R 用矢量变换矩阵中的单个数据

假设我要更改以下矩阵：

a= matrix(c(
  1:20),ncol=5)
a
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    5    9   13   17
[2,]    2    6   10   14   18
[3,]    3    7   11   15   19
[4,]    4    8   12   16   20

rvec= c(4,2)
cvec=c(1,5)
a[rvec,cvec] = c(200,500)
a
    [,1] [,2] [,3] [,4] [,5]
[1,]    1    5    9   13   17
[2,]  500    6   10   14  500
[3,]    3    7   11   15   19
[4,]  200    8   12   16  200

但我想：

    [,1] [,2] [,3] [,4] [,5]
[1,]    1    5    9   13   17
[2,]    2    6   10   14  500
[3,]    3    7   11   15   19
[4,]  200    8   12   16  20

将向量视为值的位置-分别将x，y（4,1）和x，y（2,5）更改为in

c（200500）

我可以用for循环来做，但是太慢了

 for(i in 1:length(c(200,500)))
 {
   a[rvec[i],cvec[i]] = c(200,500)[i]
 }

 a
     [,1] [,2] [,3] [,4] [,5]
[1,]    1    5    9   13   17
[2,]    2    6   10   14  500
[3,]    3    7   11   15   19
[4,]  200    8   12   16   20

##设置一个矩阵，列1中有行索引，列2中有列索引
ij
##设置一个矩阵，列1中有行索引，列2中有列索引
ij
## Set up a matrix with row-indices in column 1 & column-indices in column 2 
ij <- rbind(c(4,1), c(2,5))

## Use it to pick out individual elements of a
a[ij] <- c(200, 500)

## Check that it worked
a
#      [,1] [,2] [,3] [,4] [,5]
# [1,]    1    5    9   13   17
# [2,]    2    6   10   14  500
# [3,]    3    7   11   15   19
# [4,]  200    8   12   16   20