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)
来源：https://stackoverflow.com/questions/38990477/what-is-about-the-first-column-in-rs-dataset-mtcars