Want to share your content on R-bloggers? click here if you have a blog, or here if you don't. Columns in a data frame can be easily extracted and manipulated with the $ operator. Even new columns can be added by assigning a vector. • Extract columns from a data frame with the $.
• Replace values of existing columns in a data frame.
• Add new columns to a data frame.
___$___ ___$___  <- ___

## Extract columns with the $___$___
___$___ <- ___ Data frames are tables resulting from the combination of column vectors. Users can interact with data frames through numerous operators to extract, add or recombine values. To extract single columns from a data frame R offers a very specific operator: the dollar $. It returns the column vector as indicated by its name based on a data frame preceding the $. To see the $ operator in action let’s extract the population pop (in 1,000) from different states in the US based on the States dataset (from 1992) in the carData package:

States$pop  4041 550 3665 2351 29760 3294 3287 666 607 12938  6478 1108 1007 11431 5544 2777 2478 3685 4220 1228  4781 6016 9295 4375 2573 5117 799 1578 1202 1109  7730 1515 17990 6629 639 10847 3146 2842 11882 1003  3487 696 4877 16987 1723 563 6187 4867 1793 4892  454 The command extracts the population column as vector from the data frame. From this vector we can calculate the sum() of the total population as: sum(States$pop)
 248709

Similarly, the average salary (in $1,000) for teachers can be calculated as the mean() from the pay column: mean(States$pay)
 30.94118

## Quiz: Extract column from a data frame

      rank discipline yrs.since.phd yrs.service  sex salary
1     Prof          B            19          18 Male 139750
2     Prof          B            20          16 Male 173200
3 AsstProf          B             4           3 Male  79750
4     Prof          B            45          39 Male 115000
5     Prof          B            40          41 Male 141500
[ reached 'max' / getOption("max.print") -- omitted 392 rows ]
Which R command can be used to calculate the average salary of professors in the Salaries dataset from the carData package?
• mean(Salaries$salary) • mean(salary$Salaries)
• Salaries(mean$salary) • TitanicSurvival(age$mean)
Start Quiz

## Exercise: Extract column from a data frame

Calculate the average age of passengers in the TitanicSurvival dataset from the carData package. The carData package is already loaded.

Start Exercise

Extract or replace columns in a data frame using \$ is an excerpt from the course Introduction to R, which is available for free at quantargo.com

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