Mode = accumarray(J, M, size(J)) 

Mode =

     1
     2
     2
     0
     1
     0

From the documentation:

A = accumarray(subs,val) returns array A by accumulating elements of vector val using the subscripts subs. If subs is a column vector, then each element defines a corresponding subscript in the output, which is also a column vector. The accumarray function collects all elements of val that have identical subscripts in subs and stores their sum in the location of A corresponding to that subscript (for index i, A(i)=sum(val(subs(:)==i))). Elements of A whose subscripts do not appear in subs are equal to 0.

size(J) is used to make sure the dimension of Mode is the same as the dimension of J.

According to OP, the following code works:

A=accumarray(J,M); 
Mode(1:size(A))=A;

Upvotes: 3

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