Ruby’s Missing Data Structure

Have you ever noticed Ruby doesn’t include support for linked lists? Most computer science textbooks are filled with algorithms, examples and exercises based on linked lists: inserting or removing elements, sorting lists, reversing lists, etc. Strangely, however, there is no linked list object in Ruby.

Recently after studying Haskell and Lisp for a couple of months, I returned to Ruby and tried to use some of the functional programming ideas I had learned about. But how do I create a list in Ruby? How do I add or remove an element from a list in Ruby? Ruby contains fast, internal C implementations of the Array and Hash classes, and in the standard library you can find Ruby code implementing the Set, Matrix, and Vector classes among many other things. But no linked lists – why?

The answer is simple: Matz included features you would normally associate with linked lists in the Ruby Array class. For example, you can use Array#push and Array#unshift to add an element to an array, or Array#pop and Array#shift to remove an element from an array.

Today I’m going to look at a few common operations you would normally use a linked list for, and then take a look at how you would implement them using an array. We’ll also get some insight into how Ruby’s array object works internally.

Using an Array as a Linked List

When I think of linked lists, I think of the Lisp programming language, short for “List Processor.” In his classic article, Recursive Functions of Symbolic Expressions and Their Computation by Machine, Part I, Lisp inventor John McCarthy defined the “cons cell” as the basic building block of linked lists, and a corresponding “(cons …)” Lisp function to create these cons cells.

For example, to create a list (2, 3) in Lisp you would call:

Internally Lisp would create a linked list that looks like this:

In Ruby, since we don’t have a “List” object, we can achieve the same thing using an array and the Array#unshift method like this:

Internally, Ruby will create an array like this:

Superficially, it appears that the unshift method on the Array class is equivalent to the cons function in Lisp. Both of them add a new element to the beginning or head of a list.

Adding New Elements to a List – Shifting Array Elements

However, arrays and lists are very different things. Arrays are designed for random access of data across a large set, while linked lists instead work well inserting, removing or reordering elements.

Let’s imagine that I want to add a third value to my list – in Lisp I just call “cons” again:

Internally, Lisp just creates a new “cons cell” and sets the pointer to point to the original list:

In Ruby I would just call unshift again:

But remember, in an array there are no pointers and Ruby instead needs to shift all of the existing elements over one position – hence the method name unshift:

This means that using unshift means you pay a performance penalty every time you add a new element to the head of a list. As you might guess, adding elements to the end of the list is cheaper, since no shifting is necessary – a better way to create a list in Ruby would be to use the push or << method instead:

Adding new elements to the middle of a list works largely the same way as adding a new element to the head of the list. If you call the Array#insert method, Ruby first needs to move the remaining elements to the right to make space for the new value.

Adding More Elements to a List – Resizing Array Capacity

Now suppose I add a fourth element. In Lisp, I could continue to just call the “cons” function, adding more and more elements in precisely the same way. In Ruby, however, notice that my array has filled up and there’s no room for a fourth element:

It turns out that I didn’t choose to show three boxes in my imaginary array diagram just to make the diagram simpler. Internally, Ruby keeps track of two separate values in each array: “size” and “capacity:”

The size indicates the actual size of the array. As I repeatedly called unshift in my example above, the size of the array increased from 1 to 2 to 3.

When Ruby creates an array internally it allocates some extra memory since you are likely to add more elements to the array soon. Whenever you create a new array object, Ruby initially sets the capacity to 3; this means that it can save three values into the array without having to ask for more space. Once you add a fourth element, however, Ruby has to stop and get some new memory from the garbage collection system before saving the fourth value. This might trigger a GC operation if no memory is available.

As you add more and more elements, Ruby repeatedly increases the size of the array’s capacity in chunks whenever you cross certain array sizes: 3, 20, 37, 56, 85 elements, etc. The exact thresholds are determined by the ary_double_capa C function inside of array.c. More or less the array capacity increases by 50% each time, although the first increase, from 3 to 20, is a special case. You pay an extra performance penalty each time this happens.

Whenever you remove an element from an array, using shift or pop for example, Ruby may also reduce the capacity of the array along with the size. This means it might stop and free unused memory from the array back to the GC system, causing an additional delay.

The Linked List as a Functional Data Structure

So far, we’ve seen that Ruby arrays can function as linked lists. Ruby provides the push, unshift and insert methods to add a new element to a list, and the pop or shift methods to remove an element from a list. Not only that, with an array you also get direct, random access to all of the elements of the array, which a pure linked list structure can’t offer.

The only downside with using an array as a linked list seems to be performance: if you had many 1000s of elements Ruby would theoretically slow down repeatedly resizing the array to accommodate all of the elements as you added them.

Also, if you needed to add or remove elements to the head of the list or somewhere in the middle, then Ruby would also slow down shifting the remaining elements to the right or left. But remember, since Ruby is implemented with C internally, it is able to use very efficient C functions to move the elements around. Memmove and similar low level C functions are often implemented directly by the microprocessor hardware and work very, very quickly.

