Modulo
From Wikipedia the free encyclopedia
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.^{[1]}
For example, the expression "5 mod 2" evaluates 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.
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 (a mod 1 is always 0; a mod 0 is undefined, being a division by zero).
When exactly one of a or n is negative, the basic definition breaks down, and programming languages differ in how these values are defined.
Variants of the definition
[edit]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 nonnegative integer that belongs to that class (i.e., the remainder of the Euclidean division).^{[2]} 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:
(1) 
This still leaves a sign ambiguity if the remainder is nonzero: two possible choices for the remainder occur, one negative and the other positive; that choice determines which of the two consecutive quotients must be used to satisfy equation (1). 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.^{[a]} 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.

Many implementations use truncated division, for which the quotient is defined by
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 a so can take 2n − 1 values:

Donald Knuth^{[3]} promotes floored division, for which the quotient is defined by
where is the floor function (rounding down). Thus according to equation (1), the remainder has the same sign as the divisor n:

Raymond T. Boute^{[4]} promotes Euclidean division, for which the quotient is defined by
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:

Common Lisp and IEEE 754 use rounded division, for which the quotient is defined by
where round is the round function (rounding half to even). Thus according to equation (1), the remainder falls between and , and its sign depends on which side of zero it falls to be within these boundaries:

Common Lisp also uses ceiling division, for which the quotient is defined by
where ⌈⌉ is the ceiling function (rounding up). Thus according to equation (1), the remainder has the opposite sign of that of the divisor:
If both the dividend and divisor are positive, then the truncated, floored, and Euclidean definitions agree. If the dividend is positive and the divisor is negative, then the truncated and Euclidean definitions agree. If the dividend is negative and the divisor is positive, then the floored and Euclidean definitions agree. If both the dividend and divisor are negative, then the truncated and floored definitions agree.
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^{[5]}
However, truncated division satisfies the identity .^{[6]}
Notation
[edit]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
[edit]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; }
Performance issues
[edit]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 nontruncating definition):
x % 2^{n} == x & (2^{n}  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.^{[7]}
Compiler optimizations may recognize expressions of the form expression % constant
where constant
is a power of two and automatically implement them as expression & (constant1)
, 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 & (constant1)
will always be positive. For these languages, the equivalence x % 2^{n} == x < 0 ? x  ~(2^{n}  1) : x & (2^{n}  1)
has to be used instead, expressed using bitwise OR, NOT and AND operations.
Optimizations for general constantmodulus operations also exist by calculating the division first using the constantdivisor optimization.
Properties (identities)
[edit]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. The properties involving multiplication, division, and exponentiation generally 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
[edit]Language  Operator  Integer  Floatingpoint  Definition 

