Arity

In logic, mathematics, and computer science, the arity (synonyms include type, adicity, and rank) of a function or operation is the number of arguments or operands that the function takes. The arity of a relation is the number of domains in the corresponding Cartesian product.

The term "arity" is primarily used with reference to operations. If f is the function f : Sⁿ → S, where S is some set, then f is an operation and n is its arity.

Arities greater than 2 are seldom encountered in mathematics, except in specialized areas, and arities greater than 3 are seldom encountered in computer science.

In linguistics, arity is sometimes called valency, not to be confused with valency in mathematics.

Examples

The term "arity" is rarely employed in everyday usage. For example, rather than saying "the arity of the addition operation is 2" or "addition is an operation of arity 2" one usually says "addition is a binary operation". In general, the naming of functions or operators with a given arity follows a convention similar to the one used for n-based numeral systems such as binary and hexadecimal. One combines a Latin prefix with the -ary ending; for example:

A nullary function takes no arguments.
A unary function takes one argument.
A binary function takes two arguments.
A ternary function takes three arguments.
An n-ary function takes n arguments.

Nullary

Sometimes it is useful to consider a constant as an operation of arity 0, and hence call it nullary.

Unary

Examples of unary operators in math and in programming include the unary minus and plus, the add-one or subtract-one operator in C-style languages (not in logical languages), and the factorial function in math. The twos complement operator and the address reference operators are examples of unary operators in math and programming.

Binary

Most operators encountered in programming are of the binary form. For both programming and math these can be the multiplication operator, the addition operator, the division operator. Logical predicates such as OR, XOR, AND, IMP are typically used as binary operators with two distinct operands.

Ternary

From C, C++, C#, Java, Perl and variants comes the ternary operator ?: , which is a so-called conditional operator, taking three parameters.

Other names

Nullary means 0-ary.
Unary means 1-ary.
Binary means 2-ary.
Ternary means 3-ary.
Quaternary means 4-ary.
Quinary means 5-ary.
Sestary means 6-ary.
Polyadic or multary (or multiary) means any number of operands (or parameters).
n-ary means n operands (or parameters), but is often used as a synonym of "polyadic".

An alternative nomenclature is derived in a similar fashion from the corresponding Greek roots; for example, medadic, monadic, dyadic, triadic, polyadic, and so on. Thence derive the alternative terms adicity and adinity for the Latin-derived arity.

These words are often used to describe anything related to that number (e.g., undenary chess is a chess variant with an 11x11 board, or the Millenary Petition of 1603).

Reference

A monograph available free online:

Burris, Stanley N., and H.P. Sankappanavar, H. P., 1981. A Course in Universal Algebra. Springer-Verlag. ISBN 3540905782. Especially pp. 22-24.

Abstract algebra | Universal algebra

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