r - 如何使用多列作为键合并两个数据框？

Question

假设我有以下数据框:

DF1 <- data.frame("A" = rep(c("A","B"), 18),
                  "B" = rep(c("C","D","E"), 12),
                  "NUM"= rep(rnorm(36,10,1)),
                  "TEST" = rep(NA,36))

DF2 <- data.frame("A" = rep("A",6),
                  "B" = rep(c("C","D"),6),
                  "VAL" = rep(c(1,3),3))

*注:每个唯一的变量组合A和 B在 DF2应该有一个唯一的 VAL .

对于每一行，我想替换 NA在 TEST对应的值为 VAL在 DF1如果列中的值 A和 A匹配和列中的值 B和 B匹配该行。否则，我会离开 TEST如 NA .如果不使用 match 循环遍历每个组合，我将如何做到这一点？

理想情况下，答案将扩展到两个数据框，其中包含许多要匹配的列。

Answer 1

# this is your DF1    
DF1 <- data.frame("A" = rep(c("A","B"), 18),
                      "B" = rep(c("C","D","E"), 12),
                      "NUM"= rep(rnorm(36,10,1)),
                      "TEST" = rep(NA,36))

#this is a DF2 i created, with unique A, B, VAL
DF2 <- data.frame("A" = rep(c("A","B"),3),
                  "B" = rep(c("C","D","E"),2),
                  "VAL" = rep(1:6))

# and this is the answer of what i assume you want      
tmp <- merge(DF1,DF2, by=c("A","B"), all.x=TRUE, all.y=FALSE)
DF1[4] <- tmp[5]