我有一个看起来像这样的数据表。
ID <- c(rep("ABC",4), rep("DEF",4), rep("GHI",5))
X <- c(rep(c(1,2,3,4),3),5)
set.seed(1234)
Z <- runif(13,min=0, max =1)
a <- data.table(ID, X, Z)
a
ID X Z
1: ABC 1 0.113703411
2: ABC 2 0.622299405
3: ABC 3 0.609274733
4: ABC 4 0.623379442
5: DEF 1 0.860915384
6: DEF 2 0.640310605
7: DEF 3 0.009495756
8: DEF 4 0.232550506
9: GHI 1 0.666083758
10: GHI 2 0.514251141
11: GHI 3 0.693591292
12: GHI 4 0.544974836
13: GHI 5 0.282733584
我想生成一个在每个X子组中具有N个最高值Z的数据帧。所以我们说N是2.我想最终得到一个如下所示的数据集:
X ID Z
1: 1 DEF 0.8609154
2: 1 GHI 0.6660838
3: 2 DEF 0.6403106
4: 2 ABC 0.6222994
5: 3 GHI 0.6935913
6: 3 ABC 0.6092747
7: 4 ABC 0.6233794
8: 4 GHI 0.5449748
9: 5 GHI 0.2827336
我一直在使用这条线来实现它,但是当数据表很大(即超过1,500,000行或更多)时,我发现它特别慢。
top_n <- 2
a <- a[order(a$X, -a$Z),]
a_2 <- a[, head(.SD, top_n), by=X]
a_2
X ID Z
1: 1 DEF 0.8609154
2: 1 GHI 0.6660838
3: 2 DEF 0.6403106
4: 2 ABC 0.6222994
5: 3 GHI 0.6935913
6: 3 ABC 0.6092747
7: 4 ABC 0.6233794
8: 4 GHI 0.5449748
9: 5 GHI 0.2827336
任何帮助都将非常感谢!
谢谢!
答案 0 :(得分:5)
这应该比Allow → GET, POST, HEAD, OPTIONS
.SD
如果你需要一个有序的结果,这应该也很快
n <- 2
indx <- a[order(-Z), .I[seq_len(n)], by = X]$V1
a[indx]
# ID X Z
# 1: DEF 1 0.8609154
# 2: GHI 1 0.6660838
# 3: GHI 3 0.6935913
# 4: ABC 3 0.6092747
# 5: DEF 2 0.6403106
# 6: ABC 2 0.6222994
# 7: ABC 4 0.6233794
# 8: GHI 4 0.5449748
# 9: GHI 5 0.2827336
# 10: NA NA NA