Modulo operation
In computing, the modulo operation finds the remainder after division of one number by another (sometimes called modulus).
Given two positive numbers, a (the dividend) and n (the divisor), a modulo n (abbreviated as a mod n) is the remainder of the Euclidean division of a by n. For example, the expression "5 mod 2" would evaluate to 1 because 5 divided by 2 leaves a quotient of 2 and a remainder of 1, while "9 mod 3" would evaluate to 0 because the division of 9 by 3 has a quotient of 3 and leaves a remainder of 0; there is nothing to subtract from 9 after multiplying 3 times 3. (Note that doing the division with a calculator will not show the result referred to here by this operation; the quotient will be expressed as a decimal fraction.)
Although typically performed with a and n both being integers, many computing systems allow other types of numeric operands. The range of numbers for an integer modulo of n is 0 to n − 1. (a mod 1 is always 0; a mod 0 is undefined, possibly resulting in a division by zero error in programming languages.) See modular arithmetic for an older and related convention applied in number theory.
When either a or n is negative, the naive definition breaks down and programming languages differ in how these values are defined.
Remainder calculation for the modulo operation
Language  Operator  Result has same sign as 

ABAP  MOD 
Positive always 
ActionScript  % 
Dividend 
Ada  mod 
Divisor 
rem 
Dividend  
ALGOL 68  ÷×, mod 
Positive always 
AMPL  mod 
Dividend 
APL   ^{} 
Divisor 
AppleScript  mod 
Dividend 
AutoLISP  (rem d n) 
Remainder 
AWK  % 
Dividend 
BASIC  Mod 
Undefined 
bash  % 
Dividend 
bc  % 
Dividend 
C (ISO 1990)  % 
Implementationdefined 
div 
Dividend  
C++ (ISO 1998)  % 
Implementationdefined^{[1]} 
div 
Dividend  
C (ISO 1999)  % , div 
Dividend^{[2]} 
C++ (ISO 2011)  % , div 
Dividend 
C#  % 
Dividend 
Clarion  % 
Dividend 
Clojure  mod 
Divisor 
rem 
Dividend  
COBOL^{}  FUNCTION MOD 
Divisor 
CoffeeScript  % 
Dividend 
%% 
Divisor^{[3]}  
ColdFusion  %, MOD 
Dividend 
Common Lisp  mod 
Divisor 
rem 
Dividend  
D  % 
Dividend^{[4]} 
Dart  % 
Positive always 
remainder()  Dividend  
Eiffel  \\ 
Dividend 
Erlang  rem 
Dividend 
Euphoria  mod 
Divisor 
remainder 
Dividend  
F#  % 
Dividend 
FileMaker  Mod 
Divisor 
Forth  mod 
implementation defined 
Fortran  mod 
Dividend 
modulo 
Divisor  
Frink  mod 
Divisor 
GameMaker: Studio (GML)  mod 
Dividend 
GDScript  % 
Dividend 
Go  % 
Dividend 
Haskell  mod 
Divisor 
rem 
Dividend  
Haxe  % 
Dividend 
J   ^{} 
Divisor 
Java  % 
Dividend 
Math.floorMod 
Divisor  
JavaScript  % 
Dividend 
Julia  mod 
Divisor 
rem 
Dividend  
LabVIEW  mod 
Dividend 
LibreOffice  =MOD() 
Divisor 
Lua 5  % 
Divisor 
Lua 4  mod(x,y) 
Divisor 
Liberty BASIC  MOD 
Dividend 
Mathcad  mod(x,y) 
Divisor 
Maple  e mod m 
Positive always 
Mathematica  Mod[a, b] 
Divisor 
MATLAB  mod 
Divisor 
rem 
Dividend  
Maxima  mod 
Divisor 
remainder 
Dividend  
Maya Embedded Language  % 
Dividend 
Microsoft Excel  =MOD() 
Divisor 
Minitab  MOD 
Divisor 
mksh  % 
Dividend 
Modula2  MOD 
Divisor 
REM 
Dividend  
MUMPS  # 
Divisor 
Netwide Assembler (NASM, NASMX)  % 
Modulo operator unsigned 
%% 
Modulo operator signed  
Oberon  MOD 
Divisor^{} 
Object Pascal, Delphi  mod 
Dividend 
OCaml  mod 
Dividend 
Occam  \ 
Dividend 
Pascal (ISO7185 and 10206)  mod 
Positive always 
Perl  % 
Divisor^{} 
PHP  % 
Dividend 
PIC BASIC Pro  \\ 
Dividend 
PL/I  mod 
Divisor (ANSI PL/I) 
PowerShell  % 
Dividend 
Progress  modulo 
Dividend 
Prolog (ISO 1995)  mod 
Divisor 
rem 
Dividend  
PureBasic  %,Mod(x,y) 
Dividend 
Python  % 
Divisor 
math.fmod 
Dividend  
Racket  remainder 
Dividend 
RealBasic  MOD 
Dividend 
R  %% 
Divisor 
Rexx  // 
Dividend 
RPG  %REM 
Dividend 
Ruby  %, modulo() 
Divisor 
remainder() 
Dividend  
Rust  % 
Dividend 
Scala  % 
Dividend 
Scheme  modulo 
Divisor 
remainder 
Dividend  
Scheme R^{6}RS  mod 
Positive always^{[5]} 
mod0 
Nearest to zero^{[5]}  
Seed7  mod 
Divisor 
rem 
Dividend  
SenseTalk  modulo 
Divisor 
rem 
Dividend  
Smalltalk  \\ 
Divisor 
rem: 
Dividend  
Spin  // 
Divisor 
SQL (SQL:1999)  mod(x,y) 
Dividend 
SQL (SQL:2012)  % 
Dividend 
Standard ML  mod 
Divisor 
Int.rem 
Dividend  
Stata  mod(x,y) 
Positive always 
Swift  % 
Dividend 
Tcl  % 
Divisor 
Torque  % 
Dividend 
Turing  mod 
Divisor 
Verilog (2001)  % 
Dividend 
VHDL  mod 
Divisor 
rem 
Dividend  
VimL  % 
Dividend 
Visual Basic  Mod 
Dividend 
x86 assembly  IDIV 
Dividend 
XBase++  % 
Dividend 
Mod() 
Divisor  
Z3 theorem prover  div , mod 
Positive always 
Language  Operator  Result has same sign as 

ABAP  MOD 
Positive always 
C (ISO 1990)  fmod 
Dividend^{[6]} 
C (ISO 1999)  fmod 
Dividend 
remainder 
Nearest to zero  
C++ (ISO 1998)  std::fmod 
Dividend 
C++ (ISO 2011)  std::fmod 
Dividend 
std::remainder 
Nearest to zero  
C#  % 
Dividend 
Common Lisp  mod 
Divisor 
rem 
Dividend  
D  % 
Dividend 
Dart  % 
Positive always 
remainder()  Dividend  
F#  % 
Dividend 
Fortran  mod 
Dividend 
modulo 
Divisor  
Go  math.Mod 
Dividend 
Haskell (GHC)  Data.Fixed.mod' 
Divisor 
Java  % 
Dividend 
JavaScript  % 
Dividend 
LabVIEW  mod 
Dividend 
Microsoft Excel  =MOD() 
Divisor 
OCaml  mod_float 
Dividend 
Perl  POSIX::fmod 
Dividend 
Perl6  % 
Divisor 
PHP  fmod 
Dividend 
Python  % 
Divisor 
math.fmod 
Dividend  
Rexx  // 
Dividend 
Ruby  %, modulo() 
Divisor 
remainder() 
Dividend  
Scheme R^{6}RS  flmod 
Positive always 
flmod0 
Nearest to zero  
Standard ML  Real.rem 
Dividend 
Swift  truncatingRemainder(dividingBy:) 
Dividend 
XBase++  % 
Dividend 
Mod() 
Divisor 
In mathematics, the result of the modulo operation is 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

(1)
However, this still leaves a sign ambiguity if the remainder is nonzero: two possible choices for the remainder occur, one negative and the other positive, and two possible choices for the quotient occur. Usually, in number theory, the positive remainder is always chosen, but programming languages choose depending on the language and the signs of a or n.^{} Standard Pascal and ALGOL 68 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 for details. a modulo 0 is undefined in most systems, although some do define it as a.
