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Asked by Giorgos Papakonstantinou
on 16 Jun 2013

I want to mean the matrix a based on the logical matrix b columnwise: In other words this:

mean(a(b(:,1))) mean(a(b(:,2))) ...

a =[

7 8 5 3 1 1 4 7 9 3 8 7 10 1 8 2 7 1 2 2 8 9 5 5 8 7 5 10 4 4 7 4 7 10 8 6 2 1 8 3];

b =logical([

0 0 1 0 1 0 1 0 1 0 1 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 1 0 0 0 1]);

Answer by Jan Simon
on 16 Jun 2013

Accepted answer

Or:

m = sum(a .* b, 1) ./ sum(b, 1);

Show 1 older comment

Wayne King
on 17 Jun 2013

I agree Jan, +1, silly of me not to think of simply using the b matrix to get the correct number of nonzero elements.

Answer by Andrei Bobrov
on 17 Jun 2013

Edited by Andrei Bobrov
on 18 Jun 2013

b1 = bsxfun(@times,b,1:4); out = accumarray(b1(b),a(b),[],@mean);

or

[~,jj] = find(b); out = accumarray(jj,a(b),[],@mean);

and in line with `accumarray`:

out = accumarray(ceil(find(b)/size(b,1)),a(b),[],@mean);

Giorgos Papakonstantinou
on 17 Jun 2013

I will celebrate the day that I will understand accumarray... Thank you Andrei

Answer by Wayne King
on 16 Jun 2013

Edited by Wayne King
on 16 Jun 2013

you mean just:

mean(a.*b)

or do you want the mean of just the nonzero entries? In other words, divide by the right number of elements.

C = a.*b; for nn = 1:size(C,2) numelements(nn) = nnz(C(:,nn)); end colsumz = sum(C); meanz = colsumz./numelements;

## 1 Comment

## Jan Simon

Direct link to this comment:http://www.mathworks.com/matlabcentral/answers/79235#comment_155534

This is obviously an interesting question, which allows to shed light on different powers of Matlab. +1