I have to to game for my school project.

I have a little problem with selecting part of game screen where there is no enemy.

Please see this screens, where i tried to explain a little bit.
[First screen](http://img64.imageshack.us/img64/8290/step1c.jpg)
[, Second screen.](http://img857.imageshack.us/img857/5951/step2l.jpg)

Ok, so I have player and i can move him around a play ground - from one border to second one.
And there is also an "enemy". The thing is, that I have to select area (divided by player line) where there is no enemy.

Does anyone know kow can i do that, with some reasonable speed and memory ( i don't have to declare char array[width\_screen][height\_screen]; )

Thanks