我想要的.SD
功能与by =
在非球菌结合加入:如何选择指定列中每个组的前n行(加入后)?
data.table - select first n rows within group
示例数据:
tmp_dt1<- data.table(grp = c(1,2), time = c(0.2, 0.6, 0.4, 0.8, 0.25, 0.65))
tmp_dt2 <- data.table(grp = c(1,2), time_from = c(0.1, 0.5))
tmp_dt2 <- tmp_dt2[, time_to := time_from + 0.2]
> tmp_dt1
grp time
1: 1 0.20
2: 2 0.60
3: 1 0.40
4: 2 0.80
5: 1 0.25
6: 2 0.65
> tmp_dt2
grp time_from time_to
1: 1 0.1 0.3
2: 2 0.5 0.7
现在,我需要的输出是每个组中的第一次位于tmp_dt2
中定义的范围之间。我可以得到所有这样的时代:
> tmp_dt1[tmp_dt2, .(grp, time = x.time, time_from, time_to), on = .(grp, time >= time_from, time <= time_to)]
grp time time_from time_to
1: 1 0.20 0.1 0.3
2: 1 0.25 0.1 0.3
3: 2 0.60 0.5 0.7
4: 2 0.65 0.5 0.7
不过,我有一些麻烦,使用by
提取每个grp
第n行,没有链接。举个例子,当n = 1
,所需的输出是:
tmp_dt1[tmp_dt2, .(grp, time = x.time, time_from, time_to),
on = .(grp, time >= time_from, time <= time_to)][, .SD[1], by = grp]
grp time time_from time_to
1: 1 0.2 0.1 0.3
2: 2 0.6 0.5 0.7
但是,这样的:
> tmp_dt1[tmp_dt2, .(time = x.time[1], time_from[1], time_to[1]), on = .(grp, time >= time_from, time <= time_to), by = grp]
Error in `[.data.table`(tmp_dt1, tmp_dt2, .(time = x.time[1], time_from[1], :
object 'time_from' not found
不起作用。
使用,.SD
接近,但给我结果的混乱结束在选择的列的条款:
tmp_dt1[tmp_dt2, .SD[1], on = .(grp, time >= time_from, time <= time_to), by = grp]
grp time
1: 1 0.2
2: 2 0.6
为什么我不想做一个链的原因是因为memory issues。请注意,我只对data.table
软件包解决这个问题感兴趣。
谢谢你的回答,也是非常有用的链接,解释'x.'符号 – Alex