What ways? And why?
A knife ain't gonna help you much if someone's going to shoot you.
- - - Updated - - -
You would be arrested and charged, you are not allowed to use force that is disproportionate to the danger you are faced, i.e. if someone punches you and you stab them you will be charged. There's a line between getting a black eye and fatal injury.
I feel like having a weapon on you will cause more harm than good. Just my opinion though.