在 fread 中使用 colClasses [英] using colClasses in fread
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问题描述
我不知道如何使用 fread
中的 colClasses
选项选择特定列.我尝试以多种方式使用 NULL
但没有任何效果.这是一个最小的例子.我只想要第 1 列和第 3 列.
I don't know how to choose specific columns using the colClasses
option in fread
. I tried to use NULL
in several ways but nothing worked. Here's a minimal example. I just want columns 1 and 3.
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:
The imported dataset should look like this:
a c
1: 1 10
2: 2 11
3: 3 12
4: 4 13
5: 5 14
推荐答案
更新: 现在在 R-Forge 从提交 966 开始.来自 新闻 :
UPDATE: This is now implemented in v1.8.11 on R-Forge as of commit 966. From NEWS :
fread
的drop
、select
和colClasses
中的NULL
都实现了.到按名称或编号删除或选择列.请参阅 ?fread
中的示例.
fread
'sdrop
,select
andNULL
incolClasses
are implemented. To drop or select columns by name or by number. See examples in?fread
.
?fread
中的例子是:
data = "A,B,C,D
1,3,5,7
2,4,6,8
"
# 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
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