我不知道如何使用fread中的colClasses选项来 Select 特定的列.我试着用几种方法使用NULL，但都没用.这里有一个简单的例子.我只想要第1列和第3列.
dt <- data.table(a=1:5,b=6:10,c=10:14)
write.csv(dt,"dt.csv",row.names=F)
dt <- fread("dt.csv",colClasses=?)
packageVersion("data.table")
[1] ‘1.8.10’
getRversion()
[1] ‘3.0.1’
导入的数据集应如下所示:
a c
1: 1 10
2: 2 11
3: 3 12
4: 4 13
5: 5 14
UPDATE:这一功能现在在v1中实现.8.11自966年12月10日起生效.从NEWS开始:
fread的drop、select和NULL在colClasses中被实现.到
?fread中的例子如下:
data = "A,B,C,D\n1,3,5,7\n2,4,6,8\n"
# colClasses
fread(data, colClasses=c(B="character",C="character",D="character"))
fread(data, colClasses=list(character=c("B","C","D"))) # saves typing
fread(data, colClasses=list(character=2:4)) # same using column numbers
# drop
fread(data, colClasses=c("B"="NULL","C"="NULL")) # as read.csv
fread(data, colClasses=list(NULL=c("B","C"))) # same
fread(data, drop=c("B","C")) # same but less typing, easier to read
fread(data, drop=2:3) # same using column numbers
# select
# (in read.csv you need to work out which to drop)
fread(data, select=c("A","D")) # less typing, easier to read
fread(data, select=c(1,4)) # same using column numbers