design - Limiting a value in a user form input -

say if have text box input user key in value.

this value passed function, say:

f(x,y) = x / y; 

where x,y user inputs.

in case, if y == 0, divide 0 error occur.

my question practice, section of code should responsible error checking? it

1) read text box value, automatically change value else if equal 0.

or

2) when 0 passed function f(x,y), within function, handle potential divide 0 error.

you want errors apply (such divide 0 error) caught function itself. don't want use function in different part , need rewrite same error checking (or worse, forget to).