在fread中使用colClasses [英] using colClasses in fread

问题描述

我不知道如何使用 colClasses 选项在 fread 中选择特定的列。我尝试使用 NULL 在几种方式，但没有什么工作。这里是一个最小的例子。我只需要列1和3。

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

：现在在 R-Forge 上的v1.8.11中实现了commit 966.从新闻：

UPDATE: This is now implemented in v1.8.11 on R-Forge as of commit 966. From NEWS :

fread 的 drop 在 colClasses 中的选择和 NULL 到
按名称或按数字删除或选择列。请参阅？fread 中的示例。

fread's drop, select and NULL in colClasses are implemented. To drop or select columns by name or by number. See examples in ?fread.

？fread 是：

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

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