big

package standard library
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 Go to latest Published: Dec 16, 2025 License: BSD-3-Clause Imports: 14 Imported by: 231,945

Documentation

Overview

Package big implements arbitrary-precision arithmetic (big numbers). The following numeric types are supported: 

Int    signed integers
Rat    rational numbers
Float  floating-point numbers

The zero value for an Int, Rat, or Float correspond to 0. Thus, new values can be declared in the usual ways and denote 0 without further initialization: 

var x Int        // &x is an *Int of value 0
var r = &Rat{}   // r is a *Rat of value 0
y := new(Float)  // y is a *Float of value 0

Alternatively, new values can be allocated and initialized with factory functions of the form: 

func NewT(v V) *T

For instance, NewInt(x) returns an *Int set to the value of the int64 argument x, NewRat(a, b) returns a *Rat set to the fraction a/b where a and b are int64 values, and NewFloat(f) returns a *Float initialized to the float64 argument f. More flexibility is provided with explicit setters, for instance: 

var z1 Int
z1.SetUint64(123)                 // z1 := 123
z2 := new(Rat).SetFloat64(1.25)   // z2 := 5/4
z3 := new(Float).SetInt(z1)       // z3 := 123.0

Setters, numeric operations and predicates are represented as methods of the form: 

func (z *T) SetV(v V) *T          // z = v
func (z *T) Unary(x *T) *T        // z = unary x
func (z *T) Binary(x, y *T) *T    // z = x binary y
func (x *T) Pred() P              // p = pred(x)

with T one of Int, Rat, or Float. For unary and binary operations, the result is the receiver (usually named z in that case; see below); if it is one of the operands x or y it may be safely overwritten (and its memory reused).

Arithmetic expressions are typically written as a sequence of individual method calls, with each call corresponding to an operation. The receiver denotes the result and the method arguments are the operation's operands. For instance, given three *Int values a, b and c, the invocation 

c.Add(a, b)

computes the sum a + b and stores the result in c, overwriting whatever value was held in c before. Unless specified otherwise, operations permit aliasing of parameters, so it is perfectly ok to write 

sum.Add(sum, x)

to accumulate values x in a sum.

(By always passing in a result value via the receiver, memory use can be much better controlled. Instead of having to allocate new memory for each result, an operation can reuse the space allocated for the result value, and overwrite that value with the new result in the process.)

Notational convention: Incoming method parameters (including the receiver) are named consistently in the API to clarify their use. Incoming operands are usually named x, y, a, b, and so on, but never z. A parameter specifying the result is named z (typically the receiver).

For instance, the arguments for (*Int).Add are named x and y, and because the receiver specifies the result destination, it is called z: 

func (z *Int) Add(x, y *Int) *Int

Methods of this form typically return the incoming receiver as well, to enable simple call chaining.

Methods which don't require a result value to be passed in (for instance, Int.Sign), simply return the result. In this case, the receiver is typically the first operand, named x: 

func (x *Int) Sign() int

Various methods support conversions between strings and corresponding numeric values, and vice versa: *Int, *Rat, and *Float values implement the Stringer interface for a (default) string representation of the value, but also provide SetString methods to initialize a value from a string in a variety of supported formats (see the respective SetString documentation).

Finally, *Int, *Rat, and *Float satisfy fmt.Scanner for scanning and (except for *Rat) the Formatter interface for formatted printing.

Example (EConvergents)

This example demonstrates how to use big.Rat to compute the first 15 terms in the sequence of rational convergents for the constant e (base of natural logarithm). 

package main

import (
	"fmt"
	"math/big"
)

// Use the classic continued fraction for e
//
//	e = [1; 0, 1, 1, 2, 1, 1, ... 2n, 1, 1, ...]
//
// i.e., for the nth term, use
//
//	   1          if   n mod 3 != 1
//	(n-1)/3 * 2   if   n mod 3 == 1
func recur(n, lim int64) *big.Rat {
	term := new(big.Rat)
	if n%3 != 1 {
		term.SetInt64(1)
	} else {
		term.SetInt64((n - 1) / 3 * 2)
	}

	if n > lim {
		return term
	}

	// Directly initialize frac as the fractional
	// inverse of the result of recur.
	frac := new(big.Rat).Inv(recur(n+1, lim))

	return term.Add(term, frac)
}

// This example demonstrates how to use big.Rat to compute the
// first 15 terms in the sequence of rational convergents for
// the constant e (base of natural logarithm).
func main() {
	for i := 1; i <= 15; i++ {
		r := recur(0, int64(i))

		// Print r both as a fraction and as a floating-point number.
		// Since big.Rat implements fmt.Formatter, we can use %-13s to
		// get a left-aligned string representation of the fraction.
		fmt.Printf("%-13s = %s\n", r, r.FloatString(8))
	}

}

Output:
2/1           = 2.00000000
3/1           = 3.00000000
8/3           = 2.66666667
11/4          = 2.75000000
19/7          = 2.71428571
87/32         = 2.71875000
106/39        = 2.71794872
193/71        = 2.71830986
1264/465      = 2.71827957
1457/536      = 2.71828358
2721/1001     = 2.71828172
23225/8544    = 2.71828184
25946/9545    = 2.71828182
49171/18089   = 2.71828183
517656/190435 = 2.71828183
Example (Fibonacci)

