一种非常简单的方法是使用查找表将月份名称映射到季节：

One pretty straightforward way is to use a lookup table to map month names to seasons:

# create a named vector where names are the month names and elements are seasons
seasons <- rep(c("winter","spring","summer","fall"), each = 3)
names(seasons) <- month.abb[c(6:12,1:5)] # thanks thelatemail for pointing out month.abb
seasons
#     Jun      Jul      Aug      Sep      Oct      Nov      Dec      Jan 
#"winter" "winter" "winter" "spring" "spring" "spring" "summer" "summer" 
#     Feb      Mar      Apr      May 
#"summer"   "fall"   "fall"   "fall" 

使用它：

dt[, season := seasons[MonthName]]

数据：

dt <- setDT(read.table(text="    rrp         Year   Month Finyear hourminute AvgPriceByTOD MonthName
1: 35.27500     1999     1    1999      00:00      33.09037       Jan
2: 21.01167     1999     1    1999      00:00      33.09037       Jan
3: 25.28667     1999     2    1999      00:00      33.09037       Feb
4: 18.42334     1999     2    1999      00:00      33.09037       Feb
5: 16.67499     1999     2    1999      00:00      33.09037       Feb
6: 18.90001     1999     2    1999      00:00      33.09037       Feb",
   header = TRUE, stringsAsFactors = FALSE))