Order of operations

In mathematics and computer programming, the order of operations (sometimes called operator precedence) is a rule used to clarify unambiguously which procedures should be performed first in a given mathematical expression.

For example, in mathematics and most computer languages multiplication is done before addition; in the expression 2 + 3 × 4, the answer is 14. Brackets, which have their own rules, may be used to avoid confusion, thus the preceding expression may also be rendered 2 + (3 × 4), but the brackets are not required as multiplication still has precedence without them.

From the introduction of modern algebraic notation, where juxtaposition indicates multiplication of variables, multiplication took precedence over addition, whichever side of a number it appeared on. Thus 3 + 4 × 5 = 4 × 5 + 3 = 23. When exponents were first introduced, in the 16th and 17th centuries, exponents took precedence over both addition and multiplication, and could be placed only as a superscript to the right of their base. Thus 3 + 5² = 28 and 3 × 5² = 75. To change the order of operations, originally a vinculum (an overline or underline) was used. Today we use brackets. Thus, to force addition to precede multiplication, we write (2 + 3) × 4 = 20, and to force addition to precede exponentiation, we write (3 + 5)² = 64.

The standard order of operations

The order of operations, or precedence, used in mathematics and many programming languages is expressed here:

terms inside parentheses or brackets

exponents and roots

multiplication and division As they appear left to right

addition and subtraction As they appear left to right

This means that if a mathematical expression is preceded by one operator and followed by another, the operator higher on the list should be applied first. The commutative and associative laws of addition and multiplication allow terms to be added in any order and factors to be multiplied in any order, but mixed operations must obey the standard order of operations.

It is helpful to treat division as multiplication by the reciprocal (multiplicative inverse) and subtraction as addition of the opposite (additive inverse). Thus 3/4 = 3 ÷ 4 = 3 • ¼; in other words the quotient of 3 and 4 equals the product of 3 and  ¼. Also 3 − 4 = 3 + (−4); in other words the difference of 3 and 4 equals the sum of positive three and negative four. With this understanding, we can think of 1 - 2 + 3 as the sum of 1, negative 2, and 3, and add in any order: (1 - 2) + 3 = -1 + 3 = 2 and in reverse order (3 - 2) + 1 = 1 + 1 = 2. The important thing is to keep the negative sign with the 2.

The root symbol, √, requires a symbol of grouping around the radicand. The usual symbol of grouping is a bar (called vinculum) over the radicand.

Stacked exponents are applied from the top down.

Symbols of grouping can be used to override the usual order of operations. Grouped symbols can be treated as a single expression. Symbols of grouping can be removed using the associative and distributive laws.

Examples

${\sqrt {1+3}}+5={\sqrt {4}}+5=2+5=7.\,$

A horizontal fractional line also acts as a symbol of grouping:

${\frac {1+2}{3+4}}+5={\frac {3}{7}}+5.$

For ease in reading, other grouping symbols such as braces, sometimes called curly braces { }, or brackets, sometimes called square brackets [ ], are often used along with parentheses ( ). For example,

$[(1+2)-3]-(4-5)=[3-3]-(-1)=1.\,$

Gaps in the standard

There exist differing conventions concerning the unary operator − (usually read "minus"). In written or printed mathematics, the expression −3² is interpreted to mean −(3²) = −9, but in some applications and programming languages, notably the application Microsoft Office Excel and the programming language bc, unary operators have a higher priority than binary operators, that is, the unary minus (negation) has higher precedence than exponentiation, so in those languages −3² will be interpreted as (−3)² = 9. [1]. In any case where there is a possibility that the notation might be misinterpreted, it is advisable to use brackets to clarify which interpretation is intended.

Similarly, care must be exercised when using the slash ('/') symbol. The string of characters "1/2x" is interpreted by the above conventions ^{(which one?)} as (1/2)x. The contrary interpretation should be written explicitly as 1/(2x). Again, the use of brackets will clarify the meaning and should be used if there is any chance of misinterpretation.

Mnemonics

Mnemonics are often used to help students remember the rules, but the rules taught by the use of acronyms can be misleading. In the United States the acronym PEMDAS is common. It stands for Parentheses, Exponents, Multiplication, Division, Addition, Subtraction. In other English speaking countries, Parentheses may be called Brackets, or symbols of inclusion and Exponentiation may be called either Indices, Powers or Orders, and since multiplication and division are of equal precedence, M and D are often interchanged, leading to such acronyms as BEDMAS, BIDMAS, BODMAS, BERDMAS, PERDMAS, and BPODMAS.

These mnemonics may be misleading when written this way, especially if the user is not aware that multiplication and division are of equal precedence, as are addition and subtraction. Using any of the above rules in the order "addition first, subtraction afterward" would also give the wrong answer.

