R語言學習-Group By的實現

library(dplyr) data <- data.frame(year = rep(2016:2017,6),month = seq(1:12),sales=rep(c(10,20,30,40),3)) year month sales 1 2016 1 10 2 2017 2 20 3 2016 3 30 4 2017 4
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