Archergy
在中文版102至105介绍了reshape包的用法，但是在电脑上实验时出现问题了，望前辈们解惑。
代码如下：
library(reshape)
ID = c(1,1,2,2)
Time = c(1,2,1,2)
X1 = c(5,3,6,2)
X2 = c(6,5,1,4)
mydata = data.frame(ID, Time, X1, X2, stringsAsFactors = F)
md = melt(mydata, id.vars = c("ID", "Time"))
newdata = cast(md, ID ~ variable, mean)
错误提示为：
Error in (function (..., row.names = NULL, check.rows = FALSE, check.names = TRUE, :
arguments imply differing number of rows: 4, 0