This example demonstrates how to use big.Int to compute the smallest Fibonacci number with 100 decimal digits and to test whether it is prime. 

package main

import (
	"fmt"
	"math/big"
)

func main() {
	// Initialize two big ints with the first two numbers in the sequence.
	a := big.NewInt(0)
	b := big.NewInt(1)

	// Initialize limit as 10^99, the smallest integer with 100 digits.
	var limit big.Int
	limit.Exp(big.NewInt(10), big.NewInt(99), nil)

	// Loop while a is smaller than 1e100.
	for a.Cmp(&limit) < 0 {
		// Compute the next Fibonacci number, storing it in a.
		a.Add(a, b)
		// Swap a and b so that b is the next number in the sequence.
		a, b = b, a
	}
	fmt.Println(a) // 100-digit Fibonacci number

	// Test a for primality.
	// (ProbablyPrimes' argument sets the number of Miller-Rabin
	// rounds to be performed. 20 is a good value.)
	fmt.Println(a.ProbablyPrime(20))

}

Output:
1344719667586153181419716641724567886890850696275767987106294472017884974410332069524504824747437757
false
Example (Sqrt2)

This example shows how to use big.Float to compute the square root of 2 with a precision of 200 bits, and how to print the result as a decimal number. 

package main

import (
	"fmt"
	"math"
	"math/big"
)

func main() {
	// We'll do computations with 200 bits of precision in the mantissa.
	const prec = 200

	// Compute the square root of 2 using Newton's Method. We start with
	// an initial estimate for sqrt(2), and then iterate:
	//     x_{n+1} = 1/2 * ( x_n + (2.0 / x_n) )

	// Since Newton's Method doubles the number of correct digits at each
	// iteration, we need at least log_2(prec) steps.
	steps := int(math.Log2(prec))

	// Initialize values we need for the computation.
	two := new(big.Float).SetPrec(prec).SetInt64(2)
	half := new(big.Float).SetPrec(prec).SetFloat64(0.5)

	// Use 1 as the initial estimate.
	x := new(big.Float).SetPrec(prec).SetInt64(1)

	// We use t as a temporary variable. There's no need to set its precision
	// since big.Float values with unset (== 0) precision automatically assume
	// the largest precision of the arguments when used as the result (receiver)
	// of a big.Float operation.
	t := new(big.Float)

	// Iterate.
	for i := 0; i <= steps; i++ {
		t.Quo(two, x)  // t = 2.0 / x_n
		t.Add(x, t)    // t = x_n + (2.0 / x_n)
		x.Mul(half, t) // x_{n+1} = 0.5 * t
	}

	// We can use the usual fmt.Printf verbs since big.Float implements fmt.Formatter
	fmt.Printf("sqrt(2) = %.50f\n", x)

	// Print the error between 2 and x*x.
	t.Mul(x, x) // t = x*x
	fmt.Printf("error = %e\n", t.Sub(two, t))

}

Output:
sqrt(2) = 1.41421356237309504880168872420969807856967187537695
error = 0.000000e+00

Index

Examples

Constants

View Source 
const (
	MaxExp  = math.MaxInt32  // largest supported exponent
	MinExp  = math.MinInt32  // smallest supported exponent
	MaxPrec = math.MaxUint32 // largest (theoretically) supported precision; likely memory-limited
)

Exponent and precision limits.

View Source 
const MaxBase = 10 + ('z' - 'a' + 1) + ('Z' - 'A' + 1)

MaxBase is the largest number base accepted for string conversions.

Variables

This section is empty.

Functions

func Jacobi added in go1.5 

func Jacobi(x, y *Int) int

Jacobi returns the Jacobi symbol (x/y), either +1, -1, or 0. The y argument must be an odd integer.

Types

type Accuracy added in go1.5 

type Accuracy int8

Accuracy describes the rounding error produced by the most recent operation that generated a Float value, relative to the exact value. 

const (
	Below Accuracy = -1
	Exact Accuracy = 0
	Above Accuracy = +1
)

Constants describing the Accuracy of a Float.

func (Accuracy) String added in go1.5 

func (i Accuracy) String() string

type ErrNaN added in go1.5 

type ErrNaN struct {
	// contains filtered or unexported fields
}

An ErrNaN panic is raised by a Float operation that would lead to a NaN under IEEE 754 rules. An ErrNaN implements the error interface.

func (ErrNaN) Error added in go1.5 

func (err ErrNaN) Error() string

type Float added in go1.5 

type Float struct {
	// contains filtered or unexported fields
}

A nonzero finite Float represents a multi-precision floating point number 

sign × mantissa × 2**exponent

with 0.5 <= mantissa < 1.0, and MinExp <= exponent <= MaxExp. A Float may also be zero (+0, -0) or infinite (+Inf, -Inf). All Floats are ordered, and the ordering of two Floats x and y is defined by x.Cmp(y).

Each Float value also has a precision, rounding mode, and accuracy. The precision is the maximum number of mantissa bits available to represent the value. The rounding mode specifies how a result should be rounded to fit into the mantissa bits, and accuracy describes the rounding error with respect to the exact result.

