How to Count Specific Items in a Collection with Elixir

So, I was working on this exercise over at Exercism.io, when I stumbled up an interesting function.

To make a long story short, I needed a count of a specific character (codepoint) in a character list i.e. 'hello' contains two ls. However, this solution could apply to any collection which implements the Enumerable protocol in Elixir.

I have been reading Programming in Elixir. The author, Dave Thomas, emphasizes the use of recursion and pattern matching throughout Elixir. So, of course, I had implemented the solution using recursion and pattern matching. You can see that solution here.

You Can Count (Ha!) on Elixir

Of course, I missed the obvious. Elixir already had a very convenient function for doing exactly this: Enum.count/2.

I had anticipated that Elixir would have a function to do a straight count of all the items in a collection (which it does: Enum.count/1 doc). However, I had not considered one for looking for a specific kind of item.

Well, Enum.count/2 provides a convenient API for this exact problem. You simply pass the enumerable (in my case, the character list) and a function which takes one argument (the current item) and will return true for whatever items you want to count.

Here’s a convenient graphic to demonstrate:

Elixir Enum.count/2 Examples

Of course, the expression used to evaluate the current item does not need to be a simple equality operation. Here are some examples using a variety of different operators:

Count all the binaries (strings) which equal “price”:

Enum.count(["Total", "retail", "price", "$14.95"], &(&1 == "price"))
1

Pipe syntax:

["Total", "retail", "price", "$14.95"]
  |> ... # some other functions
  |> Enum.count(&(&1 == "price"))

Count all the binaries which are members of a list:

roles = ["User", "Customer", "Partner", "Admin"]
Enum.count(roles, &Enum.member?(["Admin", "Editor", "Developer"], &1))
1

Count all the binaries (strings) which match a regular expression:

Enum.count(["Total", "price", "$14.95"], &(&1 =~ ~r/\$\d+\.\d+/))
1

Count all the binaries (strings) in a list:

Enum.count([1, "list", 3], &is_binary(&1))
1

Count all the binaries (strings) in a multi-type list:

Enum.count([1, "list", 3], &is_binary(&1))
1

Count all the integers in a multi-type list:

Enum.count([1, "list", 3], &is_integer(&1))
2

Count all the integers greater than 3:

Enum.count([1,2,4,8,16,32], &(&1 > 3))
4

Read the documentation on Enum.count/2

This post is part of my series of Elixir Tutorials

Hey, I’m Adam. I’m guessing you just read this post from somewhere on the interwebs. Hope you enjoyed it.

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