variable x(n) binary
maximize(sum((S*x)>0))
subject to
lowerBound <= x <= upperBound
sum(x) == K

where I want the number of non zero element. This works when I use it with matlab only but it does not work with cvx. instead I get the error:
Error using sum
Invalid data type. First argument must be numeric or logical.

actually this is my first time using cvx and I did not understand the pdf in the link quit will. What I need is to maximize the non zero element of a vector I and I couldnt find something similar.

Also, I donâ€™t see how the question in the link related to my problem.

sorry for bothering you but I couldnâ€™t figure it out on my own

In the linked answer, the elements of Sx > 0 play the role of a > b. So declare a binary vector variable delta(n), whose elements are 1 or 0 according to whether the corresponding element of Xx > 0 or not. Then your objective is maximize(sum(delta)). The linked answer shows you the constraints you will need to include in your CVX program.