Unless specified otherwise, all operations (including setters) that specify a *Float variable for the result (usually via the receiver with the exception of Float.MantExp), round the numeric result according to the precision and rounding mode of the result variable.

If the provided result precision is 0 (see below), it is set to the precision of the argument with the largest precision value before any rounding takes place, and the rounding mode remains unchanged. Thus, uninitialized Floats provided as result arguments will have their precision set to a reasonable value determined by the operands, and their mode is the zero value for RoundingMode (ToNearestEven).

By setting the desired precision to 24 or 53 and using matching rounding mode (typically ToNearestEven), Float operations produce the same results as the corresponding float32 or float64 IEEE 754 arithmetic for operands that correspond to normal (i.e., not denormal) float32 or float64 numbers. Exponent underflow and overflow lead to a 0 or an Infinity for different values than IEEE 754 because Float exponents have a much larger range.

The zero (uninitialized) value for a Float is ready to use and represents the number +0.0 exactly, with precision 0 and rounding mode ToNearestEven.

Operations always take pointer arguments (*Float) rather than Float values, and each unique Float value requires its own unique *Float pointer. To "copy" a Float value, an existing (or newly allocated) Float must be set to a new value using the Float.Set method; shallow copies of Floats are not supported and may lead to errors.

Example (Shift) 
package main

import (
	"fmt"
	"math/big"
)

func main() {
	// Implement Float "shift" by modifying the (binary) exponents directly.
	for s := -5; s <= 5; s++ {
		x := big.NewFloat(0.5)
		x.SetMantExp(x, x.MantExp(nil)+s) // shift x by s
		fmt.Println(x)
	}
}

Output:
0.015625
0.03125
0.0625
0.125
0.25
0.5
1
2
4
8
16

func NewFloat added in go1.5 

func NewFloat(x float64) *Float

NewFloat allocates and returns a new Float set to x, with precision 53 and rounding mode ToNearestEven. NewFloat panics with ErrNaN if x is a NaN.

func ParseFloat added in go1.5 

func ParseFloat(s string, base int, prec uint, mode RoundingMode) (f *Float, b int, err error)

ParseFloat is like f.Parse(s, base) with f set to the given precision and rounding mode.

func (*Float) Abs added in go1.5 

func (z *Float) Abs(x *Float) *Float

Abs sets z to the (possibly rounded) value |x| (the absolute value of x) and returns z.

func (*Float) Acc added in go1.5 

func (x *Float) Acc() Accuracy

Acc returns the accuracy of x produced by the most recent operation, unless explicitly documented otherwise by that operation.

func (*Float) Add added in go1.5 

func (z *Float) Add(x, y *Float) *Float

Add sets z to the rounded sum x+y and returns z. If z's precision is 0, it is changed to the larger of x's or y's precision before the operation. Rounding is performed according to z's precision and rounding mode; and z's accuracy reports the result error relative to the exact (not rounded) result. Add panics with ErrNaN if x and y are infinities with opposite signs. The value of z is undefined in that case.

Example 
package main

import (
	"fmt"
	"math/big"
)

func main() {
	// Operate on numbers of different precision.
	var x, y, z big.Float
	x.SetInt64(1000)          // x is automatically set to 64bit precision
	y.SetFloat64(2.718281828) // y is automatically set to 53bit precision
	z.SetPrec(32)
	z.Add(&x, &y)
	fmt.Printf("x = %.10g (%s, prec = %d, acc = %s)\n", &x, x.Text('p', 0), x.Prec(), x.Acc())
	fmt.Printf("y = %.10g (%s, prec = %d, acc = %s)\n", &y, y.Text('p', 0), y.Prec(), y.Acc())
	fmt.Printf("z = %.10g (%s, prec = %d, acc = %s)\n", &z, z.Text('p', 0), z.Prec(), z.Acc())
}

Output:
x = 1000 (0x.fap+10, prec = 64, acc = Exact)
y = 2.718281828 (0x.adf85458248cd8p+2, prec = 53, acc = Exact)
z = 1002.718282 (0x.faadf854p+10, prec = 32, acc = Below)

func (*Float) Append added in go1.5 

func (x *Float) Append(buf []byte, fmt byte, prec int) []byte

Append appends to buf the string form of the floating-point number x, as generated by x.Text, and returns the extended buffer.

func (*Float) AppendText added in go1.24.0 

func (x *Float) AppendText(b []byte) ([]byte, error)

AppendText implements the encoding.TextAppender interface. Only the Float value is marshaled (in full precision), other attributes such as precision or accuracy are ignored.

func (*Float) Cmp added in go1.5 

func (x *Float) Cmp(y *Float) int

Cmp compares x and y and returns:

  • -1 if x < y;
  • 0 if x == y (incl. -0 == 0, -Inf == -Inf, and +Inf == +Inf);
  • +1 if x > y.
Example 
package main

import (
	"fmt"
	"math"
	"math/big"
)

func main() {
	inf := math.Inf(1)
	zero := 0.0

	operands := []float64{-inf, -1.2, -zero, 0, +1.2, +inf}

	fmt.Println("   x     y  cmp")
	fmt.Println("---------------")
	for _, x64 := range operands {
		x := big.NewFloat(x64)
		for _, y64 := range operands {
			y := big.NewFloat(y64)
			fmt.Printf("%4g  %4g  %3d\n", x, y, x.Cmp(y))
		}
		fmt.Println()
	}

