Question

I’m writing some code where I need to check whether all group sizes for a given input of data are equal. For example, suppose I wanted to know whether the “mpg” dataset (in the ggplot2 package) has:

• Equal numbers of cars for every manufacturer
• Equal numbers of cars for each type of drive (4-wheel, front-wheel, rear-wheel)
• Equal numbers of cars for each engine type (4-cylinder, 6-cylinder, 8-cylinder)

For data like mpg, some of those questions can be answered by inspecting the summary output

library(ggplot2)   # contains the mpg dataset
summary(mpg$drive) # shows the breakdown of cars by drive-type, 
                   # which we can verify is unequal

But I feel like I’m missing an easy way to check whether group sizes are equal. Is there some single, mythical function I can call like are.groups.of.equal.size(x)? Or another base function (or composition of them) that would return such information?

Answer 1

As Joran said we could invent 100s of ways from here till Christmas on how to do this one. I smell a microbenchmark challenge:

are.groups.of.equal.size <- function(x) {
    y <- rle(as.character(sort(x)))$lengths
    all(y%in%mean(y))
}


are.groups.of.equal.size(c(3, 3, 3))
are.groups.of.equal.size(mtcars$cyl)
are.groups.of.equal.size(CO2$Plant)
are.groups.of.equal.size(mtcars$carb)