 Many implementations use truncated division, where the quotient is defined by truncation q = trunc(a/n) and thus according to equation (1) the remainder would have same sign as the dividend. The quotient is rounded towards zero: equal to the first integer in the direction of zero from the exact rational quotient.
 Donald Knuth^{[7]} described floored division where the quotient is defined by the floor function q = ⌊a/n⌋ and thus according to equation (1) the remainder would have the same sign as the divisor. Due to the floor function, the quotient is always rounded downwards, even if it is already negative.
 Raymond T. Boute^{[8]} describes the Euclidean definition in which the remainder is nonnegative always, 0 ≤ r, and is thus consistent with the Euclidean division algorithm. This convention is denoted Positive always in the table. In this case,
or equivalently
where sgn is the sign function, and thus
 Common Lisp also defines rounddivision and ceilingdivision where the quotient is given by q = round(a/n) and q = ⌈a/n⌉ respectively.
 IEEE 754 defines a remainder function where the quotient is a/n rounded according to the round to nearest convention. Thus, the sign of the remainder is chosen to be nearest to zero.
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^{[9]}
Common pitfalls
When the result of a modulo operation has the sign of the dividend, 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 it is not 0 (because remainder 0 is the same regardless of the signs):
bool is_odd(int n) {
return n % 2 != 0;
}
Or, by understanding in the first place that for any odd number, the modulo remainder may be either 1 or −1:
bool is_odd(int n) {
return n % 2 == 1  n % 2 == 1;
}
Notation
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
or equivalent, for environments lacking a mod() function (note that 'int' inherently produces the truncated value of a/n)
a  (n * int(a/n))
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:
x % 2^{n} == x & (2^{n}  1)
Examples (assuming x is a positive 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.^{[10]}
Optimizing compilers may recognize expressions of the form expression % constant
where constant
is a power of two and automatically implement them as expression & (constant1)
. This can allow writing clearer code without compromising performance. This 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 & (constant1)
will always be positive.
Equivalencies
Some modulo operations can be factored or expanded similar to other mathematical operations. This may be useful in cryptography proofs, such as the Diffie–Hellman key exchange.
 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. Undefined otherwise.
 Inverse multiplication: [(ab mod n)(b^{−1} mod n)] mod n = a mod n.
See also
 Modulo (disambiguation) and modulo (jargon) – many uses of the word modulo, all of which grew out of Carl F. Gauss's introduction of modular arithmetic in 1801.
 Modular exponentiation
Notes
 ^ Perl usually uses arithmetic modulo operator that is machineindependent. For examples and exceptions, see the Perl documentation on multiplicative operators.^{[11]}
 ^ Mathematically, these two choices are but two of the infinite number of choices available for the inequality satisfied by a remainder.
 ^ Divisor must be positive, otherwise undefined.
 ^ As implemented in ACUCOBOL, Micro Focus COBOL, and possible others.
 ^ ^ Argument order reverses, i.e.,
αω
computes , the reminder when dividingω
byα
.
References
 ↑ "ISO/IEC 14882:2003: Programming languages – C++". 5.6.4: International Organization for Standardization (ISO), International Electrotechnical Commission (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 implementationdefined".
 ↑ openstd.org, section 6.5.5
 ↑ CoffeeScript operators
 ↑ "Expressions". D Programming Language 2.0. Digital Mars. Retrieved 29 July 2010.
 1 2 r6rs.org
 ↑ "ISO/IEC 9899:1990: Programming languages – C". 7.5.6.4: ISO, IEC. 1990. "The
fmod
function returns the valuex  i * y
, for some integeri
such that, ify
is nonzero, the result as the same sign asx
and magnitude less than the magnitude ofy
.".  ↑ Knuth, Donald. E. (1972). The Art of Computer Programming. AddisonWesley.
 ↑ Boute, Raymond T. (April 1992). "The Euclidean definition of the functions div and mod". ACM Transactions on Programming Languages and Systems. ACM Press (New York, NY, USA). 14 (2): 127–144. doi:10.1145/128861.128862.
 ↑ Leijen, Daan (December 3, 2001). "Division and Modulus for Computer Scientists" (PDF). Retrieved 20141225.
 ↑ Horvath, Adam (July 5, 2012). "Faster division and modulo operation  the power of two".
 ↑ Perl documentation