What is about the first column in R's dataset

问题:

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?

回答1:

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)