因子(Factor)
> factor = factor(rep(c(1:3),time=5))
> factor
[1] 1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
Levels: 1 2 3
> X=sample(100,15)
> tapply(X,factor,mean)
1 2 3
67.6 52.2 56.8
> rbind(X,factor)
[,1] [,2] [,3] [,4] [,5] [,6] [,7]
X 97 87 93 34 16 67 48
factor 1 2 3 1 2 3 1
[,8] [,9] [,10] [,11] [,12] [,13]
X 43 25 77 21 60 82
factor 2 3 1 2 3 1
[,14] [,15]
X 94 39
factor 2 3
> rbind(X,factor)[2,]==1
[1] TRUE FALSE FALSE TRUE FALSE FALSE
[7] TRUE FALSE FALSE TRUE FALSE FALSE
[13] TRUE FALSE FALSE
> boo = rbind(X,factor)[2,]==1
> which(boo)
[1] 1 4 7 10 13
> rbind(X,factor)[1,which(boo)]
[1] 97 34 48 77 82
> sum(rbind(X,factor)[1,which(boo)])/length(which(boo))
[1] 67.6