的R - 从data.frame

Question

Exctracting双打的向量我使用read.table()具有或不具有header=T这个问题，尝试从得到的data.frame提取与as.double(as.character())双打的载体（参见?factor）。

但这只是如何我意识到我不明白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。你能告诉我下列选项有什么区别？

1. 随着header=T相当于：

   (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相当于：

   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

的问题出在data.frame，其中的选项之一，stringsAsFactors默认设置设置为TRUE。这是您的方案中的一个问题，因为当您使用header = FALSE时，该行中字符值的存在将整个列强制转换为字符，然后转换为因子（除非您设置stringsAsFactors = FALSE）。

下面是一些例子玩：

## 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