combine Input with formula

I have written the folowing function:

function [ sort1 , sort2 ] = front(x,N)

      Y1=input('frontière inférieure: ','s') ;
      s1=eval(Y1);
      Y2=input('frontière supérieure: ','s');
      s2=eval(Y2);

      [Non important part of the function]

end

The idea is that the user can input either a number, or an expression (example given : y2 = (x^2-36) and y1=0

However, when I run the program, I get the folowing error:

frontière inférieure: 0
frontière supérieure: (36-x^2)

    ??? Error using ==> mpower
    Inputs must be a scalar and a square matrix.

    Error in ==> front at 6
        s2=eval(Y2);

    Error in ==> principal_1 at 18
[y1,y2]=front(xtab,n);

What should I use instead of Input?

Answer 1

[Andrei Bobrov]

try use

s2=eval(vectorize(Y2));

[Karel]

It worked, but I don't understand why, could you explain pplease?