In R, how do I find the indices of elements in a list in another larger list?

Question

I have a list of non-unique names in a CSV file. I want to assign an id to each unique name in this list. So if the data is something like this:

Stacy
Adam
Donald
Adam
Greg
Donald

I want the output to be something like this:

1 Stacy
2 Adam
3 Donald
2 Adam
5 Greg
3 Donald

I have tried using the match() function, but that does not seem to be working. Any help would be greatly appreciated.

data <- read.csv(file = "mock_data.csv", header = TRUE)
uniqueFirstNames <- unique(data["first_name"])
paste('Number of unique first names: ', nrow(uniqueFirstNames))
indices <- match(x = uniqueFirstNames, table = data["first_name"])

indices above currently gives me a NA

Answer 1

    df <- data.frame(names = c("Stacy", "Adam","Donald","Adam","Greg","Donald"))

    ##using factor
    df$flag <- with(df, as.numeric(factor(names,levels=unique(names) )))

    ##Using match
    df$flag2 <- with(df, match(names, unique(names)))

    names    flag  flag2
    Stacy    1     1
    Adam     2     2
    Donald   3     3
    Adam     2     2
    Greg     4     4
    Donald   3     3