Modulo operation

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"fmod" redirects here. For the computer audio library, see FMOD.

In computing, the modulo operation finds the remainder of division of one number by another.

Given two numbers, a (the dividend) and n (the divisor), a modulo n (abbreviated as a mod n) is the remainder, on division of a by n. For instance, the expression "7 mod 3" would evaluate to 1, while "9 mod 3" would evaluate to 0. Although typically performed with a and n both being integers, many computing systems allow other types of numeric operands.

See modular arithmetic for an older and related convention applied in number theory.

1 Remainder calculation for the modulo operation
2 Modulo operation expression
3 Performance issues
4 See also
5 Notes
6 References

Remainder calculation for the modulo operation

Modulo operators in various programming languages
Language	Operator	Result has the same sign as
ActionScript	`%`	Dividend
Ada	`mod`	Divisor
Ada	`rem`	Dividend
ASP	`Mod`	Not defined
C (ISO 1990)	`%`	Not defined
C (ISO 1999)	`%`	Dividend
C++	`%`	Not defined^[1]
C#	`%`	Dividend
ColdFusion	`MOD`	Dividend
Common Lisp	`mod`	Divisor
Common Lisp	`rem`	Dividend
Eiffel	`\\`	Dividend
Microsoft Excel	`=MOD()`	Divisor
Euphoria	`remainder`	Dividend
FileMaker	`Mod`	Divisor
Fortran	`mod`	Dividend
Fortran	`modulo`	Divisor
GML (Game Maker)	`mod`	Dividend
Haskell	`mod`	Divisor
Haskell	`rem`	Dividend
J	`\|~`	Divisor
Java	`%`	Dividend
JavaScript	`%`	Dividend
Lua	`%`	Divisor
MathCad	`mod(x,y)`	Divisor
Mathematica	`Mod`	Divisor
MATLAB	`mod`	Divisor
MATLAB	`rem`	Dividend
MySQL	`MOD` `%`	Dividend
Objective Caml	`mod`	Not defined
Occam	`\`	Dividend
Pascal (Delphi)	`mod`	Dividend
Perl	`%`	Divisor^[1]
PHP	`%`	Dividend
PL/I	`mod`	Divisor (ANSI PL/I)
Prolog (ISO 1995)	`mod`	Divisor
Prolog (ISO 1995)	`rem`	Dividend
Python	`%`	Divisor
QBasic	`MOD`	Dividend
R	`%%`	Divisor
RPG	`%REM`	Dividend
Ruby	`%`	Divisor
Scheme	`modulo`	Divisor
SenseTalk	`modulo`	Divisor
SenseTalk	`rem`	Dividend
Smalltalk	`\\`	Divisor
Tcl	`%`	Divisor
Verilog (2001)	`%`	Dividend
VHDL	`mod`	Divisor
VHDL	`rem`	Dividend
Visual Basic	`Mod`	Dividend

There are various ways of defining a remainder, and computers and calculators have various ways of storing and representing numbers, so what exactly constitutes the result of a modulo operation depends on the programming language and/or the underlying hardware.

In nearly all computing systems, the quotient resulting from the division is constrained to the set of integers, and the remainder r is typically constrained by either $0 \leq r < |n|$ or $-|n| < r \leq 0$ . The choice between the two possible remainders depends on the signs of a and n and the programming language being used.^[2] Some programming languages, such as C89, don't even define a result if either of n or a is negative. See the table for details.

a modulo 0 is undefined in the majority of systems, although some do define it to be a. If the definition is consistent with the division algorithm, then n = 0 implies $0 \leq r < 0$ , which is a contradiction (i.e., the usual remainder does not exist in this case).

The remainder can be calculated by using equations, in terms of other functions. Differences may arise according to the scope of the variables, which in common implementations is broader than in the definition just given. One useful equation for calculating the remainder r is

$r = a - n \left\lfloor {a \over n} \right\rfloor$

where $\lfloor x\rfloor$ is the floor function of x. See e.g. [3], [4], [5].

Raymond T. Boute^[2] analyzed several definitions of integer division and modulo, and he introduces the “Euclidean” definition. Let q be the integer quotient of a and n, then:

$q \in \mathbb{Z}$

$a = n \times q + r$

$0 \leq r < |n|.$

Two corollaries are that

$n > 0 \to q = \left\lfloor a \div n \right\rfloor$

$n < 0 \to q = \left\lceil a \div n \right\rceil.$

As described by Leijen,^[3]

Boute argues that Euclidean division is superior to the other ones in terms of regularity and useful mathematical properties, although floored division, promoted by Knuth, is also a good definition. Despite its widespread use, truncated division is shown to be inferior to the other definitions.

Modulo operation expression

Some calculators have a mod() function button, and many programming languages have a mod() function or similar, expressed as mod(a, n), for example. Some also support expressions that use "%", "mod", or "Mod" as a modulo or remainder operator, such as

a % n

a mod n.

Performance issues

Modulo operations might be implemented such that division with remainder is calculated each time. For special cases, there are faster alternatives on some hardware. For example, the modulo of powers of 2 can alternatively be expressed as a bitwise AND operation:

x % 2ⁿ == x & (2ⁿ - 1).

Examples (assuming x is an integer):

x % 2 == x & 1

x % 4 == x & 3

x % 8 == x & 7.

In devices and software that implement bitwise operations more efficiently than modulo, these alternative forms can result in faster calculations.

In the C programming language, compiling with heavy speed optimizations will typically (depending on compiler and hardware) automatically convert modulo operations to bitwise AND in the assembly file.

In some compilers, the modulo operation is implemented as mod(a, n) = a - n * floor(a / n). When performing both modulo and division on the same numbers, one can get the same result somewhat more efficiently by avoiding the actual modulo operator, and using the formula above on the result, avoiding an additional division operation.

Notes

^ Perl usually uses arithmetic modulo operator that is machine-independent. See the Perl documentation for exceptions and examples.
^ Mathematically, these two choices are but two of the infinite number of choices available for the inequality satisfied by a remainder.

References

^ ISO/IEC 14882:2003 : Programming languages -- C++, 5.6.4: ISO, IEC, 2003 . "the binary % operator yields the remainder from the division of the first expression by the second. .... If both operands are nonnegative then the remainder is nonnegative; if not, the sign of the remainder is implementation-defined".
^ Boute, Raymond T. (April 1992). "The Euclidean definition of the functions div and mod". ACM Transactions on Programming Languages and Systems (TOPLAS) 14 (2): 127 – 144. ACM Press (New York, NY, USA). doi:10.1145/128861.128862.
^ Leijen, Daan (December 3, 2001). "Division and Modulus for Computer Scientists" (PDF). Retrieved on 2006-08-27.

Retrieved from "http://en.wikipedia.org/wiki/Modulo_operation"

Categories: Computer arithmetic

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