字符变量的 order() 如何在 data.table 中工作？

Question

I want to reorder rows in my table according to a character variable ( stage in my example). If I first save the desired order ( order(dt1$stage) ) to a variable and then apply it as dt1[myorder, stage] - it works fine. But when I try to do the same inline, like dt1[order(dt1$stage), ] , the order is different. Must be something very basic I'm missing...

dt1 <- fread('
id stage pos
 1 I       1
 2 II      2
 3 III     5
 4 IV      6
 5 IIa     3
 6 IIb     7
 7 IIIa    8
 8 IIIb    4
 9 IVa     9
10 IVb    10')

sort(dt1$stage) # OK
# I II IIa IIb III IIIa IIIb IV IVa IVb

myorder <- order(dt1$stage)
dt1[myorder         , stage] # OK
# I II IIa IIb III IIIa IIIb IV IVa IVb

dt1[order(dt1$stage), stage] # different!
# I II III IIIa IIIb IIa IIb IV IVa IVb

Answer 1

It is doing a fast order instead of the base::order . According to ?data.table::order

Note that queries like x[order(.)] are optimised internally to use data.table's fast order.

Also note that data.table always reorders in "C-locale" (see Details). To sort by session locale, use x[base::order(.)].

data.table implements its own fast radix-based ordering.

data.table always reorders in "C-locale". As a consequence, the ordering may be different to that obtained by base::order. In English locales, for example, sorting is case-sensitive in C-locale. Thus, sorting c("c", "a", "B") returns c("B", "a", "c") in data.table but c("a", "B", "c") in base::ord

If we want to replicate the sort from base , use the base::order

dt1[base::order(stage)]$stage
#[1] "I"    "II"   "IIa"  "IIb"  "III"  "IIIa" "IIIb" "IV"   "IVa"  "IVb"