用A[c, ]就可以啦。请看下面的例子
> A
[,1] [,2] [,3] [,4] [,5]
[1,] 0.0759241 0.1478971 0.4773810 -1.8647390 -0.7314686
[2,] 0.8881272 -0.3949994 0.6223292 -0.2553023 0.3319910
[3,] -1.0172010 0.7953251 0.6962407 -1.6235615 -0.9614034
[4,] -1.4156303 -1.4710346 1.1651658 0.8570103 0.2618842
> c <- c(3,4)
> A[c,]
[,1] [,2] [,3] [,4] [,5]
[1,] -1.017201 0.7953251 0.6962407 -1.6235615 -0.9614034
[2,] -1.415630 -1.4710346 1.1651658 0.8570103 0.2618842