What term describes a list of exactly length 1?

Question

A container having 2 or more elements could be called a "plural container". A container with no elements could be called an "empty container".

But what is the terminology for a container whose current size is exactly 1 element?

I would use the term "unary" but that refers to operators that take exactly 1 argument.

The reason I would like a term for this is that I am writing a boolean method to check if two data structures are both of size exactly one and I want a good name for the method. Currently I have:

def length_is_1(self, x, y):
    return len(x) == 1 and len(y) == 1

Answer 1

Do not come up with a new word. Your name is perfectly fine: It is unambiguous and specific and consequently leaves no doubt to the reader what you are talking about.

By contrast, singleton, unary, 1-tuple or any other term borrowed from mathematics or software engineering carries with it a baggage of preconceptions which are confusing. A list with a single element in it is emphatically not a singleton. It has nothing to do with unary operators, and a list is clearly not a tuple in C++. It is a list with one element in it, not more, not less.

That is sometimes a perceived downside of a simple approach in programming: It seems unsophisticated, and hence of a lesser value. Look at Duff's device! Marvel at the ingenuity of boost.lambda! But then listen to Jim Radigan, who leads the VC++ compiler team:

One of the other things that happen when we go to check code into the compiler is we do peer code review. So if you survive that, it’s probably ok, it’s not too complex. But if you try to check in meta-programming constructs with 4-5 different include files and virtual methods that wind up taking you places you can’t see unless you’re in a debugger – no one is going to let you check that in.

That your peer is able to understand your code right away because it is plain and simple and calls things what they are is not a sign that you didn't realize your full programming potential. It is a sign of excellence. Do not look for a Latin word.

Answer 2

There is as far as I know, no widely accepted term in SW engineering to described a list with exactly one element

In mathematics, and more precisely in the set theory, a set with only one element is called a singleton. Unfortunately, in SW engineering, the term is so heavily associated with a design pattern, that using it for another purpose might create a confusion.

Anyway, a list is ordered, so a set is not a list. An ordered list of items is called a sequence in mathematics. And a sequence with exactly one element is called a 1-tuple. Unfortunately, a tuple corresponds to specific data structures, so this term, even converted to letters-only, would also be very ambiguous.

Finally, if mathematics don’t help, let’s look at literature! The contrary of plural is singular. According to the Meriam-Webster dictionary, it means “of, or relating to, one person, thing or instance”. So a singular list should express exactly what you want. This term is even reinforced by its ambiguity: “singular” is associated in other contexts with strangeness, and a list with only one element is indeed a strange list (as πάντα ῥεῖ already pointed out in the comments. But I’d nevertheless prefer to explain it a comment at first use ;-)

Answer 3

The term you are looking for is singleton.

In mathematics, a set of cardinality 1 is called a singleton set or just singleton. A Tuple of length one is sometimes called a singleton as well.

By analogy, collections of size 1 are typically also called singleton collections in programming, see for example the java.util.Collections.singleton, java.util.Collections.singletonList, and java.util.Collections.singletonMap methods added in Java 1.3 or this GitHub discussion titled Singleton list types (single item list), or this Python tutorial: "A list with a single object is sometimes referred to as a singleton list".

The Software Design Pattern that ensures that a class can only be instantiated once is called the Singleton Design Pattern, [a type that can only be inhabited by one value is called a singleton type.

So, a good name for your method would be is_singleton_list, or, since it is an instance method of the collection, just is_singleton.

Answer 4

There is no standard terminology for this. Even 'plural container' doesn't see any use; if I'd encounter that term in the wild, I would assume it's a group of plural words (["tests", "programs"]). Therefore, even if you would find a fitting term (e.g. 'trivial'), you should not use it to name methods if you want other people to understand from the name alone what it does.

The current name is reasonably clear, but perhaps you need a more generic method. I'm not sure about Python naming conventions, but I'm used to start methods with a verb:

def check_length(self, x, y, length):
    return len(x) == length and len(y) == length

Answer 5

Plural container is wrong to begin with. There is nothing plural about the container, it will remain to be one container no matter how many elements it contains.

Your method could be named GetNaiveCount(). The result type could be an enum NaiveCount: None, One, Many. I would make it a property NaiveCount though if the language supports that.

Answer 6

in set theory this can be called a singleton or a unit set, see also https://en.wikipedia.org/wiki/Singleton_(mathematics)

Answer 7

From a practical programmer's point of view, it is just a list. It has the interface and the behaviour of a list. It's type is list. The fact that it currently holds only one element doesn't change that, and that state may change soon anyhow.

Personally I will instantly understand if you tell me you have "a list of one (element)", but I will ask you to clarify if you use "singleton list" or one of the other special terms presented here, because most of them are already (over)loaded with a certain meaning. It may make sense within a special domain to define a term for it, to differentiate between several specializations that need to be treated differently and make communication less ambiguous, but then your peers will hopefully understand the subtleties (if not it's your issue to introduce it to them).

But don't make things more complex than they need to be.

Answer 8

In mathematics, and more precisely in the set theory, a set with only one element is called a singleton. Since lists are sets, then a list with one element can be called a singleton list.

Yes, lists are sets... or at least they can be encoded in set theory:

s is a list of type X iff there exists n in Nat such that s:[1,n] --> X

Then, s is a function and so is a set of ordered pairs. In this case the ordered pairs in s are of the form (k,x) where k in [1,n] and x in X.

In this way, the list [hello,world] is encoded as the set {(1,hellow),(2,world)}. And the singleton list [hello] becomes the singleton set {(1,hello)}. Clearly, the empty list [] is the empty set {}.

The first time I saw this encoding was in the Z formal notation (for formal specification) designed by Jean-Raymond Abrial.

Answer 9

The distinction is only meaningful in languages with Run-Time Type Information, or can have types implicitly contain other types, such as any or Object. At runtime in a function whose type is already decided, a 1-tuple of an int and an int don't have type tags, the set of operations applicable to each is identical.

A list is usually a container of dynamic length, so its size is only meaningful if you use dependent typing or contract programming, because the length is dynamic and not truly a property of the type in the mathematical sense.

The tuple syntax (1, 2) in Haskell is of type (Int, Int)

(1) is the same thing (Int) which is the same type as Int. In other words, the type of a 1-tuple of a type is the same type as that type. If you think about it, this makes sense in a language where variables can only be of one type: The same set of operations apply to a 1-tuple of a thing as to a thing itself, because the type is predetermined outside the function call.

Answer 10

If it can hold zero or one element, a common name is optional, for example C++ has std::optional.