Associative property

In standard truth-functional propositional logic, association,[4][5] or associativity[6] are two valid rules of replacement. The rules allow one to move parentheses in logical expressions in logical proofs. The rules (using logical connectives notation) are:

${\displaystyle (P\lor (Q\lor R))\Leftrightarrow ((P\lor Q)\lor R)}$

and

${\displaystyle (P\land (Q\land R))\Leftrightarrow ((P\land Q)\land R),}$

where "${\displaystyle \Leftrightarrow }$" is a metalogical symbol representing "can be replaced in a proof with."

Associativity is a property of some logical connectives of truth-functional propositional logic. The following logical equivalences demonstrate that associativity is a property of particular connectives. The following are truth-functional tautologies.[7]

Associativity of disjunction:

${\displaystyle ((P\lor Q)\lor R)\leftrightarrow (P\lor (Q\lor R))}$

${\displaystyle (P\lor (Q\lor R))\leftrightarrow ((P\lor Q)\lor R)}$

Associativity of conjunction:

${\displaystyle ((P\land Q)\land R)\leftrightarrow (P\land (Q\land R))}$

${\displaystyle (P\land (Q\land R))\leftrightarrow ((P\land Q)\land R)}$

Associativity of equivalence:

${\displaystyle ((P\leftrightarrow Q)\leftrightarrow R)\leftrightarrow (P\leftrightarrow (Q\leftrightarrow R))}$

${\displaystyle (P\leftrightarrow (Q\leftrightarrow R))\leftrightarrow ((P\leftrightarrow Q)\leftrightarrow R)}$

Joint denial is an example of a truth functional connective that is not associative.

In mathematics, addition and multiplication of real numbers is associative. By contrast, in computer science, the addition and multiplication of floating point numbers is not associative, as rounding errors are introduced when dissimilar-sized values are joined together.[8]

To illustrate this, consider a floating point representation with a 4-bit mantissa:
(1.000₂×2⁰ + 1.000₂×2⁰) + 1.000₂×2⁴ = 1.000₂×2¹ + 1.000₂×2⁴ = 1.001₂×2⁴
1.000₂×2⁰ + (1.000₂×2⁰ + 1.000₂×2⁴) = 1.000₂×2⁰ + 1.000₂×2⁴ = 1.000₂×2⁴

Even though most computers compute with a 24 or 53 bits of mantissa,[9] this is an important source of rounding error, and approaches such as the Kahan summation algorithm are ways to minimise the errors. It can be especially problematic in parallel computing.[10][11]

In general, parentheses must be used to indicate the order of evaluation if a non-associative operation appears more than once in an expression (unless the notation specifies the order in another way, like ${\displaystyle {\dfrac {2}{3/4}}}$). However, mathematicians agree on a particular order of evaluation for several common non-associative operations. This is simply a notational convention to avoid parentheses.

A left-associative operation is a non-associative operation that is conventionally evaluated from left to right, i.e.,

${\displaystyle \left.{\begin{matrix}x*y*z=(x*y)*z\qquad \qquad \quad \,\\w*x*y*z=((w*x)*y)*z\quad \\{\mbox{etc.}}\qquad \qquad \qquad \qquad \qquad \qquad \ \ \,\end{matrix}}\right\}{\mbox{for all }}w,x,y,z\in S}$

while a right-associative operation is conventionally evaluated from right to left:

${\displaystyle \left.{\begin{matrix}x*y*z=x*(y*z)\qquad \qquad \quad \,\\w*x*y*z=w*(x*(y*z))\quad \\{\mbox{etc.}}\qquad \qquad \qquad \qquad \qquad \qquad \ \ \,\end{matrix}}\right\}{\mbox{for all }}w,x,y,z\in S}$

Both left-associative and right-associative operations occur. Left-associative operations include the following:

• Subtraction and division of real numbers:[12][13][14][15][16]

${\displaystyle x-y-z=(x-y)-z}$

${\displaystyle x/y/z=(x/y)/z}$

• Function application:

${\displaystyle (f\,x\,y)=((f\,x)\,y)}$

This notation can be motivated by the currying isomorphism.

Right-associative operations include the following:

• Exponentiation of real numbers in superscript notation:

${\displaystyle x^{y^{z}}=x^{(y^{z})}}$

Exponentiation is commonly used with brackets or right-associatively because a repeated left-associative exponentiation operation is of little use. Repeated powers would mostly be rewritten with multiplication:

${\displaystyle (x^{y})^{z}=x^{(yz)}}$

Formatted correctly, the superscript inherently behaves as a set of parentheses; e.g. in the expression ${\displaystyle 2^{x+3}}$ the addition is performed before the exponentiation despite there being no explicit parentheses ${\displaystyle 2^{(x+3)}}$ wrapped around it. Thus given an expression such as ${\displaystyle x^{y^{z}}}$, the full exponent ${\displaystyle y^{z}}$ of the base ${\displaystyle x}$ is evaluated first. However, in some contexts, especially in handwriting, the difference between ${\displaystyle {x^{y}}^{z}=(x^{y})^{z}}$, ${\displaystyle x^{yz}=x^{(yz)}}$ and ${\displaystyle x^{y^{z}}=x^{(y^{z})}}$ can be hard to see. In such a case, right-associativity is usually implied.

• Function definition

${\displaystyle \mathbb {Z} \rightarrow \mathbb {Z} \rightarrow \mathbb {Z} =\mathbb {Z} \rightarrow (\mathbb {Z} \rightarrow \mathbb {Z} )}$

${\displaystyle x\mapsto y\mapsto x-y=x\mapsto (y\mapsto x-y)}$

Using right-associative notation for these operations can be motivated by the Curry–Howard correspondence and by the currying isomorphism.

Non-associative operations for which no conventional evaluation order is defined include the following.

• Exponentiation of real numbers in infix notation:[17]

${\displaystyle (x^{\wedge }y)^{\wedge }z\neq x^{\wedge }(y^{\wedge }z)}$

• Knuth's up-arrow operators:

${\displaystyle a\uparrow \uparrow (b\uparrow \uparrow c)\neq (a\uparrow \uparrow b)\uparrow \uparrow c}$

${\displaystyle a\uparrow \uparrow \uparrow (b\uparrow \uparrow \uparrow c)\neq (a\uparrow \uparrow \uparrow b)\uparrow \uparrow \uparrow c}$

usw.

• Taking the cross product of three vectors:

${\displaystyle {\vec {a}}\times ({\vec {b}}\times {\vec {c}})\neq ({\vec {a}}\times {\vec {b}})\times {\vec {c}}\qquad {\mbox{ for some }}{\vec {a}},{\vec {b}},{\vec {c}}\in \mathbb {R} ^{3}}$

• Taking the pairwise average of real numbers:

${\displaystyle {(x+y)/2+z \over 2}\neq {x+(y+z)/2 \over 2}\qquad {\mbox{for all }}x,y,z\in \mathbb {R} {\mbox{ with }}x\neq z.}$

• Taking the relative complement of sets ${\displaystyle (A\backslash B)\backslash C}$ is not the same as ${\displaystyle A\backslash (B\backslash C)}$. (Compare material nonimplication in logic.)