function map(func, tbl)
   local newtbl = {}
   for i,v in pairs(tbl) do
       newtbl[i] = func(v)
   end
   return newtbl
end

 -- filter(function, table)
 -- e.g: filter(is_even, {1,2,3,4}) -> {2,4}
 function filter(func, tbl)
   local newtbl= {}
   for _,v in pairs(tbl) do
       if func(v) then
        table.insert(newtbl, v)
    end
end
return newtbl
end