折叠data.table中的行 [英] collapse rows in data.table
问题描述
我有一个具有1M行和2列的数据表
I have one data.table with 1M rows and 2 columns
虚拟数据:
require(data.table)
ID <- c(1,2,3)
variable <- c("a,b","a,c","c,d")
dt <- data.table(ID,variable)
dt
> dt
ID variable
1 a,b
2 a,c
3 c,d
现在,我想通过"ID"将变量"列折叠到不同的行中,就像reshape2中的"melt"功能或data.table中的melt.data.table一样.
Now I want to collapse the column "variable" into different rows by "ID", just as the "melt" function in reshape2 or melt.data.table in data.table
这就是我想要的:
ID variable
1 a
1 b
2 a
2 c
3 c
3 d
PS:给定理想的结果,我知道如何执行反向步骤.
PS: Given the desired results, I know how to do the reverse step.
dt2 <- data.table(ID = c(1,1,2,2,3,3), variable = c("a","b","a","c","c","d"))
dt3 <- dt2[, list(variables = paste(variable, collapse = ",")), by = ID]
有任何提示或建议吗?
推荐答案
由于 strsplit
是矢量化的,因此这将是耗时的操作,因此我避免在每个组上使用它.相反,可以先在整个列的,
上拆分,然后按如下所示重构 data.table
:
Since strsplit
is vectorised, and that's going to be the time consuming operation here, I'd avoid using it on each group. Instead, one could first split on the ,
on the entire column and then reconstruct the data.table
as follows:
var = strsplit(dt$variable, ",", fixed=TRUE)
len = vapply(var, length, 0L)
ans = data.table(ID=rep(dt$ID, len), variable=unlist(var))
# ID variable
# 1: 1 a
# 2: 1 b
# 3: 2 a
# 4: 2 c
# 5: 3 c
# 6: 3 d
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