[Slice]

Example usage of bitwise XOR and bitwise AND:
This will give you a variable (s_iChangedKeys) with only the keys that were just pressed down.
You can, for example, do:

pawn Code:

if ( s_iChangedKeys & KEY_FIRE )
{
    // ...
}

pawn Code:

new
    g_iPreviousKeys[ MAX_PLAYERS ] // This variable will hold the previous key state for each player
;

public OnPlayerUpdate( playerid )
{
    // I use static in OnPlayerUpdate because the varaible won't have to get re-initialized every call; therefore, less work for the CPU.
    
    static
        s_iKeys,
        s_iUnused, // blah blah
        s_iChangedKeys
    ;
    
    GetPlayerKeys( playerid, s_iKeys, s_iUnused, s_iUnused );
    
    s_iChangedKeys = ( s_iKeys ^ g_iPreviousKeys[ playerid ] );
    
    // First, filter out the bits that were 1 in both variables; leaving only the bits that has changed.
    
    s_iChangedKeys = s_iChangedKeys & s_iKeys;
    
    // Second, keep only the bits that are 1 in both variables.
    // Now, s_iChangedKeys contain only bits that was off in g_iPreviousKeys and on in s_iKeys.
    
    // You can also save a line by doing: s_iChangedKeys = ( s_iKeys ^ g_iPreviousKeys[ playerid ] ) & s_iKeys;
    
    if ( s_iChangedKeys & KEY_FIRE )
    {
        SendClientMessage( playerid, -1, "you just pressed fire" );
    }
    
    g_iPreviousKeys[ playerid ] = s_iKeys;

    return 1;
}