R sorting data subset

Question

I'm learning to use R (version 3.1.2), so this may come as a noob question, but I'm having problems ordering a subset of a data frame. If I use the mtcars data frame using `attach(mtcars)`, I can easily order it using `ord.cars <- mtcars[order(hp),]`. The problem is, if I use a subset, let's say `sub.cars <- subset(mtcars, hp > 120)` and try to order it using `ord.sub <- sub.cars[order(mpg),]`, the result is the following: ``` mpg cyl disp hp drat wt qsec vs am gear carb NA NA NA NA NA NA NA NA NA NA NA NA Cadillac Fleetwood 10.4 8 472.0 205 2.93 5.250 17.98 0 0 3 4 NA.1 NA NA NA NA NA NA NA NA NA NA NA NA.2 NA NA NA NA NA NA NA NA NA NA NA NA.3 NA NA NA NA NA NA NA NA NA NA NA NA.4 NA NA NA NA NA NA NA NA NA NA NA Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4 Lincoln Continental 10.4 8 460.0 215 3.00 5.424 17.82 0 0 3 4 NA.5 NA NA NA NA NA NA NA NA NA NA NA Merc 450SL 17.3 8 275.8 180 3.07 3.730 17.60 0 0 3 3 NA.6 NA NA NA NA NA NA NA NA NA NA NA Hornet Sportabout 18.7 8 360.0 1…

Accepted Answer

This is a problem related to your use of `attach()` which is not recommended in R - for exactly this reason! The problem is, that your code is kind of ambiguous, or at least, it is something different than what you expected it to be. How to resolve this? - `detach` the data set and - don't use `attach` again. Instead, use `[` and/or `$` and if you like `with()` to subset your data. Here's how you could do it for the example: ``` detach(mtcars) ord.cars <- mtcars[order(mtcars$hp),] sub.cars <- subset(mtcars, hp > 120) #the subset could also be written as: sub.cars <- mtcars[mtcars$hp > 120,] ord.sub <- sub.cars[order(sub.cars$mpg),] head(ord.sub) # only show the first 6 rows mpg cyl disp hp drat wt qsec vs am gear carb Cadillac Fleetwood 10.4 8 472 205 2.93 5.25 18.0 0 0 3 4 Lincoln Continental 10.4 8 460 215 3.00 5.42 17.8 0 0 3 4 Camaro Z28 13.3 8 350 245 3.73 3.84 15.4 0 0 3 4 Duster 360 14.3 8 360 245 3.21 3.57 15.8 0 0 3 4 Chrysler Imperial 14.7 8 440 230 3.23 5.34 17.4 0 0 3 4 Maserati Bora 15.0 8 301 335 3.54 3.57 14.6 0 1 5 8 ``` What exactly caused the problem in your code? After you `attached` the mtcars data, whenever you call one of the column names of the attached data, like `mpg`, it will refer to the attached data set (the original mtcats data). The problem then was that you subsetted the data and stored it in a new object (sub.cars) which was not attached while mtcars was still attached. Then, when you tried to order the sub.cars data, you used `sub.cars[order(mpg),]` and as you can see, in there, you refer to `mpg` column - that is interpreted by R as the one from the attached (original) mtcars data set, with more rows than you subsetted data. All those rows in your sub.cars which were excluded by the subsetting, will now be displayed as NAs in `sub.cars`. Lesson: don't use `attach()`.

R sorting data subset

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