R in Action

Creating new variables

Use the assignment operator <- to create new variables. A wide array of operators and functions are available here.

# Three examples for doing the same computations

mydata$sum <- mydata$x1 + mydata$x2
mydata$mean <- (mydata$x1 + mydata$x2)/2

attach(mydata)
mydata$sum <- x1 + x2
mydata$mean <- (x1 + x2)/2
detach(mydata)

mydata <- transform( mydata,
sum = x1 + x2,
mean = (x1 + x2)/2
)

(To practice working with variables in R, try the first chapter of this free interactive course.)

Recoding variables

In order to recode data, you will probably use one or more of R's control structures.

# create 2 age categories
mydata$agecat <- ifelse(mydata$age > 70,
c("older"), c("younger"))

# another example: create 3 age categories
attach(mydata)
mydata$agecat[age > 75] <- "Elder"
mydata$agecat[age > 45 & age <= 75] <- "Middle Aged"
mydata$agecat[age <= 45] <- "Young"
detach(mydata)

Renaming variables

You can rename variables programmatically or interactively.

# rename interactively
fix(mydata) # results are saved on close

# rename programmatically
library(reshape)
mydata <- rename(mydata, c(oldname="newname"))

# you can re-enter all the variable names in order
# changing the ones you need to change.the limitation
# is that you need to enter all of them!
names(mydata) <- c("x1","age","y", "ses")