eliminate conversion to number ?

-- Demonstrate the difference of time cost iterating over ffi array of double versus native array both with luajit

require "os"
local ffi = require("ffi")
local num_ele = 900000
local num_pass = 50000
function elap(beg_time, msg)
  local end_time = os.clock()
  print ("elap " .. msg ..  " " .. end_time - beg_time .. "sec")
  return os.clock()
end

local begt = os.clock()
local arr = ffi.new("double[?]", num_ele + 1)
begt = elap(begt, "allocate ffi")


-- fill the array for the example, here you would fill it from a file
local begt = os.clock()
for i = 0, num_ele do
    arr[i] = i
end

begt = elap(begt, "fill ffi array")

local sum_start = ffi.cast("double", 0)
for pass = 0, num_pass do
  local sum = sum_start
  for i = 0, num_ele do
        sum = sum + arr[i]
  end
end
begt = elap(begt, "sum the ffi arrray")


for pass = 0, num_pass do
  local sum1 = 0
  for i =  num_ele,0,-1 do
        sum1 = sum1 + arr[i]
  end
end
begt = elap(begt, "sum the ffi arrray in reverse")


 --------------
 --- Test with Native Lua Table
 --------------
 tt = {}
local begt = os.clock()
for i = 0, num_ele do
    tt[i] = i
end
begt = elap(begt, "fill native table ")



for pass = 0, num_pass do
  local sum2 = 0
  for i = 0, num_ele do
        sum2 = sum2 + tt[i]
  end
end
begt = elap(begt, "sum the lua arrray")



for pass = 0, num_pass do
  local sum3 = 0
  for i = num_ele,0,-1  do
        sum3 = sum3 + tt[i]
  end
end
begt = elap(begt, "sum the lua arrray in reverse")

assert(sum == sum1, "warning sum1 ~= sum")
assert(sum == sum2,  "warning the two sum2 ~= sum")
assert(sum == sum3,  "warning the two sum3 ~= sum")