}

Output:
   x     y  cmp
---------------
-Inf  -Inf    0
-Inf  -1.2   -1
-Inf    -0   -1
-Inf     0   -1
-Inf   1.2   -1
-Inf  +Inf   -1

-1.2  -Inf    1
-1.2  -1.2    0
-1.2    -0   -1
-1.2     0   -1
-1.2   1.2   -1
-1.2  +Inf   -1

  -0  -Inf    1
  -0  -1.2    1
  -0    -0    0
  -0     0    0
  -0   1.2   -1
  -0  +Inf   -1

   0  -Inf    1
   0  -1.2    1
   0    -0    0
   0     0    0
   0   1.2   -1
   0  +Inf   -1

 1.2  -Inf    1
 1.2  -1.2    1
 1.2    -0    1
 1.2     0    1
 1.2   1.2    0
 1.2  +Inf   -1

+Inf  -Inf    1
+Inf  -1.2    1
+Inf    -0    1
+Inf     0    1
+Inf   1.2    1
+Inf  +Inf    0

func (*Float) Copy added in go1.5 

func (z *Float) Copy(x *Float) *Float

Copy sets z to x, with the same precision, rounding mode, and accuracy as x. Copy returns z. If x and z are identical, Copy is a no-op.

Example 
package main

import (
	"fmt"
	"math/big"
)

func main() {
	var x, z big.Float

	x.SetFloat64(1.23)
	r := z.Copy(&x)
	fmt.Printf("a) r = %g, z = %g, x = %g, r == z = %v\n", r, &z, &x, r == &z)

	// changing z changes r since they are identical
	z.SetInt64(42)
	fmt.Printf("b) r = %g, z = %g, r == z = %v\n", r, &z, r == &z)

	x.SetPrec(1)
	z.Copy(&x)
	fmt.Printf("c) z = %g, x = %g, z == x = %v\n", &z, &x, &z == &x)

}

Output:
a) r = 1.23, z = 1.23, x = 1.23, r == z = true
b) r = 42, z = 42, r == z = true
c) z = 1, x = 1, z == x = false

func (*Float) Float32 added in go1.5 

func (x *Float) Float32() (float32, Accuracy)

Float32 returns the float32 value nearest to x. If x is too small to be represented by a float32 (|x| < math.SmallestNonzeroFloat32), the result is (0, Below) or (-0, Above), respectively, depending on the sign of x. If x is too large to be represented by a float32 (|x| > math.MaxFloat32), the result is (+Inf, Above) or (-Inf, Below), depending on the sign of x.

func (*Float) Float64 added in go1.5 

func (x *Float) Float64() (float64, Accuracy)

Float64 returns the float64 value nearest to x. If x is too small to be represented by a float64 (|x| < math.SmallestNonzeroFloat64), the result is (0, Below) or (-0, Above), respectively, depending on the sign of x. If x is too large to be represented by a float64 (|x| > math.MaxFloat64), the result is (+Inf, Above) or (-Inf, Below), depending on the sign of x.

func (*Float) Format added in go1.5 

func (x *Float) Format(s fmt.State, format rune)

Format implements fmt.Formatter. It accepts all the regular formats for floating-point numbers ('b', 'e', 'E', 'f', 'F', 'g', 'G', 'x') as well as 'p' and 'v'. See (*Float).Text for the interpretation of 'p'. The 'v' format is handled like 'g'. Format also supports specification of the minimum precision in digits, the output field width, as well as the format flags '+' and ' ' for sign control, '0' for space or zero padding, and '-' for left or right justification. See the fmt package for details.

func (*Float) GobDecode added in go1.7 

func (z *Float) GobDecode(buf []byte) error

GobDecode implements the encoding/gob.GobDecoder interface. The result is rounded per the precision and rounding mode of z unless z's precision is 0, in which case z is set exactly to the decoded value.

func (*Float) GobEncode added in go1.7 

func (x *Float) GobEncode() ([]byte, error)

GobEncode implements the encoding/gob.GobEncoder interface. The Float value and all its attributes (precision, rounding mode, accuracy) are marshaled.

func (*Float) Int added in go1.5 

func (x *Float) Int(z *Int) (*Int, Accuracy)

Int returns the result of truncating x towards zero; or nil if x is an infinity. The result is Exact if x.IsInt(); otherwise it is Below for x > 0, and Above for x < 0. If a non-nil *Int argument z is provided, Int stores the result in z instead of allocating a new Int.

func (*Float) Int64 added in go1.5 

func (x *Float) Int64() (int64, Accuracy)

Int64 returns the integer resulting from truncating x towards zero. If math.MinInt64 <= x <= math.MaxInt64, the result is Exact if x is an integer, and Above (x < 0) or Below (x > 0) otherwise. The result is (math.MinInt64, Above) for x < math.MinInt64, and (math.MaxInt64, Below) for x > math.MaxInt64.

func (*Float) IsInf added in go1.5 

func (x *Float) IsInf() bool

IsInf reports whether x is +Inf or -Inf.

func (*Float) IsInt added in go1.5 

func (x *Float) IsInt() bool

IsInt reports whether x is an integer. ±Inf values are not integers.

func (*Float) MantExp added in go1.5 

func (x *Float) MantExp(mant *Float) (exp int)

MantExp breaks x into its mantissa and exponent components and returns the exponent. If a non-nil mant argument is provided its value is set to the mantissa of x, with the same precision and rounding mode as x. The components satisfy x == mant × 2**exp, with 0.5 <= |mant| < 1.0. Calling MantExp with a nil argument is an efficient way to get the exponent of the receiver.

