顶起来~~

Error: could not find function "cast"
reshape2包中已经没有cast函数了，换成了dcast和acast，而且dcast貌似不支持多个函数了？lz的那个换成单个函数就能执行
dcast(dfm, am+ cyl + variable ~., mean)

?dcast有段话：


     If the combination of variables you supply does not uniquely
     identify one row in the original data set, you will need to supply
     an aggregating function, 'fun.aggregate'. This function should
     take a vector of numbers and return a single summary statistic.


如果要一次返回3个值的话好像得用ddply：

不过确实可以直接用reshape包的cast函数