I have a data frame like this:
我有这样的数据框:
a=c("A","A","A","A","B","B","C","C","C","D","D")
b=c(1,2,3,4,1,2,1,2,3,1,2)
c=c(1345,645,75,8,95,678,598,95,75,4,53)
mydf <- data.frame(a,b,c) # edit note: do _not_ use cbind inside data.frame
a=
阅读背景：
按因子类别获取数据框中的最后一个值
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