Special cases are: 

(  ±0).MantExp(mant) = 0, with mant set to   ±0
(±Inf).MantExp(mant) = 0, with mant set to ±Inf

x and mant may be the same in which case x is set to its mantissa value.

func (*Float) MarshalText added in go1.6 

func (x *Float) MarshalText() (text []byte, err error)

MarshalText implements the encoding.TextMarshaler interface. Only the Float value is marshaled (in full precision), other attributes such as precision or accuracy are ignored.

func (*Float) MinPrec added in go1.5 

func (x *Float) MinPrec() uint

MinPrec returns the minimum precision required to represent x exactly (i.e., the smallest prec before x.SetPrec(prec) would start rounding x). The result is 0 for |x| == 0 and |x| == Inf.

func (*Float) Mode added in go1.5 

func (x *Float) Mode() RoundingMode

Mode returns the rounding mode of x.

func (*Float) Mul added in go1.5 

func (z *Float) Mul(x, y *Float) *Float

Mul sets z to the rounded product x*y and returns z. Precision, rounding, and accuracy reporting are as for Float.Add. Mul panics with ErrNaN if one operand is zero and the other operand an infinity. The value of z is undefined in that case.

func (*Float) Neg added in go1.5 

func (z *Float) Neg(x *Float) *Float

Neg sets z to the (possibly rounded) value of x with its sign negated, and returns z.

func (*Float) Parse added in go1.5 

func (z *Float) Parse(s string, base int) (f *Float, b int, err error)

Parse parses s which must contain a text representation of a floating- point number with a mantissa in the given conversion base (the exponent is always a decimal number), or a string representing an infinite value.

For base 0, an underscore character “_” may appear between a base prefix and an adjacent digit, and between successive digits; such underscores do not change the value of the number, or the returned digit count. Incorrect placement of underscores is reported as an error if there are no other errors. If base != 0, underscores are not recognized and thus terminate scanning like any other character that is not a valid radix point or digit.

It sets z to the (possibly rounded) value of the corresponding floating- point value, and returns z, the actual base b, and an error err, if any. The entire string (not just a prefix) must be consumed for success. If z's precision is 0, it is changed to 64 before rounding takes effect. The number must be of the form: 

number    = [ sign ] ( float | "inf" | "Inf" ) .
sign      = "+" | "-" .
float     = ( mantissa | prefix pmantissa ) [ exponent ] .
prefix    = "0" [ "b" | "B" | "o" | "O" | "x" | "X" ] .
mantissa  = digits "." [ digits ] | digits | "." digits .
pmantissa = [ "_" ] digits "." [ digits ] | [ "_" ] digits | "." digits .
exponent  = ( "e" | "E" | "p" | "P" ) [ sign ] digits .
digits    = digit { [ "_" ] digit } .
digit     = "0" ... "9" | "a" ... "z" | "A" ... "Z" .

The base argument must be 0, 2, 8, 10, or 16. Providing an invalid base argument will lead to a run-time panic.

For base 0, the number prefix determines the actual base: A prefix of “0b” or “0B” selects base 2, “0o” or “0O” selects base 8, and “0x” or “0X” selects base 16. Otherwise, the actual base is 10 and no prefix is accepted. The octal prefix "0" is not supported (a leading "0" is simply considered a "0").

A "p" or "P" exponent indicates a base 2 (rather than base 10) exponent; for instance, "0x1.fffffffffffffp1023" (using base 0) represents the maximum float64 value. For hexadecimal mantissae, the exponent character must be one of 'p' or 'P', if present (an "e" or "E" exponent indicator cannot be distinguished from a mantissa digit).

The returned *Float f is nil and the value of z is valid but not defined if an error is reported.

func (*Float) Prec added in go1.5 

func (x *Float) Prec() uint

Prec returns the mantissa precision of x in bits. The result may be 0 for |x| == 0 and |x| == Inf.

func (*Float) Quo added in go1.5 

func (z *Float) Quo(x, y *Float) *Float

Quo sets z to the rounded quotient x/y and returns z. Precision, rounding, and accuracy reporting are as for Float.Add. Quo panics with ErrNaN if both operands are zero or infinities. The value of z is undefined in that case.

func (*Float) Rat added in go1.5 

func (x *Float) Rat(z *Rat) (*Rat, Accuracy)

Rat returns the rational number corresponding to x; or nil if x is an infinity. The result is Exact if x is not an Inf. If a non-nil *Rat argument z is provided, Rat stores the result in z instead of allocating a new Rat.

func (*Float) Scan added in go1.8 

func (z *Float) Scan(s fmt.ScanState, ch rune) error

Scan is a support routine for fmt.Scanner; it sets z to the value of the scanned number. It accepts formats whose verbs are supported by fmt.Scan for floating point values, which are: 'b' (binary), 'e', 'E', 'f', 'F', 'g' and 'G'. Scan doesn't handle ±Inf.

