How do I run through a vector combining every 3 values together

Hi,

What I would like to do is run through a vector and combine every say 3 values together with some function (say the sum() function) and output a new vector. eg. A = [1 1 1 2 2 2 3 3 3]; Anew = A[sum(1:3), sum(4:6) sum(7:9)] = [1+1+1 2+2+2 3+3+3] = [3 6 9] I would like to be able to do this regardless of the length of A. Obviously I can do it in a loop, but wondered whether there was a more efficient syntax/implementation.

Thanks, Chris

[Oleg Komarov]

Does A always start with 1 and repeats the value for as many times the block is long and then increases by one?

[Oleg Komarov]

Anyway, for this particular A:

accumarray(A,A)

[Walter Roberson]

Perhaps

accumarray(A(:),A(:))

Each row forms one set of subscripts so you usually need to transform the row vector A in to a column vector.

[Oleg Komarov]

I am sloppy today!

Answer 1

[Fangjun Jiang]

A = [1 1 1 2 2 2 3 3 3];
B=reshape(A,3,[]);
C=sum(B)

Added error checking

A=[1 1 1 2 2 2 3 3 3];
N=2;
try
    B=reshape(A,N,[]);
    C=sum(B);
catch
    disp(lasterr);
end

[Jan]

+1: Very direct, very efficient.

Maybe it is necessary to catch inputs, whose length is not a multiple of 3.

[Fangjun Jiang]

Good thought! See update.

[Sean de Wolski]

or:

if ~mod(numel(a),3)

%do good stuff

else

%complain

end