[R] by group

[Val]

Hi All,

How can I generate mean by group. The sample data looks like as follow,
dat<-read.table(text="Year Sex wt
2001 M 15
2001 M 14
2001 M 16
2001 F 12
2001 F 11
2001 F 13
2002 M 14
2002 M 18
2002 M 17
2002 F 11
2002 F 15
2002 F 14
2003 M 18
2003 M 13
2003 M 14
2003 F 15
2003 F 10
2003 F 11  ",header=TRUE)

The desired  output  is,
             M        F
2001    15        12
2002    16.33   13.33
2003    15          12

Thank you,