Table value doesn't change - lua

Question

I have a 2 dim array and all it's cells filled with zeros.
What i'm trying to do is to take some randomly chosen cells and fill it with 4 or 5
but what i get is either empty gird with all value equal to zero or i get just one value that has changed to 4 or 5 and that's my code below:
local grid = {}
for i=1,10 do
grid[i] = {}
for j=1,10 do
grid[i][j] = 0
end
end
local empty={}
for i=1,10 do
for j=1,10 do
if grid[i][j]==0 then
table.insert(empty,i ..'-'.. j)
end
end
end
local fp=math.floor(table.maxn(empty)/3)
local fx,fy
for i=1,fp do
math.randomseed(os.time())
math.random(0,1)
local fo=math.random(0,1)
math.random(table.maxn(empty))
local temp= empty[math.random(table.maxn(empty))]
local dashindex=string.find(temp,'-')
fx=tonumber(string.sub(temp,1,dashindex-1))
fy=tonumber(string.sub(temp,dashindex+1,string.len(temp)))
if fo==0 then
grid[fx][fy]=4
elseif fo==1 then
grid[fx][fy]=5
end
end
for i=1,10 do
for j=1,10 do
print(grid[i][j])
end
print('\n')
end

Answer 1

I'm not sure what the for i=1,fp loop is doing with temp and fo, for example the seed should only be set once, and also, the return value on line after local fo is ignored, seems very messy. But based on your post, if you really just want to randomly select N cells from your 2D array and set those to either 4 or 5 (randomly), this should work:
-- maybe N = fp
local N = 5
math.randomseed(os.time())
local i = 1
repeat
fx = math.random(1, 10)
fy = math.random(1, 10)
if grid[fx][fy] == 0 then
grid[fx][fy] = math.random(4,5)
i = i + 1
end
until i > N
Note however that the closer N is to number of items in array (100 in your example), the longer it will take for the loop to complete. If this is a concern, then for large N values, you could do the opposite: initialize each cell to 4 or 5 randomly, and then randomly set size - N of them to 0.
math.randomseed(os.time())
local rows = 10
local columns = 10
local grid = {}
if N > rows*columns/2 then
for i=1,rows do
grid[i] = {}
for j=1,columns do
grid[i][j] = math.random(4,5)
end
end
local i = 1
repeat
fx = math.random(1, 10)
fy = math.random(1, 10)
if grid[fx][fy] ~= 0 then
grid[fx][fy] = 0
i = i + 1
end
until i > N
else
for i=1,rows do
grid[i] = {}
for j=1,columns do
grid[i][j] = 0
end
end
local i = 1
repeat
fx = math.random(1, 10)
fy = math.random(1, 10)
if grid[fx][fy] == 0 then
grid[fx][fy] = math.random(4,5)
i = i + 1
end
until i > N
end