Example 
package main

import (
	"fmt"
	"log"
	"math/big"
)

func main() {
	// The Scan function is rarely used directly;
	// the fmt package recognizes it as an implementation of fmt.Scanner.
	f := new(big.Float)
	_, err := fmt.Sscan("1.19282e99", f)
	if err != nil {
		log.Println("error scanning value:", err)
	} else {
		fmt.Println(f)
	}
}

Output:
1.19282e+99

func (*Float) Set added in go1.5 

func (z *Float) Set(x *Float) *Float

Set sets z to the (possibly rounded) value of x and returns z. If z's precision is 0, it is changed to the precision of x before setting z (and rounding will have no effect). Rounding is performed according to z's precision and rounding mode; and z's accuracy reports the result error relative to the exact (not rounded) result.

func (*Float) SetFloat64 added in go1.5 

func (z *Float) SetFloat64(x float64) *Float

SetFloat64 sets z to the (possibly rounded) value of x and returns z. If z's precision is 0, it is changed to 53 (and rounding will have no effect). SetFloat64 panics with ErrNaN if x is a NaN.

func (*Float) SetInf added in go1.5 

func (z *Float) SetInf(signbit bool) *Float

SetInf sets z to the infinite Float -Inf if signbit is set, or +Inf if signbit is not set, and returns z. The precision of z is unchanged and the result is always Exact.

func (*Float) SetInt added in go1.5 

func (z *Float) SetInt(x *Int) *Float

SetInt sets z to the (possibly rounded) value of x and returns z. If z's precision is 0, it is changed to the larger of x.BitLen() or 64 (and rounding will have no effect).

func (*Float) SetInt64 added in go1.5 

func (z *Float) SetInt64(x int64) *Float

SetInt64 sets z to the (possibly rounded) value of x and returns z. If z's precision is 0, it is changed to 64 (and rounding will have no effect).

func (*Float) SetMantExp added in go1.5 

func (z *Float) SetMantExp(mant *Float, exp int) *Float

SetMantExp sets z to mant × 2**exp and returns z. The result z has the same precision and rounding mode as mant. SetMantExp is an inverse of Float.MantExp but does not require 0.5 <= |mant| < 1.0. Specifically, for a given x of type *Float, SetMantExp relates to Float.MantExp as follows: 

mant := new(Float)
new(Float).SetMantExp(mant, x.MantExp(mant)).Cmp(x) == 0

Special cases are: 

z.SetMantExp(  ±0, exp) =   ±0
z.SetMantExp(±Inf, exp) = ±Inf

z and mant may be the same in which case z's exponent is set to exp.

func (*Float) SetMode added in go1.5 

func (z *Float) SetMode(mode RoundingMode) *Float

SetMode sets z's rounding mode to mode and returns an exact z. z remains unchanged otherwise. z.SetMode(z.Mode()) is a cheap way to set z's accuracy to Exact.

func (*Float) SetPrec added in go1.5 

func (z *Float) SetPrec(prec uint) *Float

SetPrec sets z's precision to prec and returns the (possibly) rounded value of z. Rounding occurs according to z's rounding mode if the mantissa cannot be represented in prec bits without loss of precision. SetPrec(0) maps all finite values to ±0; infinite values remain unchanged. If prec > MaxPrec, it is set to MaxPrec.

func (*Float) SetRat added in go1.5 

func (z *Float) SetRat(x *Rat) *Float

SetRat sets z to the (possibly rounded) value of x and returns z. If z's precision is 0, it is changed to the largest of a.BitLen(), b.BitLen(), or 64; with x = a/b.

func (*Float) SetString added in go1.5 

func (z *Float) SetString(s string) (*Float, bool)

SetString sets z to the value of s and returns z and a boolean indicating success. s must be a floating-point number of the same format as accepted by Float.Parse, with base argument 0. The entire string (not just a prefix) must be valid for success. If the operation failed, the value of z is undefined but the returned value is nil.

Example 
package main

import (
	"fmt"
	"math/big"
)

func main() {
	f := new(big.Float)
	f.SetString("3.14159")
	fmt.Println(f)
}

Output:
3.14159

func (*Float) SetUint64 added in go1.5 

func (z *Float) SetUint64(x uint64) *Float

SetUint64 sets z to the (possibly rounded) value of x and returns z. If z's precision is 0, it is changed to 64 (and rounding will have no effect).

func (*Float) Sign added in go1.5 

func (x *Float) Sign() int

Sign returns:

  • -1 if x < 0;
  • 0 if x is ±0;
  • +1 if x > 0.

func (*Float) Signbit added in go1.5 

func (x *Float) Signbit() bool

Signbit reports whether x is negative or negative zero.

func (*Float) Sqrt added in go1.10 

func (z *Float) Sqrt(x *Float) *Float

Sqrt sets z to the rounded square root of x, and returns it.

If z's precision is 0, it is changed to x's precision before the operation. Rounding is performed according to z's precision and rounding mode, but z's accuracy is not computed. Specifically, the result of z.Acc() is undefined.

