I am writing a function with an augument to receive some expression, like ' a>0', ' b<0' and 'a>0&b<0'. There are variables named as "a" and "b" in the function's workspace. The augument to receive the expression is char class. How could I to calculate this expression in the function? Must I use "eval"? I have got a suggestion that I should not use "eval". I thought I could turn the char expression to syms with "str2sym". What should I do next? Thank you.