How to sum a variable by factor levels in R?

We can do this by using aggregate function or with the help tapply

Example

> x <- data.frame(Category=factor(c("Graduation", "Graduation", "Post-Graduation", "Graduation","PhD", "Post-Graduation", "PhD")), Frequency=c(12,19,15,20,25,13,14)) > x Category Frequency 1 Graduation 12 2 Graduation 19 3 Post-Graduation 15 4 Graduation 20 5 PhD 25 6 Post-Graduation 13 7 PhD 14

Using aggregate

> aggregate(x$Frequency, by=list(Group=x$Category), FUN=sum) Group x 1 Graduation 51 2 PhD 39 3 Post-Graduation 28 Using tapply > tapply(x$Frequency, x$Category, FUN=sum) Graduation PhD Post-Graduation 51 39 28