buhtz buhtz - 3 months ago 19
R Question

What is about the first column in R's dataset mtcars?

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?

Answer

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)
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