Question

I am trying to merge two dataframes with ids, I want to merge first all the ids that match and then find that doesn’t match, I found the merge function which can merge the common ids.for example:

m1 = merge(df1, df2, by=c("id"))

Now I am trying to create a new dataframe with ids of dataframe 2 that do not match dataframe 1.

Could you please advise me which command should I look for?

For example:

I have the following two datasets:

df1
df2

I am trying to create a new dataframe with ids from df2 that not in df1. for example id = “a3” and “c3” in df2.

my sample data:

    df1 =data.frame(id= c("a1","a2","b1","b2","c1","c2"), value= 1:6)
df2 =data.frame(id= c("a1","a2","a3","b1","c1","c3"), value= 7:12)

Many thanks, Ayan

Answer 1

If you want to use merge, here is one way to do it:

df_merged <- merge(df2, df1, by.x="id", by.y="id", all.x=TRUE)
df_merged[is.na(df_merged$value.y),]
  id value.x value.y
3 a3       9      NA
6 c3      12      NA

Since your column names are in both data.frames identical and merge merges by common column names, you have to tell the function the column names explicitly that you want to use, here id.

But you should ask yourself if you really want to merge here? If you just want those rows in df2 that are not in df1, why not use something like this?

df2[!(df2$id %in% df1$id), ]
  id value
3 a3     9
6 c3    12