In logic, mathematics, and computer science, arity (/ˈærɪti/ ) is the number of arguments or operands taken by a function, operation or relation. In mathematics, arity may also be called rank,^{[1]}^{[2]} but this word can have many other meanings. In logic and philosophy, arity may also be called adicity and degree.^{[3]}^{[4]} In linguistics, it is usually named valency.^{[5]}
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Function Arity

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Arity Meaning

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Logical Equivalences Involving Predicates & Quantifiers (Part 1)
Transcription
Examples
In general, functions or operators with a given arity follow the naming conventions of nbased numeral systems, such as binary and hexadecimal. A Latin prefix is combined with the ary suffix. For example:
 A nullary function takes no arguments.
 Example:
 A unary function takes one argument.
 Example:
 A binary function takes two arguments.
 Example:
 A ternary function takes three arguments.
 Example:
 An nary function takes n arguments.
 Example:
Nullary
A constant can be treated as the output of an operation of arity 0, called a nullary operation.
Also, outside of functional programming, a function without arguments can be meaningful and not necessarily constant (due to side effects). Such functions may have some hidden input, such as global variables or the whole state of the system (time, free memory, etc.).
Unary
Examples of unary operators in mathematics and in programming include the unary minus and plus, the increment and decrement operators in Cstyle 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. In programming the two's complement, address reference, and the logical NOT operators are examples of unary operators.
All functions in lambda calculus and in some functional programming languages (especially those descended from ML) are technically unary, but see nary below.
According to Quine, the Latin distributives being singuli, bini, terni, and so forth, the term "singulary" is the correct adjective, rather than "unary".^{[6]} Abraham Robinson follows Quine's usage.^{[7]}
In philosophy, the adjective monadic is sometimes used to describe a oneplace relation such as 'is squareshaped' as opposed to a twoplace relation such as 'is the sister of'.
Binary
Most operators encountered in programming and mathematics are of the binary form. For both programming and mathematics, these include the multiplication operator, the radix operator, the often omitted exponentiation operator, the logarithm operator, the addition operator, and 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 is common to have two source operands (and store result in one of them).
Ternary
The computer programming language C and its various descendants (including C++, C#, Java, Julia, Perl, and others) provide the ternary conditional operator ?:
. 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 Python language has a ternary conditional expression, x if C else y
. In Elixir the equivalent would be, if(C, do: x, else: y)
.
The Forth language also contains a ternary operator, */
, which multiplies the first two (onecell) 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 Unix dc calculator has several ternary operators, such as 
, which will pop three values from the stack and efficiently compute with arbitrary precision.
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.
nary
The arithmetic mean of n real numbers is an nary function:
Similarly, the geometric mean of n positive real numbers is an nary function: Note that a logarithm of the geometric mean is the arithmetic mean of the logarithms of its n arguments
From a mathematical point of view, a function of n arguments can always be considered as a function of a single argument that is an element of some product space. However, it may be convenient for notation to consider nary 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 higherorder functions, by currying.
Varying arity
In computer science, a function that accepts 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.^{[8]}
Terminology
Latinate names are commonly used for specific arities, primarily based on Latin distributive numbers meaning "in group of n", though some are based on Latin cardinal numbers or ordinal numbers. For example, 1ary is based on cardinal unus, rather than from distributive singulī that would result in singulary.
nary  Arity (Latin based)  Adicity (Greek based)  Example in mathematics  Example in computer science 

0ary  nullary (from nūllus)  niladic  a constant  a function without arguments, True, False 
1ary  unary  monadic  additive inverse  logical NOT operator 
2ary  binary  dyadic  addition  logical OR, XOR, AND operators 
3ary  ternary  triadic  triple product of vectors  conditional operator 
4ary  quaternary  tetradic  
5ary  quinary  pentadic  
6ary  senary  hexadic  
7ary  septenary  hebdomadic  
8ary  octonary  ogdoadic  
9ary  novenary (alt. nonary)  enneadic  
10ary  denary (alt. decenary)  decadic  
more than 2ary  multary and multiary  polyadic  
varying  variadic  sum; e.g., Σ  variadic function, reduce 
nary means having n operands (or parameters), but is often used as a synonym of "polyadic".
These words are often used to describe anything related to that number (e.g., undenary chess is a chess variant with an 11×11 board, or the Millenary Petition of 1603).
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.)
In computer programming, there is often a syntactical distinction between operators and functions; syntactical operators usually have arity 1, 2, or 3 (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.
See also
 Logic of relatives
 Binary relation
 Ternary relation
 Theory of relations
 Signature (logic)
 Parameter
 padic number
 Cardinality
 Valency (linguistics)
 nary code
 nary group
 Function prototype – Declaration of a function's name and type signature but not body
 Type signature – Defines the inputs and outputs for a function, subroutine or method
 Univariate and multivariate
 Finitary
References
 ^ Hazewinkel, Michiel (2001). Encyclopaedia of Mathematics, Supplement III. Springer. p. 3. ISBN 9781402001987.
 ^ Schechter, Eric (1997). Handbook of Analysis and Its Foundations. Academic Press. p. 356. ISBN 9780126227604.
 ^ Detlefsen, Michael; McCarty, David Charles; Bacon, John B. (1999). Logic from A to Z. Routledge. p. 7. ISBN 9780415213752.
 ^ Cocchiarella, Nino B.; Freund, Max A. (2008). Modal Logic: An Introduction to its Syntax and Semantics. Oxford University Press. p. 121. ISBN 9780195366587.
 ^ Crystal, David (2008). Dictionary of Linguistics and Phonetics (6th ed.). John Wiley & Sons. p. 507. ISBN 9781405152969.
 ^ Quine, W. V. O. (1940), Mathematical logic, Cambridge, Massachusetts: Harvard University Press, p. 13
 ^ Robinson, Abraham (1966), Nonstandard Analysis, Amsterdam: NorthHolland, p. 19
 ^ Oliver, Alex (2004). "Multigrade Predicates". Mind. 113 (452): 609–681. doi:10.1093/mind/113.452.609.
External links
A monograph available free online:
 Burris, Stanley N., and H.P. Sankappanavar, H. P., 1981. A Course in Universal Algebra. SpringerVerlag. ISBN 3540905782. Especially pp. 22–24.