C/C++ 2D SDL Tile Based Game, Gravity problem

Main Entry
 Game Loop
  Player->getInput
  Background->Draw
  Map->Draw(tiles)
  Player->update
  Player->draw
  Screen->update
 End Game Loop
End Main

// player_velocity_y will equal -10, for moving up.
// GRAVITY for example will = 1 and eventually make player_velocity_y equal 0
// I simply re-wrote what I have in the actual source file to be more legible
// but everything here is exactly how I have it.

// Player is moving up
if(player_velocity_y < 0) {
     player_ypos += player_velocity_y;
     player_velocity_y += GRAVITY;
     // Player hit top of screen or a tile above him
     if(player_ypos < 0 || collision_vertical(tiles)) {
          player_ypos -= player_velocity_y;
          player_velocity_y = 0;
     }
}
// Player is moving down
else {
     player_ypos += player_velocity_y;
     player_velocity_y += GRAVITY;
     player_lockjump = true; // player may not jump in air
     // Player hit the ground tiles
     if(collision_vertical(tiles)) {
          player_ypos -= player_velocity_y; // don't fall through tile
          player_velocity_y = 0; // no more vertical speed
          player_lockjump = false; // player may jump again
     }
}

player_ypos = tilecoordinates*TILESIZE-player_height-1;