R 行绑定矩阵，除一行外，将所有行归零

给定一个矩阵

a <- matrix(c(15,2,11,16,7,12,1,8,3), nrow=3, ncol=3, byrow=T)

我可以通过rbind（a[1,0,0,0,0,a[2,0,0,0,a[3,0]）来实现这一点，但是在处理大型矩阵时，有没有更好的方法？

这个方法呢：

apply(a, 2, function(r) rbind(r, matrix(rep(0,9), nrow=3)))[1:9, ]

这个怎么样：

apply(a, 2, function(r) rbind(r, matrix(rep(0,9), nrow=3)))[1:9, ]

      [,1] [,2] [,3]
 [1,]   15    2   11
 [2,]    0    0    0
 [3,]    0    0    0
 [4,]    0    0    0
 [5,]   16    7   12
 [6,]    0    0    0
 [7,]    0    0    0
 [8,]    0    0    0
 [9,]    1    8    3

matrix(t(cbind(a,matrix(0,3,9))),ncol=3,byrow=T)[1:9,]
      [,1] [,2] [,3]
 [1,]   15    2   11
 [2,]    0    0    0
 [3,]    0    0    0
 [4,]    0    0    0
 [5,]   16    7   12
 [6,]    0    0    0
 [7,]    0    0    0
 [8,]    0    0    0
 [9,]    1    8    3