我这里class(tapply(x, f, mean))和class(tapply(x, f, mean,simplify = FALSE))的输出结果都是数组,为什么?
class(tapply(x, f, mean))和class(tapply(x, f, mean,simplify = FALSE))这两个应该是等价的吧,
simplify = true时后面就是一个向量了。