read.table自动列名

Question

我注意到当读取一个大的csv文件时

output <- read.table( ..., header = TRUE, sep = ",")

创建的数据框架有一些空列。这些列遵循命名模式。

 colnames(output)
     "Factor.1"   "Factor.2"   "etc"        "Stuff"      "X"          "X.1"        "X.2"        "X.3"        "X.4"        "X.5"       
     "X.6"        "X.7"        "X.8"        "X.9"        "X.10"       "X.11"       "X.12"       "X.13"      
     "X.14"       "X.15"       "X.16"       "X.17"       "X.18"       "X.19"       "X.20"       "X.21"      
     "X.22"       "X.23"       "X.24"       "X.25"       "X.26"       "X.27"       "X.28"       "X.29"      
     "X.30"       "X.31"       "X.32"       "X.33"

我注意到在?read.table中它声明

col.names:变量的可选名称向量。默认情况下使用"V“，后面跟着列号。

为什么它用X代替V？

编辑:这就是csv文件的样子

Date,Duration,Count,Factor 1,Factor 2,Factor 3,Hour,Day,Month,Year,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 0:00,9.99,10,GC,LS,FT,0,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 1:00,9.63125,8,GC,LS,FT,1,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 2:00,7.388888889,3,GC,LS,FT,2,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 3:00,7.087037037,9,GC,LS,FT,3,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,

..。

Answer 1

下面是来自read.table()的相关代码片段

if (header) {
    .External(C_readtablehead, file, 1L, comment.char, 
              blank.lines.skip, quote, sep, skipNul)
    if (missing(col.names)) 
        col.names <- first
    else if (length(first) != length(col.names)) 
        warning("header and 'col.names' are of different lengths")
}

重要的是if (missing(col.names)) col.names <- first。从那里，我们可以返回并得到first，在这种情况下定义为

first <- scan(textConnection(file), what = "", sep = ",", 
    nlines = 1, quiet = TRUE, skip = 0, strip.white = TRUE)

这会导致

#  [1] "Date"     "Duration" "Count"    "Factor 1" "Factor 2" "Factor 3" "Hour"     "Day"      "Month"   
# [10] "Year"     ""         ""         ""         ""         ""         ""         ""         ""        
# [19] ""         ""         ""         ""         ""         ""         ""         ""         ""        
# [28] ""         ""         ""         ""         ""         ""         ""         ""         ""        
# [37] ""         ""         ""         ""         ""         ""         ""         ""        

然后，make.names()在col.names上被调用，导致您的名字

make.names(first, unique = TRUE)
#  [1] "Date"     "Duration" "Count"    "Factor.1" "Factor.2" "Factor.3" "Hour"     "Day"      "Month"   
# [10] "Year"     "X"        "X.1"      "X.2"      "X.3"      "X.4"      "X.5"      "X.6"      "X.7"     
# [19] "X.8"      "X.9"      "X.10"     "X.11"     "X.12"     "X.13"     "X.14"     "X.15"     "X.16"    
# [28] "X.17"     "X.18"     "X.19"     "X.20"     "X.21"     "X.22"     "X.23"     "X.24"     "X.25"    
# [37] "X.26"     "X.27"     "X.28"     "X.29"     "X.30"     "X.31"     "X.32"     "X.33"    

我们获得X而不是V的原因正如文档中所指出的，是因为在if(header)之后的下一个条件是

else if (missing(col.names)) 
    col.names <- paste0("V", 1L:cols) 

但是，我们从来没有做到这一点，make.names()默认情况下连接到X。不仅仅是这个解释。最好的方法是遍历read.table源代码(它很复杂)。

数据：

file <- "Date,Duration,Count,Factor 1,Factor 2,Factor 3,Hour,Day,Month,Year,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 0:00,9.99,10,GC,LS,FT,0,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 1:00,9.63125,8,GC,LS,FT,1,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 2:00,7.388888889,3,GC,LS,FT,2,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
1/1/2012 3:00,7.087037037,9,GC,LS,FT,3,7,1,2012,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,"