I have a data frame, str(data) to show more about my data frame the result is the following:
> str(data)
'data.frame': 153 obs. of 6 variables:
$ Ozone : int 41 36 12 18 NA 28 23 19 8 NA ...
$ Solar.R: int 190 118 149 313 NA NA 299 99 19 194 ...
$ Wind : num 7.4 8 12.6 11.5 14.3 14.9 8.6 13.8 20.1 8.6 ...
$ Temp : int 67 72 74 62 56 66 65 59 61 69 ...
$ Month : int 5 5 5 5 5 5 5 5 5 5 ...
$ Day : int 1 2 3 4 5 6 7 8 9 10 ...
However, for example, when I want to subset the amounts of Ozone above 14 I use the following code which gives me an error:
> data[data$Ozone > 14 ]
Error in
[.data.frame(data, data$Ozone > 14) : undefined columns selected
You want rows where that condition is true so you need a comma:
data[data$Ozone > 14, ]
来源：https://stackoverflow.com/questions/19205806/undefined-columns-selected-when-subsetting-data-frame