Thread: gravity in 2d game

when user hit up key, jump get set to true and when user let go of up key jump get set to false.
gravity = 2
velocity = 20
y = y postion of my player


problem:

1st - if i hold up key my player go up 20 pics, but when i keep on going down for ever.
2nd - if i hit up key and let go, then hit up key and let go, and so on ..... it keep on going up for ever.

while(true){
    if(jump == true){
      velocity -= gravity;
       y -= velocity;	
    }
    else if(jump == false){
      velocity = 20;
    }
}

problem #2 is bc i am seting velocity = 20 in else if. to fix this problem i can just del the statment.
problem #1 is really hard and i cant find a good sloution for it. may be if i have a test inside of

 if(jump == true){