local t = {}
t[15.5] = "aaa"
t[3.1] = "bbb"
t[7.8] = "ccc"
t[115.5] = "aaa"
t[23.1] = "bbb"
t[557.8] = "ccc"
t[14.5] = "aaa"
t[43.1] = "bbb"
t[87.8] = "ccc"

function getFirstAndLastByKeys(t, f)
    local a = {}
    for n in pairs(t) do table.insert(a, n) end
    table.sort(a, f)
    return a[1], t[a[1]], a[#a], t[a[#a]]
end

local first_k, first_v, last_k, last_v = getFirstAndLastByKeys(t)
print(first_k.." = "..first_v)
print(last_k.." = "..last_v)

3.1 = bbb
557.8 = ccc

for k, v in pairsByKeys(t, function (a, b) return a > b end) do
    ...
end

BigTab = {}
BigTab[12.2] = {s = 11, f =22}
BigTab[11.1] = {s = 11, f =22}
BigTab[14.1] = {s = 11, f =22}
BigTab[3.32] = {s = 11, f =22}

for k, v in pairs(BigTab) do
    print(tostring(k).." = "..tostring(v))
end
print()

function pairsByKeys(t, f)
    local a = {}
    for n in pairs(t) do table.insert(a, n) end
    table.sort(a, f)
    local i = 0      -- iterator variable
    local iter = function ()   -- iterator function
        i = i + 1
        if a[i] == nil then return nil
        else return a[i], t[a[i]]
        end
    end
    return iter
end

for k, v in pairsByKeys(BigTab) do
    print(tostring(k).." = "..tostring(v))
end

11.1 = table: 005099A8
3.32 = table: 00509890
12.2 = table: 00509A98
14.1 = table: 00509868

3.32 = table: 00509890
11.1 = table: 005099A8
12.2 = table: 00509A98
14.1 = table: 00509868