lua - 如何将 Lua 字符串转换为 float

Question

我正在编写一个简单的 Lua 脚本来计算 Redis 2.8 中排序集 (http://redis.io/commands/#sorted_set) 的中位数。脚本如下

local cnt = redis.call("ZCARD", KEYS[1])

if cnt > 0 then
        if cnt%2 > 0 then
                local mid = math.floor(cnt/2)
                return redis.call("ZRANGE", KEYS[1], mid, mid)
        else
                local mid = math.floor(cnt/2)
                local vals = redis.call("ZRANGE", KEYS[1], mid-1, mid)
                return (tonumber(vals[1]) + tonumber(vals[2]))/2.0
        end
else
        return nil
end

问题是脚本总是返回一个整数，而结果应该是一个 float 。结果是错误的。

$ redis-cli zrange floats 0 100
1) "1.1"
2) "2.1"
3) "3.1"
4) "3.4975"
5) "42.63"
6) "4.1"

$ redis-cli EVAL "$(cat median.lua)" 1 floats
(integer) 3

正确的结果应该是(3.1 + 3.4975)/2.0 == 3.298

Answer 1

来自documentation for EVAL :

Lua has a single numerical type, Lua numbers. There is no distinction between integers and floats. So we always convert Lua numbers into integer replies, removing the decimal part of the number if any. If you want to return a float from Lua you should return it as a string, exactly like Redis itself does (see for instance the ZSCORE command).

因此，您应该更新脚本以便返回 float as a string :

return tostring((tonumber(vals[1]) + tonumber(vals[2]))/2.0)