ABAP  MOD  Yes  Yes  Euclidean 
ActionScript  %  Yes  No  Truncated 
Ada  mod  Yes  No  Floored^{[8]} 
rem  Yes  No  Truncated^{[8]}  
ALGOL 68  ÷× , mod  Yes  No  Euclidean 
AMPL  mod  Yes  No  Truncated 
APL   ^{[b]}  Yes  Yes  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  Varies by implementation 
bc  %  Yes  No  Truncated 
C C++  % , div  Yes  No  Truncated^{[c]} 
fmod (C)std::fmod (C++)  No  Yes  Truncated^{[11]}  
remainder (C)std::remainder (C++)  No  Yes  Rounded  
C#  %  Yes  Yes  Truncated 
Math.IEEERemainder  No  Yes  Rounded^{[12]}  
Clarion  %  Yes  No  Truncated 
Clean  rem  Yes  No  Truncated 
Clojure  mod  Yes  No  Floored^{[13]} 
rem  Yes  No  Truncated^{[14]}  
COBOL  FUNCTION MOD  Yes  No  Floored^{[15]} 
FUNCTION REM  Yes  Yes  Truncated^{[15]}  
CoffeeScript  %  Yes  No  Truncated 
%%  Yes  No  Floored^{[16]}  
ColdFusion  % , MOD  Yes  No  Truncated 
Common Intermediate Language  rem (signed)  Yes  Yes  Truncated^{[17]} 
rem.un (unsigned)  Yes  No  —  
Common Lisp  mod  Yes  Yes  Floored 
rem  Yes  Yes  Truncated  
Crystal  % , modulo  Yes  Yes  Floored 
remainder  Yes  Yes  Truncated  
D  %  Yes  Yes  Truncated^{[18]} 
Dart  %  Yes  Yes  Euclidean^{[19]} 
remainder()  Yes  Yes  Truncated^{[20]}  
Eiffel  \\  Yes  No  Truncated 
Elixir  rem/2  Yes  No  Truncated^{[21]} 
Integer.mod/2  Yes  No  Floored^{[22]}  
Elm  modBy  Yes  No  Floored^{[23]} 
remainderBy  Yes  No  Truncated^{[24]}  
Erlang  rem  Yes  No  Truncated 
math:fmod/2  No  Yes  Truncated (same as C)^{[25]}  
Euphoria  mod  Yes  No  Floored 
remainder  Yes  No  Truncated  
F#  %  Yes  Yes  Truncated 
Math.IEEERemainder  No  Yes  Rounded^{[12]}  
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 
modulo  Yes  Yes  Floored  
Frink  mod  Yes  No  Floored 
Full BASIC  MOD  Yes  Yes  Floored^{[26]} 
REMAINDER  Yes  Yes  Truncated^{[27]}  
GLSL  %  Yes  No  Undefined^{[28]} 
mod  No  Yes  Floored^{[29]}  
GameMaker Studio (GML)  mod , %  Yes  No  Truncated 
GDScript (Godot)  %  Yes  No  Truncated 
fmod  No  Yes  Truncated  
posmod  Yes  No  Euclidean  
fposmod  No  Yes  Euclidean  
Go  %  Yes  No  Truncated^{[30]} 
math.Mod  No  Yes  Truncated^{[31]}  
big.Int.Mod  Yes  No  Euclidean^{[32]}  
big.Int.Rem  Yes  No  Truncated^{[33]}  
Groovy  %  Yes  No  Truncated 
Haskell  mod  Yes  No  Floored^{[34]} 
rem  Yes  No  Truncated^{[34]}  
Data.Fixed.mod' (GHC)  No  Yes  Floored  
Haxe  %  Yes  No  Truncated 
HLSL  %  Yes  Yes  Undefined^{[35]} 
J   ^{[b]}  Yes  No  Floored 
Java  %  Yes  Yes  Truncated 
Math.floorMod  Yes  No  Floored  
JavaScript TypeScript  %  Yes  Yes  Truncated 
Julia  mod  Yes  Yes  Floored^{[36]} 
% , rem  Yes  Yes  Truncated^{[37]}  
Kotlin  % , rem  Yes  Yes  Truncated^{[38]} 
mod  Yes  Yes  Floored^{[39]}  
ksh  %  Yes  No  Truncated (same as POSIX sh) 
fmod  No  Yes  Truncated  
LabVIEW  mod  Yes  Yes  Truncated 
LibreOffice  =MOD()  Yes  No  Floored 
Logo  MODULO  Yes  No  Floored 
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 
rem  Yes  No  Truncated  
Maxima  mod  Yes  No  Floored 
remainder  Yes  No  Truncated  
Maya Embedded Language  %  Yes  No  Truncated 
Microsoft Excel  =MOD()  Yes  Yes  Floored 
Minitab  MOD  Yes  No  Floored 
Modula2  MOD  Yes  No  Floored 
REM  Yes  No  Truncated  
MUMPS  #  Yes  No  Floored 
Netwide Assembler (NASM, NASMX)  % , div (unsigned)  Yes  No  — 
%% (signed)  Yes  No  Implementationdefined^{[40]}  
Nim  mod  Yes  No  Truncated 
Oberon  MOD  Yes  No  Flooredlike^{[d]} 
ObjectiveC  %  Yes  No  Truncated (same as C99) 
Object Pascal, Delphi  mod  Yes  No  Truncated 
OCaml  mod  Yes  No  Truncated^{[41]} 
mod_float  No  Yes  Truncated^{[42]}  
Occam  \  Yes  No  Truncated 
Pascal (ISO7185 and 10206)  mod  Yes  No  Euclideanlike^{[e]} 
Perl  %  Yes  No  Floored^{[f]} 
POSIX::fmod  No  Yes  Truncated  
Phix  mod  Yes  No  Floored 
remainder  Yes  No  Truncated  
PHP  %  Yes  No  Truncated^{[44]} 
fmod  No  Yes  Truncated^{[45]}  
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 
rem  Yes  No  Truncated  
PureBasic  % , Mod(x,y)  Yes  No  Truncated 
PureScript  `mod`  Yes  No  Euclidean^{[46]} 
Pure Data  %  Yes  No  Truncated (same as C) 
