Is there a way to create Stata's _merge indicator variable with R's merge()?

Question

Stata automatically creates a variable called \"_merge\" indicating the matched variables in both datasets after merge. Is there a way to get such variable generated by R\'s

Answer 1

I have written the following function based on @Metrics answer. It creates a variable "merge" in the resulting dataset that indicates observations as Stata does.

stata.merge <- function(x,y, by = intersect(names(x), names(y))){

x[is.na(x)] <- Inf
y[is.na(y)] <- Inf

matched <- merge(x, y, by.x = by, by.y = by, all = TRUE)
matched <- matched[complete.cases(matched),]
matched$merge <- "matched"
master <- merge(x, y, by.x = by, by.y = by, all.x = TRUE)
master <- master[!complete.cases(master),]
master$merge <- "master"
using <- merge(x, y, by.x = by, by.y = by, all.y = TRUE)
using <- using[!complete.cases(using),]
using$merge <- "using"

df <- rbind(matched, master,using)
df[sapply(df, is.infinite)] <- NA
df
}

Test.

df1 <- data.frame(id = letters[c(1:5,8:9)], v1=c(1:5,8:9))
df1

   id v1
1  a  1
2  b  2
3  c  3
4  d  4
5  e  5
6  h  8
7  i  9

df2 <- data.frame(id = letters[1:8], v1=c(1:7,NA))
df2

  id v1
1  a  1
2  b  2
3  c  3
4  d  4
5  e  5
6  f  6
7  g  7
8  h NA

stata.merge(df1,df2, by = "id")

   id v1.x v1.y   merge
1   a    1    1 matched
2   b    2    2 matched
3   c    3    3 matched
4   d    4    4 matched
5   e    5    5 matched
6   h    8   NA matched
7   i    9   NA  master
71  f   NA    6   using
8   g   NA    7   using