基於兩列之間的匹配的數據幀中的新列

Question

我試圖通過比較列x的值與不同數據框（df2）中列y的值在現有數據框（df1）中創建一個新列。

結果應該看起來像df_end。如果有匹配，則應返回第x列的值。如果沒有匹配，則應退還NA。

df1 <- data.frame(x = c("blue2", "blue6", "green9", "green7")) 
df2 <- data.frame(y = c("blue2", "green9")) 

df_end <- data.frame(x = c("blue2", "blue6", "green9", "green7"), 
        match = c("blue2", NA, "green9", NA)) 

我已經試驗過merge，match和if，else語句，但我無法弄清楚。有人對我有一個建議嗎？

#Attempt 1: Merge 
df1$match <- merge(df1, df2, by.x = x, all = TRUE) 

這不起作用，因爲df1和df2長度不同。

Answer 1

我做了以下內容：

df1 <- data.frame(x = c("blue2", "blue6", "green9", "green7")) 
df2 <- data.frame(y = c("blue2", "green9")) 

end <- sapply(df1$x, function(x) { # for each value in df1$x 
    j <- which(df2$y == x) # check if df2$y has a match 
    ifelse(length(j) > 0, j, NA) # if there is, give the location in the vector 
}) # if not give NA 

cbind(df1,df2, match = df2$y[end]) # subset the df2 with the location to get the characters 

#  x  y match 
#1 blue2 blue2 blue2 
#2 blue6 green9 <NA> 
#3 green9 blue2 green9 
#4 green7 green9 <NA> 

編輯： 看到索托斯的最佳答案評論：merge`與`所有= TRUE`參數df2$y[match(df1$x, df2$y)]