How to Calculate Combinations & Permutations in R

Combinations and permutations are key mathematical concepts used extensively in probability theory and statistics, and are essential for data analysis and computational statistics. These are both ways of counting or grouping items, and are particularly useful in situations where you need to calculate the number of ways to select items from a larger set.

Before diving into calculating these concepts in R, it’s important to understand the difference between combinations and permutations:

• Combinations refer to the selection of items without regard to the order in which they are selected. For example, if you are choosing two items from the set {A, B, C}, AB and BA are considered the same combination.
• Permutations, on the other hand, are all the possible arrangements of items where the order does matter. Using the same example as above, AB and BA are considered different permutations.

In this article, we will guide you through the process of calculating combinations and permutations in R, with the help of some illustrative examples.

Calculating Combinations in R

In R, the function to calculate combinations is choose(). This function takes two arguments: n and k. It calculates the number of ways to choose k items from a set of n items, without regard to the order of selection.

Let’s say we have a set of 5 items (A, B, C, D, E), and we want to know how many ways we can choose 3 items from this set. We can calculate this in R as follows:

n = 5
k = 3
combinations = choose(n, k)
print(combinations)

This will return the value 10, indicating that there are 10 ways to choose 3 items from a set of 5.

Calculating Permutations in R

R does not have a built-in function to calculate permutations, but we can easily define one ourselves.

Remember that the number of permutations of k items from a set of n items (also written as “nPk”) is calculated as n! / (n-k)!, where ! denotes the factorial of a number.

Here’s how we can define a function to calculate permutations in R:

permute <- function(n, k) {
  return(factorial(n) / factorial(n-k))
}

With this function defined, we can now calculate permutations. Let’s find the number of ways to arrange 3 items from our set of 5:

n = 5
k = 3
permutations = permute(n, k)
print(permutations)

This will return the value 60, indicating that there are 60 different ways to arrange 3 items from a set of 5.

Calculating Combinations and Permutations for Vector Elements

The choose() function and our permute() function work well when we want to choose items from a known set of n items. But what if we have a vector of items in R, and we want to calculate combinations or permutations of these items directly?

In this case, we can use the combn() function to generate all combinations of the elements in a vector. This function takes two arguments: a vector x and a number m indicating the number of items to choose. The function returns a matrix where each column is a combination of m items.

Here’s an example with our set of 5 items:

items = c("A", "B", "C", "D", "E")
m = 3
combinations = combn(items, m)
print(combinations)

To calculate permutations of the elements in a vector, we can use the permutations() function from the gtools package. This function generates all permutations of the elements in a vector.

# Install the gtools package if you haven't already
install.packages("gtools")

# Load the package
library(gtools)

items = c("A", "B", "C", "D", "E")
m = 3
permutations = permutations(length(items), m, items)
print(permutations)

Advanced Combinations and Permutations with Replacement

In all the examples so far, we’ve assumed that once an item is selected, it can’t be selected again. This is called “without replacement”. But what if we’re dealing with a situation where items can be selected more than once?

For example, let’s say we’re choosing numbers from 1 to 5, and we can choose the same number more than once. In this case, we would say we’re choosing “with replacement”.

R does not have built-in functions to handle combinations or permutations with replacement, but we can create them ourselves.

Here’s how we can calculate combinations with replacement. The formula to calculate combinations with replacement is (n + k - 1) choose k.

# Function to calculate combinations with replacement
comb_repl <- function(n, k) {
  return(choose(n + k - 1, k))
}

n = 5
k = 3
combinations = comb_repl(n, k)
print(combinations)

And here’s how we can calculate permutations with replacement. The formula to calculate permutations with replacement is simply n^k.

# Function to calculate permutations with replacement
perm_repl <- function(n, k) {
  return(n^k)
}

n = 5
k = 3
permutations = perm_repl(n, k)
print(permutations)

Conclusion

Understanding and being able to calculate combinations and permutations is an important skill for anyone working in statistics or data analysis. R provides straightforward ways to calculate these quantities, and with a little bit of additional work, you can extend these calculations to handle cases “with replacement”.

Whether you’re designing an experiment, performing a statistical test, or building a machine learning model, chances are you’ll encounter combinations and permutations at some point. Hopefully, this guide will help you handle these situations with ease in R.