However, there’s another important feature of linked lists which isn’t immediately apparent: they work nicely as functional data structures. What does this mean? In functional programming language such as Lisp, you specify your program as a series of “pure functions;” these are bits of code that always return the same output given the same input.

Functional programming languages also discourage the use of “mutable values” – variables or objects that can change from one value to another in place. The primary benefit of using pure functions and immutable values is to allow your software to run more reliably and robustly since your code will have no side effects. Functional languages also work well in multithreaded environments since there’s no need to synchronize or lock code that modifies data – you just don’t modify data at all.

Data structures that exclusively use immutable values are known as “functional data structures.” Aside from the large benefits of robustness and removing the need to synchronize access different threads, functional data structures also allow for data to be easily shared. Since you never modify data in place, it’s easy for many different functional data structures to use the same copy of data internally.

The linked list is a great example of this: different lists can share the same “cons” cells or data values internally. For example, suppose in Lisp I create one list:

And then create a second list by adding another element to the first list:

Now I have two linked lists, which share the same cons cells for 2 and 3. As long as you never actually change one of the elements in place, it’s no problem at all for the elements of different linked lists to be shared!

Functional data structures allow you – or language implementors at least – to apply optimizations to share and save memory.

Copy on Write Optimization in Ruby Arrays

At first glance, it seems that using arrays as lists in Ruby makes space optimization impossible. In the first place, the array in Ruby is not a functional data structure – you can, of course, change any value in an array whenever you want. But let’s suppose, for a moment, that Ruby arrays were immutable – or that there was an “ImmutableArray” object in Ruby. How could two separate arrays even share the same data? For example, suppose I created an array with six elements:

… and a second array as a subset:

In order to use space optimizations similar to Lisp and other functional languages, we would have to imagine that Ruby could share the same copy of the common elements in both arrays:

As a matter of fact, it turns out that Ruby does exactly that! Using a series of tricks known as “copy on write optimization,” Ruby shares values across different arrays! Ruby does this by maintaining a single copy of the larger array with all the data values, and creating the second array using a simple pointer or reference to the middle of larger array as shown above.

This is called a “copy on write” optimization because Ruby has to break down the sharing and copy the data elements over when you change or write to one of the values in the shared array.

Practically speaking, you don’t need to worry about this: Ruby will automatically handle everything for you internally, saving you the memory when it can, and paying the price to copy around the data when it must.

However, one simple case related to linked lists that benefits from copy on write optimization is using shift repeatedly to iterate through a list:

As I explained above, theoretically Ruby needs to shift or copy all of the existing elements over to the left whenever you remove an element from the head of a list. For a small array like this one that won’t matter. But if the array contained 1000s of elements, it’s possible this could be a slow operation.

But because of Ruby’s copy on write optimization inside of the Array object, Ruby is smart enough to preserve the original array and to simply move a pointer or reference around as you call shift:

Why did I bother to draw an array with 16 elements in these diagrams? It turns out that when you call shift Ruby doesn’t use this optimization until there are at least 16 elements. If you shift a value off of an array with 15 or fewer elements, then Ruby will just move all the elements to the left, since memmove is so fast with a small array.

Implementing Your Own Linked List

To wrap things up today, let’s take a look at how you would actually implement a linked list in Ruby. As we’ve seen, arrays can often serve quite well as lists. Sometimes, however, you may actually need to repeatedly insert, remove and reorder elements in a large list. To avoid the performance penalties associated with array resizing and shifting, a real linked list might be what you need.

One simple way to create linked list would be to use the Struct object to represent a cons cell, like this:

See this interesting 2012 article for more details on this approach. The author also shows some benchmarks comparing arrays and lists.

Another approach would be to create separate Entry and List objects, as described by Eno Compton his excellent article: Reversing a Linked List in Ruby. Here’s the code:

Finally, if you’re interested in learning more about functional data structures and trying them out in Ruby, see Simon Harris’s Hamster library of immutable data structures implemented in Ruby. Simon also wrote this excellent article in 2010 about how functional programming has caused him to rethink how he writes OO classes: Functional programming in object oriented languages.

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  • http://freeldssheetmusic.org roger

    Hmm…I wouldn’t trust an array as a linked list since insertion in the middle might be expensive, I presume :)

  • Sam

    @roger did you even read the whole article? Ruby’s array insertion is quick because of how array’s are optimized once they get to 16 elements or more. The only real hit is taken when array’s need to be re-sized and memory allocated.

  • doel

    great article Pat… using unshift/shift push/pop/insert in Ruby-Array to be used as a linked list is a great learning :)

    • http://patshaughnessy.net Pat Shaughnessy

      Thanks Doel :)

  • Justin Hellings

    Interesting and very clear. I really enjoyed it. Thanks.

    I think you meant “shift” when you said “push” in the following para…

    Why did I bother to draw an array with 16 elements in these diagrams? It turns out that when you call push Ruby doesn’t use this optimization until there are at least 16 elements. If you push a value off of an array with 15 or fewer elements, then Ruby will just shift all the elements to the left, since memmove is so fast with a small array.

    • http://patshaughnessy.net Pat Shaughnessy

      Yes – good catch. Thanks! I’ll update the text.