> mean(c(2,3,4,5,6,7))
[1] 4.5

> cardata
                   mpg cyl disp  hp drat    wt  qsec
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02
Datsun 710        22.8   4  108  93 3.85 2.320 18.61
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02
Valiant           18.1   6  225 105 2.76 3.460 20.22

> apply(cardata,1,mean)#按行求均值
        Mazda RX4     Mazda RX4 Wag        Datsun 710    Hornet 4 Drive 
         45.71143          45.82786          36.08286          60.16214 
Hornet Sportabout           Valiant 
         83.61571          54.36286 

> apply(cardata,2,mean)#按列统计
       mpg        cyl       disp         hp       drat         wt       qsec 
 20.500000   6.000000 211.833333 117.166667   3.440000   2.988333  18.128333 

> x<-c(2,3,4,5,6,7,NA)
> mean(x)
[1] NA
> mean(x,na.rm=TRUE)
[1] 4.5