如果 data.table 中的选定行与另一个 data.table 中的值匹配，如何更新它们

Question

I would like to find a data.table solution for updating selected rows from values in another table:

Table 1 looks like this

id  var grade
1   var1    X
2   var2    B
3   var3    X
4   var4    D
5   var5    X

Table 2

id  var grade
1   var1    Y
2   var2    B2
3   var3    Y
4   var4    D2
5   var5    Y

the updated table 1 looks like this

id  var grade
1   var1    Y
2   var2    B
3   var3    Y
4   var4    D
5   var5    Y

any help is much appreciated

Answer 1

We can just use second dataset and remove the digits at the end

 df2$grade <- sub("\\d+$", "", df2$grade)

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Or in data.table , convert the first data.frame to 'data.table', and assign 'grade' with the second datasets 'grade' joined by 'id', 'var' and remove the digits at the end

library(data.table)
setDT(df1)[df2, grade := i.grade, on = .(id, var)][, grade := sub("\\d+$", "", grade][]

Or as @IceCreamToucan commented

setDT(df1)[df2, grade := sub("\\d+$", "", i.grade), on = .(id, var)]