我们可以对'Year'、'Code'进行联接,然后更新'dfGermany'、'Sales'列
library(data.table)
setDT(dfGermany)[dfJapan, Sales := Sales + i.Sales/2, on = .(Year, Code)]
dfGermany
# Country Sales Year Code
#1: Germany 2250 2000 221
#2: Germany 1500 2001 150
#3: Germany 2650 2002 270
dfGermany <- structure(list(Country = c("Germany", "Germany", "Germany"),
Sales = c(2000, 1500, 2150), Year = 2000:2002, Code = c(221L,
150L, 270L)), row.names = c(NA, -3L), class = "data.frame")
dfJapan <- structure(list(Country = c("Japan", "Japan", "Japan", "Japan"
), Sales = c(500L, 750L, 800L, 1000L), Year = c(2000L, 2001L,
2001L, 2002L), Code = c(221L, 221L, 270L, 270L)),
class = "data.frame", row.names = c(NA, -4L))