problem in calculating mean from the required element of matrix??

I have a matrix mat6 of dimension 1-by-210. j =210. I am applying a threshold condition on mat6 and then i need to calculate the mean of next twenty elements . I want that the mean should be calculated from that first element of mat6 at which threshold condition was met at first time. But it is calculating mean from that last element of mat6 at which the threshold condition was met.

for k  = 1:j-20
if mat6(k)>0.5
    w=k+20;
   mea = sum(mat6(1,k:w))/20;
end
 end

can anyone please sort out this problem ??

Thanks

Navneet Nayan

[Jan]

k:k+20 are 21 elements, not 20.

Answer 1

[KL]

k = 1;
while k <j-20
    if mat6(k)>0.5
        mea = mean(mat6(1,k:k+19))
        k=k+20;
    else
        k = k+1;
    end
end

[NAVNEET NAYAN]

thanks KL... It worked....

Answer 2

[Jan]

index = find(mat6 > 0.5, 1);  % Find 1 index only
mea   = mean(mat6(index:index+19));

Consider that index+19 might exceed the data size. Handle this situation e.g. by an error, e.g.:

index = find(mat6 > 0.5, 1);
if index + 19 > size(mat6, 2)
  error('Index out of range in mat6');
end

[KL]

Jan, I have a follow up question of my own. When index is a vector, is it possible to avoid loop?

[Andrei Bobrov]

@KL: index -> vector

mea  = mean( mat6(index(:)' + (0:19)') );              % MATLAB >= R2016b
mea  = mean( mat6( bsxfun(@plus,index(:)',(0:19)')) ); % MATLAB < R2016b

[NAVNEET NAYAN]

thanks Jan, it worked .....nice one.....

[KL]

Thanks Andrei!

[NAVNEET NAYAN]

tell me one thing Jan, if i want to use the subsequent indices for eg. i want to use the second or 3rd index, how can I proceed??

[KL]

index = find(mat6 > 0.5);
%this finds all the indices and then you could follow Andrei's command. (with the error check by Jan)
mea  = mean( mat6( bsxfun(@plus,index(:)',(0:19)')) );