Arrays 基于向量的r子集数组

我觉得这个问题应该已经得到了回答，但我没有找到答案。我有一个数组，我想用一个向量来子集它。我知道怎么做很难，但我相信一定有一个简单的方法。有什么想法吗

下面是我的例子：

dat <- data.frame(a = rep(letters[1:3], 2), b = rep(letters[1:2], 3), c = c(rep("a", 5), "b"), x = rnorm(6), stringsAsFactors = FALSE)

l <- by(dat[ , "x"], dat[ , 1:3], mean)

l["a", "a", "a"] # works  
l[c("a", "a", "a")] # does not work

---

dat可以使用矩阵代替向量：

l[matrix(rep("a", 3), nrow=1)]

这已经得到了回答，但我想让事情变得更清楚一点。让我们以您为例：

dat <- data.frame(a = rep(letters[1:3], 2), b = rep(letters[1:2], 3), c = c(rep("a", 
    5), "b"), x = rnorm(6), stringsAsFactors = FALSE)

l <- by(dat[, "x"], dat[, 1:3], mean)

l["a", "a", "a"]  # works  

## [1] 1.246

l[c("a", "a", "a")]  # does not work

## [1] NA NA NA

a.mat
和
a.vec
在屏幕上打印时看起来是一样的，但它们不是
以相同的方式处理，因为R创建矩阵是因为它逐列写入和读取矩阵。当您使用矩阵进行子集设置时，它将使用每列作为不同的维度。如果矩阵中的列数与要子集的对象中的维度数匹配，则它将为每个后续维度使用每列

如果列数不匹配，R将把矩阵折叠成一个向量，并尝试以这种方式匹配元素索引。以下是更多的例子：

a.mat[, -1]  # Now only two columns

## [1] "a" "a"

l[a.mat[, -1]]  # Notice you get NA twice here.

## [1] NA NA

l[matrix(rep("a", 4), nrow = 1)]  # Using a matrix with 4 columns.

## [1] NA NA NA NA

作为旁注，当您使用字符向量子集时，R将尝试匹配任何元素名称。如果它们不存在，您将得到一个
NA
或一个错误：

# Vector example:
x <- letters
x[1]

## [1] "a"

x["a"]

## [1] NA

names(x) <- letters
x[1]

##   a 
## "a"

x["a"]

##   a 
## "a"

x[c("a", "a", "a")]

##   a   a   a 
## "a" "a" "a"

x[a.mat]  # collapsing matrix down to a vector.

##   a   a   a 
## "a" "a" "a"
# Matrix example:
x <- matrix(letters[1:9], nrow = 3, ncol = 3)
x

##      [,1] [,2] [,3]
## [1,] "a"  "d"  "g" 
## [2,] "b"  "e"  "h" 
## [3,] "c"  "f"  "i"

x[c(1, 1)]

## [1] "a" "a"

x[1, 1]

## [1] "a"

x[c("a", "a")]

## [1] NA NA

x["a", "a"]

## Error: no 'dimnames' attribute for array

rownames(x) <- letters[1:3]
colnames(x) <- letters[1:3]
x

##   a   b   c  
## a "a" "d" "g"
## b "b" "e" "h"
## c "c" "f" "i"

x[c(1, 1)]

## [1] "a" "a"

x[1, 1]

## [1] "a"

x[c("a", "a")]

## [1] NA NA

x["a", "a"]

## [1] "a"

或者使用
t（c（“a”、“a”、“a”））
# Vector example:
x <- letters
x[1]

## [1] "a"

x["a"]

## [1] NA

names(x) <- letters
x[1]

##   a 
## "a"

x["a"]

##   a 
## "a"

x[c("a", "a", "a")]

##   a   a   a 
## "a" "a" "a"

x[a.mat]  # collapsing matrix down to a vector.

##   a   a   a 
## "a" "a" "a"
# Matrix example:
x <- matrix(letters[1:9], nrow = 3, ncol = 3)
x

##      [,1] [,2] [,3]
## [1,] "a"  "d"  "g" 
## [2,] "b"  "e"  "h" 
## [3,] "c"  "f"  "i"

x[c(1, 1)]

## [1] "a" "a"

x[1, 1]

## [1] "a"

x[c("a", "a")]

## [1] NA NA

x["a", "a"]

## Error: no 'dimnames' attribute for array

rownames(x) <- letters[1:3]
colnames(x) <- letters[1:3]
x

##   a   b   c  
## a "a" "d" "g"
## b "b" "e" "h"
## c "c" "f" "i"

x[c(1, 1)]

## [1] "a" "a"

x[1, 1]

## [1] "a"

x[c("a", "a")]

## [1] NA NA

x["a", "a"]

## [1] "a"

l[c(1,1,1)]

## [1] 1.246 1.246 1.246

l[1, 1, 1]

## [1] 1.246