fb-r-20230202

library(data.table)

# 原本的資料
(md <- data.table(ID = rep(c("A", "B"), each=3),
                 Subject = rep(c("English", "Math","Chinese"), 2),
                 Grade = c(90, 80, 100, 70, 65, 80)))
#>        ID Subject Grade
#>    <char>  <char> <num>
#> 1:      A English    90
#> 2:      A    Math    80
#> 3:      A Chinese   100
#> 4:      B English    70
#> 5:      B    Math    65
#> 6:      B Chinese    80

md[, rank:=frank(-Grade), by=ID]

md
#>        ID Subject Grade  rank
#>    <char>  <char> <num> <int>
#> 1:      A English    90     2
#> 2:      A    Math    80     3
#> 3:      A Chinese   100     1
#> 4:      B English    70     2
#> 5:      B    Math    65     3
#> 6:      B Chinese    80     1


# 當有科目同分時
(md2 <- data.table(ID = rep(c("A", "B"), each=3),
                 Subject = rep(c("English", "Math","Chinese"), 2),
                 Grade = c(90, 90, 100, 70, 65, 65)))
#>        ID Subject Grade
#>    <char>  <char> <num>
#> 1:      A English    90
#> 2:      A    Math    90
#> 3:      A Chinese   100
#> 4:      B English    70
#> 5:      B    Math    65
#> 6:      B Chinese    65

md2[, rank:=frank(-Grade, ties.method = "dense"), by=ID]
md2
#>        ID Subject Grade  rank
#>    <char>  <char> <num> <int>
#> 1:      A English    90     2
#> 2:      A    Math    90     2
#> 3:      A Chinese   100     1
#> 4:      B English    70     1
#> 5:      B    Math    65     2
#> 6:      B Chinese    65     2

<sup>Created on 2023-02-02 with [reprex v2.0.2](https://reprex.tidyverse.org)</sup>