[Calgon]

You would have to create some code which detects whether a player is actually using their knife at the time (best bet would be to check if they have a knife under OnPlayerKeyStateChange, for KEY_FIRE), then check how close they are to a player, if they're close enough, then you could get that players ID and subtract 50 from their HP.

I've created some code for you, but I haven't tested it. (FYI I started scripting this after I posted, added about 5-10 minutes later).

pawn Код:

// PRESSED(keys) - From the SA-MP wiki
#define PRESSED(%0) \
    (((newkeys & (%0)) == (%0)) && ((oldkeys & (%0)) != (%0)))

public OnPlayerKeyStateChange(playerid, newkeys, oldkeys) {
    if(PRESSED(KEY_FIRE)) { // Check if they pressed KEY_FIRE
        if(GetPlayerWeapon(playerid) == 4) { // Check if they have a knife
            new
                Float: _curHealth, // Variable for storing the looping players health to subtract from. 
                Float: _knifeDamagePos[3]; // Create our variable to store our current players position.

            GetPlayerPos(playerid, _knifeDamagePos[0], _knifeDamagePos[1], _knifeDamagePos[2]);
            foreach(Player, _i) { // Perform a loop to see who is in range
                if(IsPlayerInRangeOfPoint(_i, 1, _knifeDamagePos[0], _knifeDamagePos[1], _knifeDamagePos[2])) {
                    // This code isn't exactly 100% accurate, others close may be damaged by the knife, I've added a return though so it only gets 1 player.
                    GetPlayerHealth(_i, _curHealth); // Get the players current health.
                    SetPlayerHealth(_i, _curHealth-50); // Subtract 50.
                    return 1;
                }
            }
        }
    }
    return 1;
}

Grab foreach and include it for this, or replace the foreach line with:

pawn Код:

for(new _i = 0; _i < MAX_PLAYERS; _i++)

Enjoy.