在R中多个数据框中的一列中创建唯一值列表

Question

Suppose I have 3 data frames (df1, df2, df3) that explain the color, year and make of a car. Each data frame has a column named "id". Not all of the ids match in each data frame, but I want to create a list of unique ids across all three data frames.

 df1 looks like this:  

 id     color
 5A     black
 7T     green
 8Q     gold
 4D     white

 df2 looks like this:  

 id     make
 5A     BMW
 6N     Benz
 8Q     GM
 3G     Toyota

 df3 looks like this:  

 id     year
 5A     2003
 2B     2010
 8Q     1999
 4D     2007

Here's the code to make the 3 data frames:

 df1 <- data.frame(id=c("5A", "7T", "8Q", "4D"), color=c("black", "green", "gold", "white"))
 df2 <- data.frame(id=c("5A", "6N", "8Q", "3G"), make=c("BMW", "Benz", "GM", "Toyota"))
 df3 <- data.frame(id=c("5A", "2B", "8Q", "4D"), year=c(2003,2010,1999,2007))

I would like my final list to have the following values:

 id
 5A
 7T
 8Q
 4D
 6N
 3G
 2B

I tried using unique() but that only works for one vector? Any thoughts?

Answer 1

unique(c(df1$id, df2$id, df3$id)) 

works for numeric values.

unique(c(as.character(df1$id), as.character(df2$id), as.character(df3$id)))

returns the values as characters, and you could manipulate them back to factors if that's desired.

Answer 2

This may be a way of doing it

lst <- list(df1, df2, df3)
unique(unlist(lapply(lst, function(x) unique(x[,1]))))

[1] 5A 7T 8Q 4D 6N 3G 2B
Levels: 4D 5A 7T 8Q 3G 6N 2B