local fmt = require "fmt"

local function middle3(n)
    if n < 0 then n = -n end
    local s = tostring(n)
    local c = #s
    if c < 3 then return $"Minimum is 3 digits, only has {c}." end
    if c % 2 == 0 then return $"Number of digits must be odd, {c} is even." end
    if c == 3 then return s end
    local d = (c - 1) / 2
    return s:sub(d, d + 2)
end

local a = {123, 12345, 1234567, 987654321, 10001, -10001, -123, -100, 100, -12345,
           1, 2, -1, -10, 2002, -2002, 0}

for a as e do
    fmt.print("%9d -> %s", e, middle3(e))
end