I have done quite a few martial arts. Anyone who tells you you can learn X and fight against someone who is armed (knife or gun) is simply spouting B.S.

If someone pulls out a gun on you, give that person what he wants and pray you are not going to end up shot anyway.

If someone pulls out a knife on you, do the same. Don’t play hero, especially if the guy holding the knife seems to know what he is doing.

Martial arts are just a way to train your body and your mind, both trainings are valuable in and out of themselves. They will keep you calm in a tense situation, they may even save your life since no one wants to mess with a dude that keeps his cool. Ultimately, a street fight can be avoided just by looking calm and composed.