Question

是否有一种优雅的方式将同一行的多个副本添加到矩阵？下面是一个解决问题的例子。但是，这个例子很慢而且不优雅。

m <- matrix(c(1,2,3,4,5,6,7,8,9),ncol=3)
m0 <- m

r <- c(10,20,30)

#Correct example
for (i in c(1,2))
  m[i,] <- r

print(m)

#    [,1] [,2] [,3]
#[1,]   10   20   30
#[2,]   10   20   30
#[3,]    3    6    9


#Attempts below look straightforward but lead to incorrect results
m <- m0

m[1:2,] <- apply(m[1:2,],1,function(x)r)
print(m)

#  [,1] [,2] [,3]
#[1,]   10   30   20
#[2,]   20   10   30
#[3,]    3    6    9

m <- m0
m[1:2,] <- r
print(m)

#  [,1] [,2] [,3]
#[1,]   10   30   20
#[2,]   20   10   30
#[3,]    3    6    9

Answer 1

您可以从向量r构建第二个矩阵，并将其插入m，如下所示：

m[1:2, ] <- matrix(r, ncol = 3, nrow = 2, byrow = TRUE)
m
#     [,1] [,2] [,3]
#[1,]   10   20   30
#[2,]   10   20   30
#[3,]    3    6    9

另一个选择是t转机m，再次插入r和t：

m <- t(m)
m[, 1:2] <- r
m <- t(m)
m
#     [,1] [,2] [,3]
#[1,]   10   20   30
#[2,]   10   20   30
#[3,]    3    6    9

但我没有对性能进行基准测试。

Answer 2

据我所知， for-loop 最快解决问题的方法。更快的解决方案是在R中选择不同的数据结构，其中记录在列而不是在行中。

以下是@ docendo-discimus和我的解决方案的速度测量以及记录在列而不是行中的其他示例。在这种情况下，直接分配可以正常工作。

m <- matrix(c(1,2,3,4,5,6,7,8,9),ncol=3)
m0 <- m

r <- c(10,20,30)

#For loop
print(
  system.time(
    for (k in 1:100000){  
      for (i in c(1,2))
        m[i,] <- r
    }))
print(m)

#     User      System verstrichen 
#     0.524       0.011       0.552 

#     [,1] [,2] [,3]
#     [1,]   10   20   30
#     [2,]   10   20   30
#     [3,]    3    6    9


#@docendo-discimus: Create new matrix
m <- m0
print(
  system.time(
  for (k in 1:100000){
    m[1:2, ] <- matrix(r, ncol = 3, nrow = 2, byrow = TRUE)})
)
print(m)

#     User      System verstrichen 
#     0.818       0.011       0.833 

#     [,1] [,2] [,3]
#     [1,]   10   20   30
#     [2,]   10   20   30
#     [3,]    3    6    9

#@docendo-discimus Transpose, modify columns, transpose again
m <- m0
print(
  system.time(
    for (k in 1:100000){
      m <- t(m)
      m[, 1:2] <- r
      m <- t(m)
      m}))

print(m)

#     User      System verstrichen 
#     1.870       0.010       1.895 

#     [,1] [,2] [,3]
#     [1,]   10   20   30
#     [2,]   10   20   30
#     [3,]    3    6    9

#Usage of different data structure with a transposed matrix
m <- matrix(c(1,2,3,4,5,6,7,8,9),nrow=3)
m0 <- m
print(m)

#     [,1] [,2] [,3]
#     [1,]    1    4    7
#     [2,]    2    5    8
#     [3,]    3    6    9

print(
  system.time(
    for (k in 1:100000){
      m[,1:2] <- r
    }))

print(m)

#     User      System verstrichen 
#     0.241       0.013       0.255

#     [,1] [,2] [,3]
#     [1,]   10   10    7
#     [2,]   20   20    8
#     [3,]   30   30    9

将多个行副本添加到矩阵

2 个答案: