Modulo facts for kids

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This page is about the binary operation mod( $a,n$ ). For the (mod $n$ ) notation, see Modular arithmetic. For other uses, see Modulo (disambiguation).

In computing, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another (called the modulus of the operation).

Given two positive numbers $a$ and $n$ , $a$ modulo $n$ (often abbreviated as $a mod n$ ) is the remainder of the Euclidean division of $a$ by $n$ , where $a$ is the dividend and $n$ is the divisor.

For example, the expression "5 mod 2" would evaluate to 1, because 5 divided by 2 has a quotient of 2 and a remainder of 1, while "9 mod 3" would evaluate to 0, because 9 divided by 3 has a quotient of 3 and a remainder of 0; there is nothing to subtract from 9 after multiplying 3 times 3.

Although typically performed with $a$ and $n$ both being integers, many computing systems now allow other types of numeric operands. The range of values for an integer modulo operation of $n$ is 0 to $n - 1$ inclusive ( $a$ mod 1 is always 0; $a mod 0$ is undefined, possibly resulting in a division by zero error in some programming languages). See Modular arithmetic for an older and related convention applied in number theory.

When exactly one of $a$ or $n$ is negative, the naive definition breaks down, and programming languages differ in how these values are defined.

Variants of the definition
Notation
Common pitfalls
Performance issues
Properties (identities)
In programming languages
Generalizations
- Modulo with offset
- Implementing other modulo definitions using truncation
See also

Variants of the definition

In mathematics, the result of the modulo operation is an equivalence class, and any member of the class may be chosen as representative; however, the usual representative is the least positive residue, the smallest non-negative integer that belongs to that class (i.e., the remainder of the Euclidean division). However, other conventions are possible. Computers and calculators have various ways of storing and representing numbers; thus their definition of the modulo operation depends on the programming language or the underlying hardware.

In nearly all computing systems, the quotient $q$ and the remainder $r$ of $a$ divided by $n$ satisfy the following conditions:

Template:Repeat/0

$\begin{align} &q, r \in \mathbb{Z} \\ &a = n q + r \\ &|r| < |n| \end{align}$

(1)

Template:Repeat/0

However, this still leaves a sign ambiguity if the remainder is non-zero: two possible choices for the remainder occur, one negative and the other positive, and two possible choices for the quotient occur. In number theory, the positive remainder is always chosen, but in computing, programming languages choose depending on the language and the signs of $a$ or $n$ . Standard Pascal and ALGOL 68, for example, give a positive remainder (or 0) even for negative divisors, and some programming languages, such as C90, leave it to the implementation when either of $n$ or $a$ is negative (see the table under § In programming languages for details). $a$ modulo 0 is undefined in most systems, although some do define it as $a$ .

Quotient ( $q$ ) and remainder ( $r$ ) as functions of dividend ( $a$ ), using truncated division

Many implementations use truncated division, for which the quotient is defined by

$q = \left[\frac{a}{n}\right]$

where [] is the integral part function (rounding toward zero), i.e. the truncation to zero significant digits. Thus according to equation (1), the remainder has the same sign as the dividend:

$r = a - n \left[\frac{a}{n}\right]$
Quotient and remainder using floored division

Donald Knuth promotes floored division, for which the quotient is defined by

$q = \left\lfloor\frac{a}{n}\right\rfloor$

where ⌊⌋ is the floor function (rounding down). Thus according to equation (1), the remainder has the same sign as the divisor:

$r = a - n \left\lfloor\frac{a}{n}\right\rfloor$
Quotient and remainder using Euclidean division

Raymond T. Boute promotes Euclidean division, for which the quotient is defined by

$q = \sgn(n) \left\lfloor\frac{a}{\left|n\right|}\right\rfloor = \begin{cases} \left\lfloor\frac{a}{n}\right\rfloor & \text{if } n > 0 \\ \left\lceil\frac{a}{n}\right\rceil & \text{if } n < 0 \\ \end{cases}$

where $sgn$ is the sign function, ⌊⌋ is the floor function (rounding down), and ⌈⌉ is the ceiling function (rounding up). Thus according to equation (1), the remainder is non negative:

$r = a - |n| \left\lfloor\frac{a}{\left|n\right|}\right\rfloor$
Quotient and remainder using rounded division

Common Lisp and IEEE 754 use rounded division, for which the quotient is defined by

$q = \operatorname{round}\left(\frac{a}{n}\right)$

where $round$ is the round function (rounding half to even). Thus according to equation (1), the remainder falls between $-\frac{n}{2}$ and $\frac{n}{2}$ , and its sign depends on which side of zero it falls to be within these boundaries:

$r = a - n \operatorname{round}\left(\frac{a}{n}\right)$
Quotient and remainder using ceiling division

Common Lisp also uses ceiling division, for which the quotient is defined by

$q = \left\lceil\frac{a}{n}\right\rceil$

where ⌈⌉ is the ceiling function (rounding up). Thus according to equation (1), the remainder has the opposite sign of that of the divisor:

$r = a - n \left\lceil\frac{a}{n}\right\rceil$

As described by Leijen,

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.

—Daan Leijen, Division and Modulus for Computer Scientists

However, truncated division satisfies the identity $({-a})/b = {-(a/b)} = a/({-b})$ .

Notation

This page is about the binary mod operation. For the (mod m) notation, see congruence relation.

Some calculators have a $mod()$ function button, and many programming languages have a similar function, 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 or a mod n.

For environments lacking a similar function, any of the three definitions above can be used.

Common pitfalls

When the result of a modulo operation has the sign of the dividend (truncated definition), it can lead to surprising mistakes.

For example, to test if an integer is odd, one might be inclined to test if the remainder by 2 is equal to 1:

bool is_odd(int n) {
    return n % 2 == 1;
}

But in a language where modulo has the sign of the dividend, that is incorrect, because when $n$ (the dividend) is negative and odd, $n$ mod 2 returns −1, and the function returns false.

One correct alternative is to test that the remainder is not 0 (because remainder 0 is the same regardless of the signs):

bool is_odd(int n) {
    return n % 2 != 0;
}

Another alternative is to use the fact that for any odd number, the remainder may be either 1 or −1:

bool is_odd(int n) {
    return n % 2 == 1 || n % 2 == -1;
}

A simpler alternative is to treat the result of n % 2 as if it is a boolean value, where any non-zero value is true:

bool is_odd(int n) {
    return n % 2;
}

Performance issues

Modulo operations might be implemented such that a division with a remainder is calculated each time. For special cases, on some hardware, faster alternatives exist. For example, the modulo of powers of 2 can alternatively be expressed as a bitwise AND operation (assuming $x$ is a positive integer, or using a non-truncating definition):

x % 2ⁿ == x & (2ⁿ - 1)

Examples:

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.

Compiler optimizations may recognize expressions of the form expression % constant where constant is a power of two and automatically implement them as expression & (constant-1), allowing the programmer to write clearer code without compromising performance. This simple optimization is not possible for languages in which the result of the modulo operation has the sign of the dividend (including C), unless the dividend is of an unsigned integer type. This is because, if the dividend is negative, the modulo will be negative, whereas expression & (constant-1) will always be positive. For these languages, the equivalence x % 2ⁿ == x < 0 ? x | ~(2ⁿ - 1) : x & (2ⁿ - 1) has to be used instead, expressed using bitwise OR, NOT and AND operations.

Optimizations for general constant-modulus operations also exist by calculating the division first using the constant-divisor optimization.

Properties (identities)

Some modulo operations can be factored or expanded similarly to other mathematical operations. This may be useful in cryptography proofs, such as the Diffie–Hellman key exchange. Some of these properties require that $a$ and $n$ are integers.

Identity:
- $(a mod n) mod n = a mod n$ .
- $n x mod n = 0$ for all positive integer values of $x$ .
- If $p$ is a prime number which is not a divisor of $b$ , then $ab p -1 mod p = a mod p$ , due to Fermat's little theorem.
Inverse:
- $[(- a mod n) + (a mod n)] mod n = 0$ .
- $b -1 mod n$ denotes the modular multiplicative inverse, which is defined if and only if $b$ and $n$ are relatively prime, which is the case when the left hand side is defined: $[(b -1 mod n)(b mod n)] mod n = 1$ .
Distributive:
- $(a + b) mod n = [(a mod n) + (b mod n)] mod n$ .
- $ab mod n = [(a mod n)(b mod n)] mod n$ .
Division (definition): $a / b mod n = [(a mod n)(b -1 mod n)] mod n$ , when the right hand side is defined (that is when $b$ and $n$ are coprime), and undefined otherwise.
Inverse multiplication: $[(ab mod n)(b -1 mod n)] mod n = a mod n$ .

In programming languages

Modulo operators in various programming languages
Language	Operator	Integer	Floating-point	Definition
ABAP	`MOD`	Yes	Yes	Euclidean
ActionScript	`%`	Yes	No	Truncated
Ada	`mod`	Yes	No	Floored
Ada	`rem`	Yes	No	Truncated
ALGOL 68	`÷×`, `mod`	Yes	No	Euclidean
AMPL	`mod`	Yes	No	Truncated
APL	`\|`	Yes	No	Floored
AppleScript	`mod`	Yes	No	Truncated
AutoLISP	`(rem d n)`	Yes	No	Truncated
AWK	`%`	Yes	No	Truncated
bash	`%`	Yes	No	Truncated
BASIC	`Mod`	Yes	No	Undefined
bc	`%`	Yes	No	Truncated
C C++	`%`, `div`	Yes	No	Truncated
	`fmod` (C) `std::fmod` (C++)	No	Yes	Truncated
	`remainder` (C) `std::remainder` (C++)	No	Yes	Rounded
C#	`%`	Yes	Yes	Truncated
C#	`Math.IEEERemainder`	No	Yes	Rounded
Clarion	`%`	Yes	No	Truncated
Clean	`rem`	Yes	No	Truncated
Clojure	`mod`	Yes	No	Floored
Clojure	`rem`	Yes	No	Truncated
COBOL	`FUNCTION MOD`	Yes	No	Floored
CoffeeScript	`%`	Yes	No	Truncated
CoffeeScript	`%%`	Yes	No	Floored
ColdFusion	`%`, `MOD`	Yes	No	Truncated
Common Lisp	`mod`	Yes	Yes	Floored
Common Lisp	`rem`	Yes	Yes	Truncated
Crystal	`%`, `modulo`	Yes	Yes	Floored
Crystal	`remainder`	Yes	Yes	Truncated
D	`%`	Yes	Yes	Truncated
Dart	`%`	Yes	Yes	Euclidean
Dart	`remainder()`	Yes	Yes	Truncated
Eiffel	`\\`	Yes	No	Truncated
Elixir	`rem/2`	Yes	No	Truncated
Elixir	`Integer.mod/2`	Yes	No	Floored
Elm	`modBy`	Yes	No	Floored
Elm	`remainderBy`	Yes	No	Truncated
Erlang	`rem`	Yes	No	Truncated
Erlang	`math:fmod/2`	No	Yes	Truncated (same as C)
Euphoria	`mod`	Yes	No	Floored
Euphoria	`remainder`	Yes	No	Truncated
F#	`%`	Yes	Yes	Truncated
F#	`Math.IEEERemainder`	No	Yes	Rounded
Factor	`mod`	Yes	No	Truncated
FileMaker	`Mod`	Yes	No	Floored
Forth	`mod`	Yes	No	Implementation defined
	`fm/mod`	Yes	No	Floored
	`sm/rem`	Yes	No	Truncated
Fortran	`mod`	Yes	Yes	Truncated
Fortran	`modulo`	Yes	Yes	Floored
Frink	`mod`	Yes	No	Floored
GLSL	`%`	Yes	No	Undefined
GLSL	`mod`	No	Yes	Floored
GameMaker Studio (GML)	`mod`, `%`	Yes	No	Truncated
GDScript (Godot)	`%`	Yes	No	Truncated
	`fmod`	No	Yes	Truncated
	`posmod`	Yes	No	Floored
	`fposmod`	No	Yes	Floored
Go	`%`	Yes	No	Truncated
	`math.Mod`	No	Yes	Truncated
	`big.Int.Mod`	Yes	No	Euclidean
Groovy	`%`	Yes	No	Truncated
Haskell	`mod`	Yes	No	Floored
	`rem`	Yes	No	Truncated
	`Data.Fixed.mod'` (GHC)	No	Yes	Floored
Haxe	`%`	Yes	No	Truncated
HLSL	`%`	Yes	Yes	Undefined
J	`\|`	Yes	No	Floored
Java	`%`	Yes	Yes	Truncated
Java	`Math.floorMod`	Yes	No	Floored
JavaScript TypeScript	`%`	Yes	Yes	Truncated
Julia	`mod`	Yes	Yes	Floored
Julia	`%`, `rem`	Yes	Yes	Truncated
Kotlin	`%`, `rem`	Yes	Yes	Truncated
Kotlin	`mod`	Yes	Yes	Floored
ksh	`%`	Yes	No	Truncated (same as POSIX sh)
ksh	`fmod`	No	Yes	Truncated
LabVIEW	`mod`	Yes	Yes	Truncated
LibreOffice	`=MOD()`	Yes	No	Floored
Logo	`MODULO`	Yes	No	Floored
Logo	`REMAINDER`	Yes	No	Truncated
Lua 5	`%`	Yes	Yes	Floored
Lua 4	`mod(x,y)`	Yes	Yes	Truncated
Liberty BASIC	`MOD`	Yes	No	Truncated
Mathcad	`mod(x,y)`	Yes	No	Floored
Maple	`e mod m` (by default), `modp(e, m)`	Yes	No	Euclidean
	`mods(e, m)`	Yes	No	Rounded
	`frem(e, m)`	Yes	Yes	Rounded
Mathematica	`Mod[a, b]`	Yes	No	Floored
MATLAB	`mod`	Yes	No	Floored
MATLAB	`rem`	Yes	No	Truncated
Maxima	`mod`	Yes	No	Floored
Maxima	`remainder`	Yes	No	Truncated
Maya Embedded Language	`%`	Yes	No	Truncated
Microsoft Excel	`=MOD()`	Yes	Yes	Floored
Minitab	`MOD`	Yes	No	Floored
Modula-2	`MOD`	Yes	No	Floored
Modula-2	`REM`	Yes	No	Truncated
MUMPS	`#`	Yes	No	Floored
Netwide Assembler (NASM, NASMX)	`%`, `div` (unsigned)	Yes	No	N/A
Netwide Assembler (NASM, NASMX)	`%%` (signed)	Yes	No	Implementation-defined
Nim	`mod`	Yes	No	Truncated
Oberon	`MOD`	Yes	No	Floored-like
Objective-C	`%`	Yes	No	Truncated (same as C99)
Object Pascal, Delphi	`mod`	Yes	No	Truncated
OCaml	`mod`	Yes	No	Truncated
OCaml	`mod_float`	No	Yes	Truncated
Occam	`\`	Yes	No	Truncated
Pascal (ISO-7185 and -10206)	`mod`	Yes	No	Euclidean-like
Programming Code Advanced (PCA)	`\`	Yes	No	Undefined
Perl	`%`	Yes	No	Floored
Perl	`POSIX::fmod`	No	Yes	Truncated
Phix	`mod`	Yes	No	Floored
Phix	`remainder`	Yes	No	Truncated
PHP	`%`	Yes	No	Truncated
PHP	`fmod`	No	Yes	Truncated
PIC BASIC Pro	`\\`	Yes	No	Truncated
PL/I	`mod`	Yes	No	Floored (ANSI PL/I)
PowerShell	`%`	Yes	No	Truncated
Programming Code (PRC)	`MATH.OP - 'MOD; (\)'`	Yes	No	Undefined
Progress	`modulo`	Yes	No	Truncated
Prolog (ISO 1995)	`mod`	Yes	No	Floored
Prolog (ISO 1995)	`rem`	Yes	No	Truncated
PureBasic	`%`, `Mod(x,y)`	Yes	No	Truncated
PureScript	`mod`	Yes	No	Euclidean
Pure Data	`%`	Yes	No	Truncated (same as C)
Pure Data	`mod`	Yes	No	Floored
Python	`%`	Yes	Yes	Floored
Python	`math.fmod`	No	Yes	Truncated
Q#	`%`	Yes	No	Truncated
R	`%%`	Yes	Yes	Floored
Racket	`modulo`	Yes	No	Floored
Racket	`remainder`	Yes	No	Truncated
Raku	`%`	No	Yes	Floored
RealBasic	`MOD`	Yes	No	Truncated
Reason	`mod`	Yes	No	Truncated
Rexx	`//`	Yes	Yes	Truncated
RPG	`%REM`	Yes	No	Truncated
Ruby	`%`, `modulo()`	Yes	Yes	Floored
Ruby	`remainder()`	Yes	Yes	Truncated
Rust	`%`	Yes	Yes	Truncated
Rust	`rem_euclid()`	Yes	Yes	Euclidean
SAS	`MOD`	Yes	No	Truncated
Scala	`%`	Yes	No	Truncated
Scheme	`modulo`	Yes	No	Floored
Scheme	`remainder`	Yes	No	Truncated
Scheme R⁶RS	`mod`	Yes	No	Euclidean
	`mod0`	Yes	No	Rounded
	`flmod`	No	Yes	Euclidean
	`flmod0`	No	Yes	Rounded
Scratch	`mod`	Yes	Yes	Floored
Seed7	`mod`	Yes	Yes	Floored
Seed7	`rem`	Yes	Yes	Truncated
SenseTalk	`modulo`	Yes	No	Floored
SenseTalk	`rem`	Yes	No	Truncated
`sh` (POSIX) (includes bash, mksh, &c.)	`%`	Yes	No	Truncated (same as C)
Smalltalk	`\\`	Yes	No	Floored
Smalltalk	`rem:`	Yes	No	Truncated
Snap!	`mod`	Yes	No	Floored
Spin	`//`	Yes	No	Floored
Solidity	`%`	Yes	No	Floored
SQL (SQL:1999)	`mod(x,y)`	Yes	No	Truncated
SQL (SQL:2011)	`%`	Yes	No	Truncated
Standard ML	`mod`	Yes	No	Floored
	`Int.rem`	Yes	No	Truncated
	`Real.rem`	No	Yes	Truncated
Stata	`mod(x,y)`	Yes	No	Euclidean
Swift	`%`	Yes	No	Truncated
	`remainder(dividingBy:)`	No	Yes	Rounded
	`truncatingRemainder(dividingBy:)`	No	Yes	Truncated
Tcl	`%`	Yes	No	Floored
tcsh	`%`	Yes	No	Truncated
Torque	`%`	Yes	No	Truncated
Turing	`mod`	Yes	No	Floored
Verilog (2001)	`%`	Yes	No	Truncated
VHDL	`mod`	Yes	No	Floored
VHDL	`rem`	Yes	No	Truncated
VimL	`%`	Yes	No	Truncated
Visual Basic	`Mod`	Yes	No	Truncated
WebAssembly	`i32.rem_u`, `i64.rem_u` (unsigned)	Yes	No	N/A
WebAssembly	`i32.rem_s`, `i64.rem_s` (signed)	Yes	No	Truncated
x86 assembly	`IDIV`	Yes	No	Truncated
XBase++	`%`	Yes	Yes	Truncated
XBase++	`Mod()`	Yes	Yes	Floored
Zig	`%`, `@mod`, `@rem`	Yes	Yes	Truncated
Z3 theorem prover	`div`, `mod`	Yes	No	Euclidean

In addition, many computer systems provide a divmod functionality, which produces the quotient and the remainder at the same time. Examples include the x86 architecture's IDIV instruction, the C programming language's div() function, and Python's divmod() function.

Generalizations

Modulo with offset

Sometimes it is useful for the result of $a$ modulo $n$ to lie not between 0 and $n - 1$ , but between some number $d$ and $d + n - 1$ . In that case, $d$ is called an offset. There does not seem to be a standard notation for this operation, so let us tentatively use $a mod d n$ . We thus have the following definition: $x = a mod d n$ just in case $d \leq x \leq d + n - 1$ and $x mod n = a mod n$ . Clearly, the usual modulo operation corresponds to zero offset: $a mod n = a mod 0 n$ . The operation of modulo with offset is related to the floor function as follows:

a \operatorname{mod}_d n = a - n \left\lfloor\frac{a-d}{n}\right\rfloor.

(To see this, let ${\textstyle x = a - n \left\lfloor\frac{a-d}{n}\right\rfloor}$ . We first show that $x mod n = a mod n$ . It is in general true that $(a + bn) mod n = a mod n$ for all integers $b$ ; thus, this is true also in the particular case when ${\textstyle b = -\!\left\lfloor\frac{a-d}{n}\right\rfloor}$ ; but that means that ${\textstyle x \bmod n = \left(a - n \left\lfloor\frac{a-d}{n}\right\rfloor\right)\! \bmod n = a \bmod n}$ , which is what we wanted to prove. It remains to be shown that $d \leq x \leq d + n - 1$ . Let $k$ and $r$ be the integers such that $a - d = kn + r$ with $0 \leq r \leq n - 1$ (see Euclidean division). Then ${\textstyle \left\lfloor\frac{a-d}{n}\right\rfloor = k}$ , thus ${\textstyle x = a - n \left\lfloor\frac{a-d}{n}\right\rfloor = a - n k = d +r}$ . Now take $0 \leq r \leq n - 1$ and add $d$ to both sides, obtaining $d \leq d + r \leq d + n - 1$ . But we've seen that $x = d + r$ , so we are done. □)

The modulo with offset $a mod d n$ is implemented in Mathematica as Mod[a, n, d] .

Implementing other modulo definitions using truncation

Despite the mathematical elegance of Knuth's floored division and Euclidean division, it is generally much more common to find a truncated division-based modulo in programming languages. Leijen provides the following algorithms for calculating the two divisions given a truncated integer division:

/* Euclidean and Floored divmod, in the style of C's ldiv() */
typedef struct {
  /* This structure is part of the C stdlib.h, but is reproduced here for clarity */
  long int quot;
  long int rem;
} ldiv_t;

/* Euclidean division */
inline ldiv_t ldivE(long numer, long denom) {
  /* The C99 and C++11 languages define both of these as truncating. */
  long q = numer / denom;
  long r = numer % denom;
  if (r < 0) {
    if (denom > 0) {
      q = q - 1;
      r = r + denom;
    } else {
      q = q + 1;
      r = r - denom;
    }
  }
  return (ldiv_t){.quot = q, .rem = r};
}

/* Floored division */
inline ldiv_t ldivF(long numer, long denom) {
  long q = numer / denom;
  long r = numer % denom;
  if ((r > 0 && denom < 0) || (r < 0 && denom > 0)) {
    q = q - 1;
    r = r + denom;
  }
  return (ldiv_t){.quot = q, .rem = r};
}

For both cases, the remainder can be calculated independently of the quotient, but not vice versa. The operations are combined here to save screen space, as the logical branches are the same.