Carrying a knife for defense, what a horrible, horrible idea. If you use it against someone who is unarmed you should get arrested. If you use it against someone who has a gun you will get shot. If you use it against someone with a knife, well, you are an idiot by definition. Only idiots get into knife fights. No one wins. Unless you are leaving out that you are the reincarnation of Bruce Lee or that you trained as a ninja.
BTW do not pull out a deadly weapon unless you are in a situation where you have to kill someone to protect the life of yourself or someone else. If you pull a knife because someone is bothering you then expect a long stay in jail. If you pull it on someone with training or talent it might just piss them off so then expect a severe beating or worse.