[英]R data.table compute new column, but insert at beginning
In R data.table
s, I can use this syntax to add a new column: 在R data.table
,我可以使用此语法添加新列:
> dt <- data.table(a=c(1,2), b=c(3,4))
> dt[, c := a + b]
> dt
a b c
1: 1 3 4
2: 2 4 6
But how would I insert c at the front of the dt like so: 但是我如何在dt的前面插入c,如下所示:
c a b
1: 4 1 3
2: 6 2 4
I looked on SO, and found some people suggesting cbind
for data.frame
s, but it's more convenient for me to use the :=
syntax here, so I was wondering if there was a data.table
sanctioned way of doing this. 我看着SO,并发现了一些人建议cbind
为data.frame
S中,但它更方便,我用:=
这里的语法,所以我在想,如果有一个data.table
这样的制裁方式。 My data.table
has around 100 columns, so I don't want to list them all out. 我的data.table
有大约100列,所以我不想全部列出。
Update: This feature has now been merged into the latest CRAN version of
data.table
(starting with v1.11.0), so installing the development version is no longer necessary to use this feature. 更新:此功能现已合并到data.table
的最新CRAN版本(从v1.11.0开始),因此不再需要安装开发版本才能使用此功能。 From the release notes: 从发行说明:
- setcolorder() now accepts less than ncol(DT) columns to be moved to the front, #592. setcolorder()现在接受少于ncol(DT)列移动到前面,#592。 Thanks @MichaelChirico for the PR. 感谢@MichaelChirico的PR。
Current development version of data.table
(v1.10.5) has updates to setcolorder()
that make this way more convenient by accepting a partial list of columns. data.table
(v1.10.5)的当前开发版本对setcolorder()
进行了更新,通过接受部分列列表使这种方式更加方便。 The columns provided are placed first, and then all non-specified columns are added after in the existing order. 首先放置提供的列,然后在现有顺序之后添加所有未指定的列。
Installation instructions for development branch here. 这里有开发分支的安装说明。
Note regarding development branch stability: I've been running it for several months now to utilize the multi-threaded version in fread()
in v1.10.5 (that alone is worth the update if you deal with multi-GB .csv files) and I have not noticed any bugs or regressions for my usage. 关于开发分支稳定性的注意事项:我已经运行了几个月,现在在v1.10.5中的fread()
中使用多线程版本fread()
如果你处理多GB .csv文件,那么单独值得更新)和我没有注意到我的使用有任何错误或回归。
library(data.table)
DT <- as.data.table(mtcars)
DT[1:5]
gives 给
mpg cyl disp hp drat wt qsec vs am gear carb
1: 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
2: 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
3: 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
4: 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
5: 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
re-order columns based on a partial list: 根据部分列表重新排序列:
setcolorder(DT,c("gear","carb"))
DT[1:5]
now gives 现在给
gear carb mpg cyl disp hp drat wt qsec vs am
1: 4 4 21.0 6 160 110 3.90 2.620 16.46 0 1
2: 4 4 21.0 6 160 110 3.90 2.875 17.02 0 1
3: 4 1 22.8 4 108 93 3.85 2.320 18.61 1 1
4: 3 1 21.4 6 258 110 3.08 3.215 19.44 1 0
5: 3 2 18.7 8 360 175 3.15 3.440 17.02 0 0
If for any reason you don't want to update to the development branch, the following works in previous (and current CRAN) versions. 如果由于任何原因您不想更新到开发分支,则以下(以及当前的CRAN)版本中的以下内容适用。
newCols <- c("gear","carb")
setcolorder(DT,c(newCols, setdiff(newCols,colnames(DT)) ## (Per Frank's advice in comments)
## the long way I'd always done before seeing setdiff()
## setcolorder(DT,c(newCols,colnames(DT)[which(!colnames(DT) %in% newCols)]))
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