CODEWITHSUNDEEP
rpackagesubset
user5765809
user5765809

Reputation:

R, finding installed packages

I have used installed.packages() to see installed packages in R. And I want to extract two columns from the output, "Packages" and "Version" by using installed.packages()[c("Package","Version")] but it does not show what I expect. But if I put a "," just before "Package" (installed.packages()[,c("Package","Version")]) it works! Why is a "," necessary in the statement?

Upvotes: 2

Views: 214

Answers (3)

vahid-dan
vahid-dan

Reputation: 322

For future references, if you want to know which exact GitHub commit is used for a GitHub package:

install.packages("devtools")
library("devtools")
package_info("<package_name>")

Upvotes: 0

Thomas
Thomas

Reputation: 44525

You need to give ? Extract a look to understand indexing in R. Here are some hints for how to understand what the object you're looking at is structured:

> class(installed.packages())
[1] "matrix"
> dim(installed.packages())
[1] 173  16
> str(installed.packages())
 chr [1:173, 1:16] "aws.s3" "aws.signature" "BH" "bit" "bit64" ...
 - attr(*, "dimnames")=List of 2
  ..$ : chr [1:173] "aws.s3" "aws.signature" "BH" "bit" ...
  ..$ : chr [1:16] "Package" "LibPath" "Version" "Priority" ...

So, what that tells us is that the object is a matrix, with 173 rows and 16 columns.

  • To extract from a matrix, you use notation like matrix[rows, columns].
  • To get all rows but only some columns, you can shortcut that to matrix[, columns].
  • To get all columns but some rows, you can shortcut that to matrix[rows,]

You're probably expecting that the object is a data.frame instead. A data.frame allows various other forms of indexing/extraction with which you might be more familiar:

> str(mtcars["mpg"])
'data.frame':   32 obs. of  1 variable:
 $ mpg: num  21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
> str(mtcars[["mpg"]])
 num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
> str(mtcars[, "mpg"])
 num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
> str(mtcars$mpg)
 num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...

Upvotes: 2

MikeRSpencer
MikeRSpencer

Reputation: 1316

The contents of installed.packages() have columns and rows and in subsetting [a, b], a are rows and b are columns. You're asking for the columns named 'package' and 'version', so you need to tell it to look at columns for those names.

More guidance here: http://statmethods.net/management/subset.html and http://adv-r.had.co.nz/Subsetting.html.

Upvotes: 3

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