How can I solve a problem with subs?

I have to substitute values in a matrix with 20 variables. I substitute variable after variable using a for loop, but in some cases the matrix does not contain the variable the loop is substituting, and in these cases the problem is Matlab replaces the zeros in the matrix wiht the variable. For instance if I am trying to replace the symbolic variable u3 subs(mat,{u3},{0}) and the matrix does not contain u3,the result is that u3 will appear in the matrix positions which were originally 0.(If originally mat(10,10)=0, after substitution mat(10,10)=u3).

Answer 1

[Walter Roberson]

http://www.mathworks.com/help/symbolic/subs.html

subs(mat, {u3}, {0}, 0)

[Carlos]

Thank you very much Walter, you have saved my life.