Rownames as column in list of dataframes

Question

I'd like to create a column year for each dataframe in a list based on its rownames. This question has been asked before on SO but unfortunately the answer doesn't help. So, is there a way to do it?

mylist <- list(structure(list(a = 1:10), .Names = "a", row.names = 1991:2000, class = "data.frame"), 
            structure(list(a = 1:10), .Names = "a", row.names = 1992:2001, class = "data.frame"))

Expected outcome:

[[1]]
      a year
1991  1 1991
1992  2 1992
1993  3 1993
1994  4 1994
1995  5 1995
1996  6 1996
1997  7 1997
1998  8 1998
1999  9 1999
2000 10 2000

[[2]]
      a year
1992  1 1992
1993  2 1993
1994  3 1994
1995  4 1995
1996  5 1996
1997  6 1997
1998  7 1998
1999  8 1999
2000  9 2000
2001 10 2001

Why doesn't it help? What question are you referencing? – franklin Mar 03 '16 at 20:32 — franklin, Mar 03 '16 at 20:32

talat · Answer 1 · 2016-02-16T13:48:43.600

5

Here's another option:

lapply(mylist, function(df) transform(df, year = rownames(df)))

Just for fun, you could also use dplyr's add_rownames function:

lapply(mylist, dplyr::add_rownames, 'year')

edited Feb 16 '16 at 13:48

answered Feb 16 '16 at 11:15

talat

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akrun · Accepted Answer · 2016-02-16T11:22:26.310

3

We can use Map

Map(cbind, mylist, year= lapply(mylist, rownames))

edited Feb 16 '16 at 11:22

answered Feb 16 '16 at 11:14

akrun

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I guess your original answer with `Map` + `lapply` will be less efficient – talat Feb 16 '16 at 11:19
@docendodiscimus I didn't see that you posted a similar answer. I will delete it. – akrun Feb 16 '16 at 11:22

Rownames as column in list of dataframes

2 Answers2