12

I think I am missing a fundamental concept about R's data frames.

head(mtcars)
                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1

The names of the cars here. Is this a column? I don't think so, because I am not able to access them via mtcars[,1]. And there is no column name/header for it.

How could I create a data frame like that? How could I use that special column e.g. to describe the data in a plot for example?

starball
  • 20,030
  • 7
  • 43
  • 238
buhtz
  • 10,774
  • 18
  • 76
  • 149

4 Answers4

12

They are row names, to access them use:

rownames(mtcars)

For column names use colnames, to see both row and column names, we can use:

dimnames(mtcars)

To modify, for example the first row:

rownames(mtcars)[1] <- "myNewName"

When data frame is created with data.frame, row names are assigned with 1:n numbers.

mydata <- data.frame(x = 1:5)

Then we can modify them:

rownames(mydata) <- paste0("MyName", 1:5)

Or we can add rownames when creating the data.frame:

mydata <- data.frame(x = 1:5, row.names = paste0("MyName", 1:5))

Note: rownames are not very reliable, for example see this post. (this could be subjective opinion and I avoid them by reassigning rownames to columns)

data.table and dplyr packages prefer not to have them. You can always reassign rownames into a columns as:

mydata$myNames <- rownames(mydata)
zx8754
  • 52,746
  • 12
  • 114
  • 209
  • 2
    **row names**?! This is out of my Excel-universe! Great! ;) – buhtz Aug 17 '16 at 07:37
  • btw: Some export librarys (for csv or xlsx files) use the rownames as first column. Can I delete the rownames? – buhtz Aug 17 '16 at 07:38
  • When I do `mtcars$mpg` the rownames are missing. Can this be fixed? – buhtz Aug 17 '16 at 07:44
  • 1
    to delete rownames you can just do `rownames(mtcars) <- NULL`. Also, when you do `mtcars$mpg`, you are extracting the `mpg` variable as a vector. – Sotos Aug 17 '16 at 07:47
  • 2
    Because by doing above we are dropping data.frame attributes and converting it into a vector, try `mtcars["mpg"]` instead. – zx8754 Aug 17 '16 at 07:47
  • Note that the formal attribute is `row.names` (`names(attributes(mtcars))`). While `rownames` (actually `dimnames`) knows how to handle a "data.frame", there could be cases that will not behave correctly: `structure(list(1, 2), class = "data.frame", names = c("a", "b"), row.names = .set_row_names(1))` VS `structure(list(1, 2), class = "data.frame", names = c("a", "b"), rownames = .set_row_names(1))` – alexis_laz Aug 17 '16 at 11:17
  • @alexis_laz feel free to edit or post another answer. – zx8754 Aug 17 '16 at 11:20
  • So, the final solution if you would like to add a new column is col1<-dimnames(mtcars)[[1]] ver<-as.data.frame(col1) colnames(ver)[1]<-"col1" mtcars<-data.frame(mtcars,ver) – lolo Sep 08 '17 at 17:27
  • 2
    Just thougth this simple argument would do well. ``df <- cbind(new_col_name =rownames(mtcars), data.frame(mtcars, row.names= NULL)).`` Assigning `new_col_name` will replace the default `rn` column name – linkonabe Jul 20 '20 at 18:04
4

A shorter one liner argument with data.tablePackage will make the rowname a column.

library(data.table)
setDT(mtcars, keep.rownames = TRUE[])

head(mtcars)

      rn           mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1
linkonabe
  • 661
  • 7
  • 23
Flo
  • 53
  • 6
3

This works too using tibble.

library(tibble)
mtcars %>%
  rownames_to_column(var="carnames")
william3031
  • 1,653
  • 1
  • 18
  • 39
0
  • How could you create a data frame like that? => you can transform a column to a row names using textshape package. see exemple below

> column to row names

library(textshape)

state_dat <- data.frame(state.name, state.area, state.center, state.division)
column_to_rownames(state_dat)
#making 'state.name' to row names in new data 'new_state_dat'
new_state_dat<-column_to_rownames(state_dat, 'state.name')

I advise you not to use row.names() to transform column into row names

How could I use that special column e.g. to describe the data in a plot for example?

you can use superheat package, for more information, see https://rlbarter.github.io/superheat/index.html , it's more simple and more powerful if you use textshape package instead row.names() to transform column into rownames

Justino Mory
  • 1