r - 折叠 data.table 中的行

Question

我有一个包含 100 万行和 2 列的 data.table

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 variable1      a,b2      a,c3      c,d

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

Here's what I want:

ID variable1  a1  b2  a2  c3  c3  d 

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]

任何提示或建议？

Answer 1

自 strsplit是矢量化的，这将是这里的耗时操作，我会避免在每个组上使用它。相反，可以先在 , 上拆分在整个列上，然后重建 data.table如下:

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