我不能完全得到正確的語法。我有一個data.table我想先按分組列g1（有序因子）排序，然后按另一列降序排序的地方n。唯一的問題是我希望第三列的標記為“其他”的行g2出現在每組的底部，而不管它們的n.
例子：
library(data.table)
dt <- data.table(g1 = factor(rep(c('Australia', 'Mexico', 'Canada'), 3), levels = c('Australia', 'Canada', 'Mexico')),
g2 = rep(c('stuff', 'things', 'other'), each = 3),
n = c(1000, 2000, 3000, 5000, 100, 3500, 10000, 10000, 0))
這是預期的輸出，在 each 中g1，n除了g2 == 'other'始終位于底部的行之外，我們有降序排列：
g1 g2 n
1: Australia things 5000
2: Australia stuff 1000
3: Australia other 10000
4: Canada things 3500
5: Canada stuff 3000
6: Canada other 0
7: Mexico stuff 2000
8: Mexico things 100
9: Mexico other 10000
uj5u.com熱心網友回復：
利用data.table::order和它的-反向排序：
dt[order(g1, g2 == "other", -n), ]
# g1 g2 n
# <fctr> <char> <num>
# 1: Australia things 5000
# 2: Australia stuff 1000
# 3: Australia other 10000
# 4: Canada things 3500
# 5: Canada stuff 3000
# 6: Canada other 0
# 7: Mexico stuff 2000
# 8: Mexico things 100
# 9: Mexico other 10000
我們添加g2 == "other"是因為您說過“其他”應該始終放在最后。例如，如果"stuff"是"abc"，那么我們可以看到行為的差異：
dt[ g2 == "stuff", g2 := "abc" ]
dt[order(g1, -n), ]
# g1 g2 n
# <fctr> <char> <num>
# 1: Australia other 10000
# 2: Australia things 5000
# 3: Australia abc 1000
# 4: Canada things 3500
# 5: Canada abc 3000
# 6: Canada other 0
# 7: Mexico other 10000
# 8: Mexico abc 2000
# 9: Mexico things 100
dt[order(g1, g2 == "other", -g2), ]
# g1 g2 n
# <fctr> <char> <num>
# 1: Australia things 5000
# 2: Australia abc 1000
# 3: Australia other 10000
# 4: Canada things 3500
# 5: Canada abc 3000
# 6: Canada other 0
# 7: Mexico things 100
# 8: Mexico abc 2000
# 9: Mexico other 10000
這樣做的一個缺點是setorder不能直接作業：
setorder(dt, g1, g2 == "other", -n)
# Error in setorderv(x, cols, order, na.last) :
# some columns are not in the data.table: ==,other
所以我們需要重新排序并重新分配回dt.
順便說一句：這是有效的，因為g2 == "other"決議為logical，是的，但是在對它們進行排序時被視為0（假）和1（真），因此假條件將出現在真條件之前。
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