who is x?
That message is the result of trying to access an non-existent element of the array, such as x(0).
Make sure x has the right size.
By the way, your for-loop code
x= rand(3,4); % sample data
[M,N]=size(x);
y=zeros(1,N);
for l=1:N
sum1=sum(x(:,l).*log(x(:,l)));
sum1=-1*sum1;
y(1,l)=sum1;
end
Is the same that this, in a single line:
for l=1:N
y(l) = -( sum(x(:,l).*log(x(:,l))) );
end
