R - 从 data.frame 中提取双精度向量

Question

我使用read.table() 和header=T 得到了这个问题，试图从data.frame 和as.double(as.character()) 中提取双精度向量（参见?factor）。

但这只是如何我意识到我不理解 R 的逻辑。所以你不会看到例如read.table 在下面的代码中，只有必要的部分。你能告诉我以下选项有什么区别吗？

1. 与header=T 等效：

   (a <- data.frame(array(c(0.5,0.5,0.5,0.5), c(1,4))))
   as.character(a)
   # [1] "0.5" "0.5" "0.5" "0.5"
   

2. 没有header=T 等效：

   b <- data.frame(array(c("a",0.5,"b",0.5,"c",0.5,"d",0.5), c(2,4)))
   (a <- b[2,])
   as.character(a)
   # [1] "1" "1" "1" "1"
   
   (a <- data.frame(a, row.names=NULL)) # now there's not even a visual difference
   as.character(a)
   # [1] "1" "1" "1" "1"
   

Answer 1

问题在于data.frame 的默认设置，其中一个选项stringsAsFactors 设置为TRUE。这是您的方案中的一个问题，因为当您使用 header = FALSE 时，该行中存在的字符值会将整个列强制转换为字符，然后将其转换为因子（除非您设置 stringsAsFactors = FALSE）。

这里有一些可以玩的例子：

## Two similar `data.frame`s -- just one argument different

b <- data.frame(array(c("a",0.5,"b",0.5,"c",0.5,"d",0.5), c(2,4)))
b2 <- data.frame(array(c("a",0.5,"b",0.5,"c",0.5,"d",0.5), c(2,4)),
                stringsAsFactors = FALSE)

## First with "b"

as.character(b[2, ])
# [1] "1" "1" "1" "1"

sapply(b[2, ], as.character)
#    X1    X2    X3    X4 
# "0.5" "0.5" "0.5" "0.5"
as.matrix(b)[2, ]
#    X1    X2    X3    X4 
# "0.5" "0.5" "0.5" "0.5"
as.double(as.matrix(b)[2, ])
# [1] 0.5 0.5 0.5 0.5

## Now with "b2"

as.character(b2[2, ])
# [1] "0.5" "0.5" "0.5" "0.5"
as.double(as.character(b2[2, ]))
# [1] 0.5 0.5 0.5 0.5