Going back in For Next Loop

Thu, 05 Oct 2017 19:42:53 UTC

Hello,
I'm a noob at programming and need some help please.
I'm try to write a for next loop that randomly fills an 2D array (my game grid), but I also want it to check that if that cell already has been used, then knock the counter back -1 and tries again. So in this way, at the end of the loop there should be a set number of cells filled.
My code works great except that when it detaches a duplicate cell, it doesn't roll the counter back.

So say at loop count 50, there's a duplicate cell, it doesn't alter the cell (which is correct), but it doesn't roll the loop counter back, it just continues.
Here's my code. I was hoping that the i-=1 would do the trick, but it doesn't.

function make_grid()
for i=1,level*100+1 do
y=flr(rnd(gridheight-6))
x=flr(rnd(gridwidth-6))
--is the space empty? then add
--a bomb. if not rollback counter
if grid[1][y+3][x+1]==0 then
grid[1][y+3][x+1]=2
else
i-=1
end
end
end

I've had print statements in there print the value of I as it went along, but the I-=1 doesn't effect the loop counter I
Any ideas and thanks in advance.

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Going back in For Next Loop