mod  Yes  No  Floored  
Python  %  Yes  Yes  Floored 
math.fmod  No  Yes  Truncated  
math.remainder  No  Yes  Rounded  
Q#  %  Yes  No  Truncated^{[47]} 
R  %%  Yes  Yes  Floored^{[48]} 
Racket  modulo  Yes  No  Floored 
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 
remainder()  Yes  Yes  Truncated  
Rust  %  Yes  Yes  Truncated 
rem_euclid()  Yes  Yes  Euclidean^{[49]}  
SAS  MOD  Yes  No  Truncated 
Scala  %  Yes  Yes  Truncated 
Scheme  modulo  Yes  No  Floored 
remainder  Yes  No  Truncated  
Scheme R^{6}RS  mod  Yes  No  Euclidean^{[50]} 
mod0  Yes  No  Rounded^{[50]}  
flmod  No  Yes  Euclidean  
flmod0  No  Yes  Rounded  
Scratch  mod  Yes  Yes  Floored 
Seed7  mod  Yes  Yes  Floored 
rem  Yes  Yes  Truncated  
SenseTalk  modulo  Yes  No  Floored 
rem  Yes  No  Truncated  
sh (POSIX) (includes bash, mksh, &c.)  %  Yes  No  Truncated (same as C)^{[51]} 
Smalltalk  \\  Yes  No  Floored 
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^{[52]} 
remainder(dividingBy:)  No  Yes  Rounded^{[53]}  
truncatingRemainder(dividingBy:)  No  Yes  Truncated^{[54]}  
Tcl  %  Yes  No  Floored 
fmod()  No  Yes  Truncated (as C)  
tcsh  %  Yes  No  Truncated 
Torque  %  Yes  No  Truncated 
Turing  mod  Yes  No  Floored 
Verilog (2001)  %  Yes  No  Truncated 
VHDL  mod  Yes  No  Floored 
rem  Yes  No  Truncated  
VimL  %  Yes  No  Truncated 
Visual Basic  Mod  Yes  No  Truncated 
WebAssembly  i32.rem_u , i64.rem_u (unsigned)  Yes  No  —^{[55]} 
i32.rem_s , i64.rem_s (signed)  Yes  No  Truncated^{[55]}  
x86 assembly  IDIV  Yes  No  Truncated 
XBase++  %  Yes  Yes  Truncated 
Mod()  Yes  Yes  Floored  
Zig  % ,
 Yes  Yes  Truncated^{[56]} 
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
[edit]Modulo with offset
[edit]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 and d = 1 is particularly common.
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:^{[57]} x = a mod_{d} n just in case d ≤ x ≤ 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:
To see this, let . 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 ; but that means that , which is what we wanted to prove. It remains to be shown that d ≤ x ≤ d + n − 1. Let k and r be the integers such that a − d = kn + r with 0 ≤ r ≤ n − 1 (see Euclidean division). Then , thus . Now take 0 ≤ r ≤ n − 1 and add d to both sides, obtaining d ≤ d + r ≤ 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]
.^{[57]}
Implementing other modulo definitions using truncation
[edit]Despite the mathematical elegance of Knuth's floored division and Euclidean division, it is generally much more common to find a truncated divisionbased modulo in programming languages. Leijen provides the following algorithms for calculating the two divisions given a truncated integer division:^{[5]}
/* 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.
See also
[edit] Modulo (disambiguation) – many uses of the word modulo, all of which grew out of Carl F. Gauss's introduction of modular arithmetic in 1801.
 Modulo (mathematics), general use of the term in mathematics
 Modular exponentiation
 Turn (angle)
Notes
[edit] ^ Mathematically, these two choices are but two of the infinite number of choices available for the inequality satisfied by a remainder.
 ^ ^{a} ^{b} Argument order reverses, i.e.,
αω
computes , the remainder when dividingω
byα
.  ^ C99 and C++11 define the behavior of
%
to be truncated.^{[9]} The standards before then leave the behavior implementationdefined.^{[10]}  ^ Divisor must be positive, otherwise undefined.
 ^ As discussed by Boute, ISO Pascal's definitions of
div
andmod
do not obey the Division Identity of D = d · (D / d) + D % d, and are thus fundamentally broken.  ^ Perl usually uses arithmetic modulo operator that is machineindependent. For examples and exceptions, see the Perl documentation on multiplicative operators.^{[43]}
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{{cite book}}
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External links
[edit] Different kinds of integer division
 Modulorama, animation of a cyclic representation of multiplication tables (explanation in French)