的R - 從data.frame

Question

Exctracting雙打的向量我使用read.table()具有或不具有header=T這個問題，嘗試從得到的data.frame提取與as.double(as.character())雙打的載體（參見?factor）。

但這只是如何我意識到我不明白R的邏輯。所以你不會看到例如read.table在下面的代碼中，只有必要的部分。你能告訴我下列選項有什麼區別？

1. 隨着header=T相當於：

   (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. 沒有header=T相當於：

   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

的問題出在data.frame，其中的選項之一，stringsAsFactors默認設置設置爲TRUE。這是您的方案中的一個問題，因爲當您使用header = FALSE時，該行中字符值的存在將整個列強制轉換爲字符，然後轉換爲因子（除非您設置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