在R中使用merge()函数，有没有办法创建一个指示符列来指示哪一行有匹配，哪行没有匹配？

# 创建两个数据框
df1 <- data.frame(ID = c(1, 2, 3), Value1 = c("A", "B", "C"))
df2 <- data.frame(ID = c(2, 3, 4), Value2 = c("X", "Y", "Z"))

# 使用merge()函数合并两个数据框，并创建指示符列
merged <- merge(df1, df2, by = "ID", all = TRUE, indicator = TRUE)

# 查看合并结果
print(merged)

  ID Value1 Value2      _merge
1  1      A   <NA>   left_only
2  2      B      X        both
3  3      C      Y        both
4  4   <NA>      Z right_only