The function panics if z < 0. The value of z is undefined in that case.

func (*Float) String added in go1.5 

func (x *Float) String() string

String formats x like x.Text('g', 10). (String must be called explicitly, Float.Format does not support %s verb.)

func (*Float) Sub added in go1.5 

func (z *Float) Sub(x, y *Float) *Float

Sub sets z to the rounded difference x-y and returns z. Precision, rounding, and accuracy reporting are as for Float.Add. Sub panics with ErrNaN if x and y are infinities with equal signs. The value of z is undefined in that case.

func (*Float) Text added in go1.5 

func (x *Float) Text(format byte, prec int) string

Text converts the floating-point number x to a string according to the given format and precision prec. The format is one of: 

'e'	-d.dddde±dd, decimal exponent, at least two (possibly 0) exponent digits
'E'	-d.ddddE±dd, decimal exponent, at least two (possibly 0) exponent digits
'f'	-ddddd.dddd, no exponent
'g'	like 'e' for large exponents, like 'f' otherwise
'G'	like 'E' for large exponents, like 'f' otherwise
'x'	-0xd.dddddp±dd, hexadecimal mantissa, decimal power of two exponent
'p'	-0x.dddp±dd, hexadecimal mantissa, decimal power of two exponent (non-standard)
'b'	-ddddddp±dd, decimal mantissa, decimal power of two exponent (non-standard)

For the power-of-two exponent formats, the mantissa is printed in normalized form: 

'x'	hexadecimal mantissa in [1, 2), or 0
'p'	hexadecimal mantissa in [½, 1), or 0
'b'	decimal integer mantissa using x.Prec() bits, or 0

Note that the 'x' form is the one used by most other languages and libraries.

If format is a different character, Text returns a "%" followed by the unrecognized format character.

The precision prec controls the number of digits (excluding the exponent) printed by the 'e', 'E', 'f', 'g', 'G', and 'x' formats. For 'e', 'E', 'f', and 'x', it is the number of digits after the decimal point. For 'g' and 'G' it is the total number of digits. A negative precision selects the smallest number of decimal digits necessary to identify the value x uniquely using x.Prec() mantissa bits. The prec value is ignored for the 'b' and 'p' formats.

func (*Float) Uint64 added in go1.5 

func (x *Float) Uint64() (uint64, Accuracy)

Uint64 returns the unsigned integer resulting from truncating x towards zero. If 0 <= x <= math.MaxUint64, the result is Exact if x is an integer and Below otherwise. The result is (0, Above) for x < 0, and (math.MaxUint64, Below) for x > math.MaxUint64.

func (*Float) UnmarshalText added in go1.6 

func (z *Float) UnmarshalText(text []byte) error

UnmarshalText implements the encoding.TextUnmarshaler interface. The result is rounded per the precision and rounding mode of z. If z's precision is 0, it is changed to 64 before rounding takes effect.

type Int 

type Int struct {
	// contains filtered or unexported fields
}

An Int represents a signed multi-precision integer. The zero value for an Int represents the value 0.

Operations always take pointer arguments (*Int) rather than Int values, and each unique Int value requires its own unique *Int pointer. To "copy" an Int value, an existing (or newly allocated) Int must be set to a new value using the Int.Set method; shallow copies of Ints are not supported and may lead to errors.

Note that methods may leak the Int's value through timing side-channels. Because of this and because of the scope and complexity of the implementation, Int is not well-suited to implement cryptographic operations. The standard library avoids exposing non-trivial Int methods to attacker-controlled inputs and the determination of whether a bug in math/big is considered a security vulnerability might depend on the impact on the standard library.

func NewInt 

func NewInt(x int64) *Int

NewInt allocates and returns a new Int set to x.

func (*Int) Abs 

func (z *Int) Abs(x *Int) *Int

Abs sets z to |x| (the absolute value of x) and returns z.

func (*Int) Add 

func (z *Int) Add(x, y *Int) *Int

Add sets z to the sum x+y and returns z.

func (*Int) And 

func (z *Int) And(x, y *Int) *Int

And sets z = x & y and returns z.

func (*Int) AndNot 

func (z *Int) AndNot(x, y *Int) *Int

AndNot sets z = x &^ y and returns z.

func (*Int) Append added in go1.6 

func (x *Int) Append(buf []byte, base int) []byte

Append appends the string representation of x, as generated by x.Text(base), to buf and returns the extended buffer.

func (*Int) AppendText added in go1.24.0 

func (x *Int) AppendText(b []byte) (text []byte, err error)

AppendText implements the encoding.TextAppender interface.

func (*Int) Binomial 

func (z *Int) Binomial(n, k int64) *Int

Binomial sets z to the binomial coefficient C(n, k) and returns z.

func (*Int) Bit 

func (x *Int) Bit(i int) uint

Bit returns the value of the i'th bit of x. That is, it returns (x>>i)&1. The bit index i must be >= 0.

func (*Int) BitLen 

func (x *Int) BitLen() int

BitLen returns the length of the absolute value of x in bits. The bit length of 0 is 0.

func (*Int) Bits 

func (x *Int) Bits() []Word

Bits provides raw (unchecked but fast) access to x by returning its absolute value as a little-endian Word slice. The result and x share the same underlying array. Bits is intended to support implementation of missing low-level Int functionality outside this package; it should be avoided otherwise.

func (*Int) Bytes 

func (x *Int) Bytes() []byte

Bytes returns the absolute value of x as a big-endian byte slice.

