In logic, mathematics, and computer science, the arity // (listen) of a function or operation is the number of arguments or operands that the function takes. The arity of a relation (or predicate) is the dimension of the domain in the corresponding Cartesian product. (A function of arity n thus has arity n+1 considered as a relation.) The term springs from words like unary, binary, ternary, etc. Unary functions or predicates may be also called "monadic"; similarly, binary functions may be called "dyadic".
In mathematics, arity may also be named rank, but this word can have many other meanings in mathematics. In logic and philosophy, arity is also called adicity and degree. In linguistics, arity is usually named valency.
In computer programming, there is often a syntactical distinction between operators and functions; syntactical operators usually have arity 0, 1, or 2 (the ternary operator ?: is also common). Functions vary widely in the number of arguments, though large numbers can become unwieldy. Some programming languages also offer support for variadic functions, i.e., functions syntactically accepting a variable number of arguments.
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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.
Sometimes it is useful to consider a constant to be an operation of arity 0, and hence call it nullary.
Also, in non-functional programming, a function without arguments can be meaningful and not necessarily constant (due to side effects). Often, such functions have in fact some hidden input which might be global variables, including the whole state of the system (time, free memory, ...). The latter are important examples which usually also exist in "purely" functional programming languages.
Examples of unary operators in mathematics and in programming include the unary minus and plus, the increment and decrement operators in C-style languages (not in logical languages), and the successor, factorial, reciprocal, floor, ceiling, fractional part, sign, absolute value, square root (the principal square root), complex conjugate (unary of "one" complex number, that however has two parts at a lower level of abstraction), and norm functions in mathematics. The two's complement, address reference and the logical NOT operators are examples of unary operators in math and programming.
Most operators encountered in programming and mathematics are of the binary form. For both programming and mathematics these can be the multiplication operator, the radix operator, the often omitted exponentiation operator, the logarithm 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. In CISC architectures, it's common to have two source operands (and store result in one of them).
The computer programming language C and its various descendants (including C++, C#, Java, Julia, Perl, and others) provides the ternary operator
?:, also known as the conditional operator, taking three operands. The first operand (the condition) is evaluated, and if it is true, the result of the entire expression is the value of the second operand, otherwise it is the value of the third operand. The Forth language also contains a ternary operator,
*/, which multiplies the first two (one-cell) numbers, dividing by the third, with the intermediate result being a double cell number. This is used when the intermediate result would overflow a single cell. The Python language has a ternary conditional expression,
x if C else y. The Unix dc calculator has several ternary operators, such as
|, which will pop three values from the stack and efficiently compute with arbitrary precision. Additionally, many (RISC) assembly language instructions are ternary (as opposed to only two operands specified in CISC); or higher, such as
MOV %AX, (%BX, %CX), which will load (MOV) into register AX the contents of a calculated memory location that is the sum (parenthesis) of the registers BX and CX.
From a mathematical point of view, a function of n arguments can always be considered as a function of one single argument which is an element of some product space. However, it may be convenient for notation to consider n-ary functions, as for example multilinear maps (which are not linear maps on the product space, if n≠1).
The same is true for programming languages, where functions taking several arguments could always be defined as functions taking a single argument of some composite type such as a tuple, or in languages with higher-order functions, by currying.
In computer science, a function accepting a variable number of arguments is called variadic. In logic and philosophy, predicates or relations accepting a variable number of arguments are called multigrade, anadic, or variably polyadic.
There are Latinate names for specific arities, primarily based on Latin distributive numbers meaning "in group of n", though some are based on cardinal numbers or ordinal numbers. Only binary and ternary are both commonly used and derived from distributive numbers.
- Nullary means 0-ary (from nūllus).
- Unary means 1-ary (from cardinal unus, rather than singulary from distributive singulī).
- Binary means 2-ary.
- Ternary means 3-ary.
- Quaternary means 4-ary.
- Quinary means 5-ary.
- Senary means 6-ary.
- Septenary means 7-ary.
- Octonary means 8-ary (alternatively octary).
- Novenary means 9-ary (alternatively nonary, from ordinal).
- Denary means 10-ary (alternatively decenary)
- Polyadic, multary and multiary mean 2 or more 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, niladic (or medadic), monadic, dyadic, triadic, polyadic, and so on. Thence derive the alternative terms adicity and adinity for the Latin-derived arity.
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