Golang Float.SetFloat64方法代碼示例

func TestSqrt(t *testing.T) {
	tests := []struct {
		prec uint
		in   float64
	}{
		{16, 0},
		{16, 1},
		{16, 4},
		{16, 10000},
		{16, 2},
		{64, 2},
		{256, 2},
		{1024, 1.5},
	}
	for _, test := range tests {
		x := new(big.Float).SetPrec(test.prec)
		x.SetFloat64(test.in)
		var got, got2, diff big.Float
		pslq := New(test.prec)
		pslq.Sqrt(x, &got)
		got2.SetPrec(test.prec).Mul(&got, &got)
		diff.Sub(&got2, x)
		if diff.MinPrec() > 1 {
			t.Errorf("sqrt(%f) prec %d wrong got %.20f square %.20f expecting %f diff %g minprec %d", test.in, test.prec, &got, &got2, x, &diff, diff.MinPrec())
		}
	}
}

func (sed StringEncoderDecoder) Decode(r io.Reader, n *big.Float) error {
	n.SetFloat64(0)
	n.SetPrec(ENCODER_DECODER_PREC)

	buf := make([]byte, 256)
	if _, err := r.Read(buf); err != nil {
		return err
	}
	_, _, err := n.Parse(string(buf), 10)
	return err
}

func ExampleFloat_Add() {
	// Operating 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 = %s (%s, prec = %d, acc = %s)\n", &x, x.Format('p', 0), x.Prec(), x.Acc())
	fmt.Printf("y = %s (%s, prec = %d, acc = %s)\n", &y, y.Format('p', 0), y.Prec(), y.Acc())
	fmt.Printf("z = %s (%s, prec = %d, acc = %s)\n", &z, z.Format('p', 0), z.Prec(), z.Acc())
	// Output:
	// x = 1000 (0x.fap10, prec = 64, acc = Exact)
	// y = 2.718281828 (0x.adf85458248cd8p2, prec = 53, acc = Exact)
	// z = 1002.718282 (0x.faadf854p10, prec = 32, acc = Below)
}

func (p *exporter) float(x constant.Value) {
	if x.Kind() != constant.Float {
		log.Fatalf("gcimporter: unexpected constant %v, want float", x)
	}
	// extract sign (there is no -0)
	sign := constant.Sign(x)
	if sign == 0 {
		// x == 0
		p.int(0)
		return
	}
	// x != 0

	var f big.Float
	if v, exact := constant.Float64Val(x); exact {
		// float64
		f.SetFloat64(v)
	} else if num, denom := constant.Num(x), constant.Denom(x); num.Kind() == constant.Int {
		// TODO(gri): add big.Rat accessor to constant.Value.
		r := valueToRat(num)
		f.SetRat(r.Quo(r, valueToRat(denom)))
	} else {
		// Value too large to represent as a fraction => inaccessible.
		// TODO(gri): add big.Float accessor to constant.Value.
		f.SetFloat64(math.MaxFloat64) // FIXME
	}

	// extract exponent such that 0.5 <= m < 1.0
	var m big.Float
	exp := f.MantExp(&m)

	// extract mantissa as *big.Int
	// - set exponent large enough so mant satisfies mant.IsInt()
	// - get *big.Int from mant
	m.SetMantExp(&m, int(m.MinPrec()))
	mant, acc := m.Int(nil)
	if acc != big.Exact {
		log.Fatalf("gcimporter: internal error")
	}

	p.int(sign)
	p.int(exp)
	p.string(string(mant.Bytes()))
}

// Exp returns a big.Float representation of exp(z). Precision is
// the same as the one of the argument. The function returns +Inf
// when z = +Inf, and 0 when z = -Inf.
func Exp(z *big.Float) *big.Float {

	// exp(0) == 1
	if z.Sign() == 0 {
		return big.NewFloat(1).SetPrec(z.Prec())
	}

	// Exp(+Inf) = +Inf
	if z.IsInf() && z.Sign() > 0 {
		return big.NewFloat(math.Inf(+1)).SetPrec(z.Prec())
	}

	// Exp(-Inf) = 0
	if z.IsInf() && z.Sign() < 0 {
		return big.NewFloat(0).SetPrec(z.Prec())
	}

	guess := new(big.Float)

	// try to get initial estimate using IEEE-754 math
	zf, _ := z.Float64()
	if zfs := math.Exp(zf); zfs == math.Inf(+1) || zfs == 0 {
		// too big or too small for IEEE-754 math,
		// perform argument reduction using
		//     e^{2z} = (e^z)²
		halfZ := new(big.Float).Mul(z, big.NewFloat(0.5))
		halfExp := Exp(halfZ.SetPrec(z.Prec() + 64))
		return new(big.Float).Mul(halfExp, halfExp).SetPrec(z.Prec())
	} else {
		// we got a nice IEEE-754 estimate
		guess.SetFloat64(zfs)
	}

	// f(t)/f'(t) = t*(log(t) - z)
	f := func(t *big.Float) *big.Float {
		x := new(big.Float)
		x.Sub(Log(t), z)
		return x.Mul(x, t)
	}

	x := newton(f, guess, z.Prec())

	return x
}

func main() {
	i1, i2, i3 := 12, 45, 68
	intSum := i1 + i2 + i3

	fmt.Println("Integer Sum:", intSum)

	f1, f2, f3 := 23.5, 65.1, 76.3
	floatSum := f1 + f2 + f3
	fmt.Println("Float Sum:", floatSum)

	var b1, b2, b3, bigSum big.Float
	b1.SetFloat64(23.5)
	b2.SetFloat64(65.1)
	b3.SetFloat64(76.3)
	bigSum.Add(&b1, &b2).Add(&bigSum, &b3)
	fmt.Printf("BigSum = %.10g\n", &bigSum)

	circleRadius := 15.5
	circumference := 2 * circleRadius * math.Pi
	fmt.Printf("Circumference: %.2f\n", circumference)
}