R中data.frame中sort（）如何工作

Question

How does sort work, that is using what method to sort column in

data.frame (barley$site, barley$year, barley$variety)

as following

library(lattice)
barley <- barley[order(barley$site, barley$year, barley$variety), ] 

Answer 1

You probably want:

barley[order(as.character(barley$site), as.numeric(barley$year), as.character(barley$variety)),] 

As you have it you are ordering by the underlying levels of the data.frame, which leads to really odd stuff. Look at the structure of the data frame:

 'data.frame':  120 obs. of  4 variables:
 $ yield  : num  27 48.9 27.4 39.9 33 ...
 $ variety: Factor w/ 10 levels "Svansota","No. 462",..: 3 3 3 3 3 3 7 7 7 7 ...
 $ year   : Factor w/ 2 levels "1932","1931": 2 2 2 2 2 2 2 2 2 2 ...
 $ site   : Factor w/ 6 levels "Grand Rapids",..: 3 6 4 5 1 2 3 6 4 5 ...

Notice how the levels for year are in the opposite order you would expect. The documentation for order discusses this very briefly:

For factors, this sorts on the internal codes, which is particularly appropriate for ordered factors.

I personally think this terribly confusing, but it is what it is. factor are very useful in most contexts, but incredibly dangerous in others if you're not careful. Having numbers represented as factors (as year was here) is particularly bad.

See ?factor for more details.

Answer 2

By default, sort doesn't know how to do anything with a data frame. You can sort the individual columns within a data frame, with something like df$x <- sort(df$x) but you almost certainly don't want to do that; it will just mess up your data.

You order the rows in the data frame by using order as in the example code you have there. This orders the rows by values in the column site , breaking ties with year , and then with variety .