$10-3+2\,$

The correct answer is 9, which is best understood by thinking of the problem as the sum of positive ten, negative three, and positive two.

$10+(-3)+2\,$

An alternative way to write the mnemonic is:

P

E

MD

AS

This makes the equivalence of multiplication and division, and addition and subtraction, clear.

Once logs are introduced the mnemonic can be written as:

P

EL

MD

AS

Special cases

An exclamation mark indicates that one should compute the factorial of the term immediately to its left, before computing any of the lower-precedence operations, unless grouping symbols dictate otherwise. But 2³! means (2³)! = 8! = 40320 while 2^3! = 2⁶ = 64; a factorial in an exponent applies to the exponent, while a factorial not in the exponent applies to the entire power.

If exponentiation is indicated by stacked symbols, the rule is to work from the top down, thus:

$a^{b^{c}}=a^{(b^{c})}\neq (a^{b})^{c}.\,$

A function name usually applies to the monomial following the name, thus "sin xy" means sin (xy) but sin x + y means (sin x) + y. Calculators usually require brackets, and brackets should be used in complicated expressions to prevent misunderstanding.

Sometimes a dash or a heavy dot is used as a multiplication sign which has higher precedence than division. For example, J/kg K, J/kg-K, and J/kg · K are all equivalent.

Calculators

Main article: Calculator input methods

Different calculators follow different orders of operations. Most non-scientific calculators without a stack work left to right without any priority given to different operators, for example giving

$1+2\times 3=9,\;$

while more sophisticated calculators will use a more standard priority, for example giving

$1+2\times 3=7.\;$

The Microsoft Calculator program uses the former in its standard view and the latter in its scientific view.

The non-scientific calculator expects two operands and an operator. When the next operator is pressed, the expression is immediately evaluated and the answer becomes the left hand of the next operator. Advanced calculators allow entry of the whole expression, grouped as necessary, and only evaluates when the user uses the equals sign.

Calculators may associate exponents to the left or to the right depending on the model. For example, the expression a ^ b ^ c on the TI-92 and TI-30XII (both Texas Instruments calculators) associates two different ways:

The TI-92 associates to the right, that is

a ^ b ^ c = a ^ (b ^ c) = $a^{(b^{c})}=a^{b^{c}}$

whereas, the TI-30XII associates to the left, that is

a ^ b ^ c = (a ^ b) ^ c = $(a^{b})^{c}.$

An expression like 1/2x is interpreted as 1/(2x) by TI-82, but as (1/2)x by TI-83. While the first interpretation may be expected by some users, only the latter is in agreement with the standard rules stated above.

Programming languages

Many programming languages use precedence levels that conform to the order commonly used in mathematics, though some, such as APL and Smalltalk, have no operator precedence rules (in APL evaluation is strictly right to left, in Smalltalk it's strictly left to right).

The logical bitwise operators in C (and all programming languages that borrowed precedence rules from C, for example, C++, Perl and PHP) have a precedence level that the creator of the C language considers to be unsatisfactory. However, many programmers have become accustomed to this order. The relative precedence levels of operators found in many C-style languages are as follows:

1	()   []   ->   .   ::	Grouping, scope, array/member access
2	!   ~   -   +   *   &   sizeof   type cast ++x   --x	(most) unary operations, sizeof and type casts
3	*   /   %	Multiplication, division, modulo
4	+   -	Addition and subtraction
5	<<   >>	Bitwise shift left and right
6	<   <=   >   >=	Comparisons: less-than, ...
7	==   !=	Comparisons: equal and not equal
8	&	Bitwise AND
9	^	Bitwise exclusive OR
10	|	Bitwise inclusive (normal) OR
11	&&	Logical AND
12	||	Logical OR
13	?:	Conditional expression (ternary operator)
14	=   +=   -=   *=   /=   %=   &=   |=   ^=   <<=   >>=	Assignment operators
15	,	Comma operator

Examples:

• !A + !B $\equiv$ (!A) + (!B)
• ++A + !B $\equiv$ (++A) + (!B)
• A * B + C $\equiv$ (A * B) + C
• A || B && C $\equiv$ A || (B && C)
• ( A && B == C ) $\equiv$ ( A && ( B == C ) )

The accuracy of software developer knowledge about binary operator precedence has been found to closely follow their frequency of occurrence in source code

See also

• Common operator notation (for a more formal description)
• Operator associativity
• Associativity
• Commutativity
• Distributivity
• Operator (programming)
• Operator overloading
• Operator precedence in C and C++
• Reverse Polish Notation

External links

• "Order of operations". PlanetMath.