To use a fixed length slice, or a preallocated one, use Int.FillBytes.

func (*Int) Cmp 

func (x *Int) Cmp(y *Int) (r int)

Cmp compares x and y and returns:

  • -1 if x < y;
  • 0 if x == y;
  • +1 if x > y.

func (*Int) CmpAbs added in go1.10 

func (x *Int) CmpAbs(y *Int) int

CmpAbs compares the absolute values of x and y and returns:

  • -1 if |x| < |y|;
  • 0 if |x| == |y|;
  • +1 if |x| > |y|.

func (*Int) Div 

func (z *Int) Div(x, y *Int) *Int

Div sets z to the quotient x/y for y != 0 and returns z. If y == 0, a division-by-zero run-time panic occurs. Div implements Euclidean division (unlike Go); see Int.DivMod for more details.

func (*Int) DivMod 

func (z *Int) DivMod(x, y, m *Int) (*Int, *Int)

DivMod sets z to the quotient x div y and m to the modulus x mod y and returns the pair (z, m) for y != 0. If y == 0, a division-by-zero run-time panic occurs.

DivMod implements Euclidean division and modulus (unlike Go): 

q = x div y  such that
m = x - y*q  with 0 <= m < |y|

(See Raymond T. Boute, “The Euclidean definition of the functions div and mod”. ACM Transactions on Programming Languages and Systems (TOPLAS), 14(2):127-144, New York, NY, USA, 4/1992. ACM press.) See Int.QuoRem for T-division and modulus (like Go).

func (*Int) Exp 

func (z *Int) Exp(x, y, m *Int) *Int

Exp sets z = x**y mod |m| (i.e. the sign of m is ignored), and returns z. If m == nil or m == 0, z = x**y unless y <= 0 then z = 1. If m != 0, y < 0, and x and m are not relatively prime, z is unchanged and nil is returned.

Modular exponentiation of inputs of a particular size is not a cryptographically constant-time operation.

func (*Int) FillBytes added in go1.15 

func (x *Int) FillBytes(buf []byte) []byte

FillBytes sets buf to the absolute value of x, storing it as a zero-extended big-endian byte slice, and returns buf.

If the absolute value of x doesn't fit in buf, FillBytes will panic.

func (*Int) Float64 added in go1.21.0 

func (x *Int) Float64() (float64, Accuracy)

Float64 returns the float64 value nearest x, and an indication of any rounding that occurred.

func (*Int) Format 

func (x *Int) Format(s fmt.State, ch rune)

Format implements fmt.Formatter. It accepts the formats 'b' (binary), 'o' (octal with 0 prefix), 'O' (octal with 0o prefix), 'd' (decimal), 'x' (lowercase hexadecimal), and 'X' (uppercase hexadecimal). Also supported are the full suite of package fmt's format flags for integral types, including '+' and ' ' for sign control, '#' for leading zero in octal and for hexadecimal, a leading "0x" or "0X" for "%#x" and "%#X" respectively, specification of minimum digits precision, output field width, space or zero padding, and '-' for left or right justification.

func (*Int) GCD 

func (z *Int) GCD(x, y, a, b *Int) *Int

GCD sets z to the greatest common divisor of a and b and returns z. If x or y are not nil, GCD sets their value such that z = a*x + b*y.

a and b may be positive, zero or negative. (Before Go 1.14 both had to be > 0.) Regardless of the signs of a and b, z is always >= 0.

If a == b == 0, GCD sets z = x = y = 0.

If a == 0 and b != 0, GCD sets z = |b|, x = 0, y = sign(b) * 1.

If a != 0 and b == 0, GCD sets z = |a|, x = sign(a) * 1, y = 0.

func (*Int) GobDecode 

func (z *Int) GobDecode(buf []byte) error

GobDecode implements the encoding/gob.GobDecoder interface.

func (*Int) GobEncode 

func (x *Int) GobEncode() ([]byte, error)

GobEncode implements the encoding/gob.GobEncoder interface.

func (*Int) Int64 

func (x *Int) Int64() int64

Int64 returns the int64 representation of x. If x cannot be represented in an int64, the result is undefined.

func (*Int) IsInt64 added in go1.9 

func (x *Int) IsInt64() bool

IsInt64 reports whether x can be represented as an int64.

func (*Int) IsUint64 added in go1.9 

func (x *Int) IsUint64() bool

IsUint64 reports whether x can be represented as a uint64.

func (*Int) Lsh 

func (z *Int) Lsh(x *Int, n uint) *Int

Lsh sets z = x << n and returns z.

func (*Int) MarshalJSON added in go1.1 

func (x *Int) MarshalJSON() ([]byte, error)

MarshalJSON implements the encoding/json.Marshaler interface.

func (*Int) MarshalText added in go1.3 

func (x *Int) MarshalText() (text []byte, err error)

MarshalText implements the encoding.TextMarshaler interface.

func (*Int) Mod 

func (z *Int) Mod(x, y *Int) *Int

Mod sets z to the modulus x%y for y != 0 and returns z. If y == 0, a division-by-zero run-time panic occurs. Mod implements Euclidean modulus (unlike Go); see Int.DivMod for more details.

func (*Int)