Using switch–case statements to verify user input

Currently, I am trying to test something but I am stuck. Now I do not have to use switch–case. I could use if–else instead, but I just figured that switch–case statements are easier.

Anyway, I want to use either switch–case statements or if–else statements to verify that the user only inputs numbers and then re-direct them somewhere depending on if the verification failed or passed. Now I have tested this. It works, but it like crashes when the user enters in anything other than only numbers. I would like to put the verification code under the comment /*Error message for if the user entered any non-numbers.*/ if possible. How would I accomplish this?