转换 data.table 中的列类 [英] Convert column classes in data.table
问题描述
我在使用 data.table 时遇到问题:如何转换列类?这是一个简单的示例:使用 data.frame 转换它没有问题,使用 data.table 我只是不知道如何:
I have a problem using data.table: How do I convert column classes? Here is a simple example: With data.frame I don't have a problem converting it, with data.table I just don't know how:
df <- data.frame(ID=c(rep("A", 5), rep("B",5)), Quarter=c(1:5, 1:5), value=rnorm(10))
#One way: http://stackoverflow.com/questions/2851015/r-convert-data-frame-columns-from-factors-to-characters
df <- data.frame(lapply(df, as.character), stringsAsFactors=FALSE)
#Another way
df[, "value"] <- as.numeric(df[, "value"])
library(data.table)
dt <- data.table(ID=c(rep("A", 5), rep("B",5)), Quarter=c(1:5, 1:5), value=rnorm(10))
dt <- data.table(lapply(dt, as.character), stringsAsFactors=FALSE)
#Error in rep("", ncol(xi)) : invalid 'times' argument
#Produces error, does data.table not have the option stringsAsFactors?
dt[, "ID", with=FALSE] <- as.character(dt[, "ID", with=FALSE])
#Produces error: Error in `[<-.data.table`(`*tmp*`, , "ID", with = FALSE, value = "c(1, 1, 1, 1, 1, 2, 2, 2, 2, 2)") :
#unused argument(s) (with = FALSE)
我在这里错过了什么明显的东西吗?
Do I miss something obvious here?
由于 Matthew 的帖子而更新:我之前使用的是旧版本,但即使在更新到 1.6.6(我现在使用的版本)之后,我仍然收到错误.
Update due to Matthew's post: I used an older version before, but even after updating to 1.6.6 (the version I use now) I still get an error.
更新 2:假设我想将因子"类的每一列转换为字符"列,但事先不知道哪一列属于哪个类.使用 data.frame,我可以执行以下操作:
Update 2: Let's say I want to convert every column of class "factor" to a "character" column, but don't know in advance which column is of which class. With a data.frame, I can do the following:
classes <- as.character(sapply(df, class))
colClasses <- which(classes=="factor")
df[, colClasses] <- sapply(df[, colClasses], as.character)
我可以对 data.table 做类似的事情吗?
Can I do something similar with data.table?
更新 3:
会话信息()R 版本 2.13.1 (2011-07-08)平台:x86_64-pc-mingw32/x64(64位)
sessionInfo() R version 2.13.1 (2011-07-08) Platform: x86_64-pc-mingw32/x64 (64-bit)
locale:
[1] C
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] data.table_1.6.6
loaded via a namespace (and not attached):
[1] tools_2.13.1
推荐答案
对于单列:
dtnew <- dt[, Quarter:=as.character(Quarter)]
str(dtnew)
Classes ‘data.table’ and 'data.frame': 10 obs. of 3 variables:
$ ID : Factor w/ 2 levels "A","B": 1 1 1 1 1 2 2 2 2 2
$ Quarter: chr "1" "2" "3" "4" ...
$ value : num -0.838 0.146 -1.059 -1.197 0.282 ...
<小时>
使用 lapply
和 as.character
:
dtnew <- dt[, lapply(.SD, as.character), by=ID]
str(dtnew)
Classes ‘data.table’ and 'data.frame': 10 obs. of 3 variables:
$ ID : Factor w/ 2 levels "A","B": 1 1 1 1 1 2 2 2 2 2
$ Quarter: chr "1" "2" "3" "4" ...
$ value : chr "1.487145280568" "-0.827845218358881" "0.028977182770002" "1.35392750102305" ...
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