Ah ok wouldn't mind seeing that either .
On Space Invaders here's how I did the bottom enemy shooting. Put it within spoilers in case you'd rather work it out yourself:
You just do a for loop going column by column starting with the bottom most invaders (whether their alive or dead) and have it subtract the amount of columns which gets you the invader directly above it (so if we were on enemy 51 lets say, and columns were 11 the next above would be enemy 40). If its alive, add that to a list of alive invaders and move on to the next column. Once you've looped through all of the invader columns pick one from that list randomly to shoot. And hey presto.
There's probably an easier way, but it works 100% of the time with only a few lines of code.