练习 2.1： 向tempconv包添加类型、常量和函数用来处理Kelvin绝对温度的转换，Kelvin 绝对零度是−273.15°C，Kelvin绝对温度1K和摄氏度1°C的单位间隔是一样的
package tempconv
import "fmt"
type Celsius float64
type Fahrenheit float64
type Kelvin float64
const (
AbsoluteZeroC Celsius = -273.15
FreezingC Celsius = 0
BoilingC Celsius = 100
)
func (c Celsius) String() string { return fmt.Sprintf("%g°C", c) }
func (f Fahrenheit) String() string { return fmt.Sprintf("%g°F", f) }
func (k Kelvin) String() string { return fmt.Sprintf("%gK", k) }
func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9/5 + 32) }
func FToC(f Fahrenheit) Celsius { return Celsius((f - 32) * 5 / 9) }
func CToK(c Celsius) Kelvin {return Kelvin(c + AbsoluteZeroC)}
func KToC(k Kelvin) Celsius { return Celsius(k) - AbsoluteZeroC}
package main
import (
"fmt"
"./tempconv"
)
func main() {
fmt.Println("AbsoluteZeroK:",tempconv.CToK(tempconv.AbsoluteZeroC))
fmt.Println("FreezingK:",tempconv.CToK(tempconv.FreezingC))
fmt.Println("BoilinigK:",tempconv.CToK(tempconv.BoilingC))
}
练习 2.2： 写一个通用的单位转换程序，用类似cf程序的方式从命令行读取参数，如果缺省的话则是从标准输入读取参数，然后做类似Celsius和Fahrenheit的单位转换，长度单位可以对应英尺和米，重量单位可以对应磅和公斤等。
package main
import (
"fmt"
"os"
"bufio"
"strings"
"strconv"
"../ex01/tempconv"
)
type Meter float64
type Feet float64
type Pound float64
type Kilogram float64
func (m Meter) String() string {return fmt.Sprintf("%gm",m)}
func (f Feet) String() string {return fmt.Sprintf("%gft",f)}
func (p Pound) String() string {return fmt.Sprintf("%glb",p)}
func (k Kilogram) String() string {return fmt.Sprintf("%gkg",k)}
func MToF(m Meter) Feet {return Feet(m * 1200 / 3937)}
func FToM(f Feet) Meter {return Meter(f * 3937 / 1200)}
func PToK(p Pound) Kilogram {return Kilogram( p * 0.45359237)}
func KToP(k Kilogram) Pound { return Pound(k / 0.45359237)}
func main() {
var args []string
if len(os.Args) > 1 {
args = os.Args[1:]
}else{
r := bufio.NewReader(os.Stdin)
s, _ := r.ReadString('\n')
args = []string{strings.TrimSpace(s)}
}
for _,arg := range args {
v, err := strconv.ParseFloat(arg, 64)
if err != nil {
fmt.Fprintf(os.Stderr,"unitconv: %v\n",err)
os.Exit(1)
}
{
f := tempconv.Fahrenheit(v)
c := tempconv.Celsius(v)
fmt.Printf("%s = %s,%s = %s\n",f,tempconv.FToC(f),c,tempconv.CToF(c))
}
{
m := Meter(v)
f := Feet(v)
fmt.Printf("%s = %s,%s = %s\n",m,MToF(m),f,FToM(f))
}
{
p := Pound(v)
k := Kilogram(v)
fmt.Printf("%s = %s, %s = %s\n",p,PToK(p),k,KToP(k))
}
}
}
练习 2.3： 重写PopCount函数，用一个循环代替单一的表达式。比较两个版本的性能。（11.4节将展示如何系统地比较两个不同实现的性能。
package popcount
// pc[i] is the population count of i.
var pc [256]byte
func init() {
for i := range pc {
pc[i] = pc[i/2] + byte(i&1)
}
}
// PopCount returns the population count (number of set bits) of x.
func PopCount(x uint64) int {
return int(pc[byte(x>>(0*8))] +
pc[byte(x>>(1*8))] +
pc[byte(x>>(2*8))] +
pc[byte(x>>(3*8))] +
pc[byte(x>>(4*8))] +
pc[byte(x>>(5*8))] +
pc[byte(x>>(6*8))] +
pc[byte(x>>(7*8))])
}
func PopCountByLoop(x uint64) int {
n := 0
for i := byte(0); i < 8; i++ {
n += int(pc[byte(x >>(i*8))])
}
return n
}
package popcount
import (
"testing"
"reflect"
)
func assert(t *testing.T,expected,actual interface{}){
if !reflect.DeepEqual(expected,actual){
t.Errorf("(expected,actual) = (%v,%v)\n",expected,actual)
}
}
func TestPopCount(t *testing.T) {
assert(t,32,PopCount(0x1234567890ABCDEF))
}
func TestPopCountByLoop(t *testing.T) {
assert(t, 32, PopCountByLoop(0x1234567890ABCDEF))
}
func BenchmarkPopCount(b *testing.B) {
for i := 0; i < b.N; i++ {
PopCount(0x1234567890ABCDEF)
}
}
func BenchmarkPopCountByLoop(b *testing.B) {
for i := 0; i < b.N; i++ {
PopCountByLoop(0x1234567890ABCDEF)
}
}
练习 2.4： 用移位算法重写PopCount函数，每次测试最右边的1bit，然后统计总数。比较和查表算法的性能差异。
package popcount
// pc[i] is the population count of i.
var pc [256]byte
func init() {
for i := range pc {
pc[i] = pc[i/2] + byte(i&1)
}
}
// PopCount returns the population count (number of set bits) of x.
func PopCount(x uint64) int {
return int(pc[byte(x>>(0*8))] +
pc[byte(x>>(1*8))] +
pc[byte(x>>(2*8))] +
pc[byte(x>>(3*8))] +
pc[byte(x>>(4*8))] +
pc[byte(x>>(5*8))] +
pc[byte(x>>(6*8))] +
pc[byte(x>>(7*8))])
}
func PopCountByBitShift(x uint64) int {
n := 0
for i := uint(0); i < 64; i++ {
if (x>>i)&1 != 0 {
n++
}
}
return n
}
package popcount
import (
"reflect"
"testing"
)
func assert(t *testing.T, expected, actual interface{}) {
if !reflect.DeepEqual(expected, actual) {
t.Errorf("(expected, actual) = (%v, %v)\n", expected, actual)
}
}
func TestPopCount(t *testing.T) {
assert(t, 32, PopCount(0x1234567890ABCDEF))
}
func TestPopCountByBitShift(t *testing.T) {
assert(t, 32, PopCountByBitShift(0x1234567890ABCDEF))
}
func BenchmarkPopCount(b *testing.B) {
for i := 0; i < b.N; i++ {
PopCount(0x1234567890ABCDEF)
}
}
func BenchmarkPopCountByBitShift(b *testing.B) {
for i := 0; i < b.N; i++ {
PopCountByBitShift(0x1234567890ABCDEF)
}
}
练习 2.5： 表达式x&(x-1)用于将x的最低的一个非零的bit位清零。使用这个算法重写PopCount函数，然后比较性能。
package popcount
// pc[i] is the population count of i.
var pc [256]byte
func init() {
for i := range pc {
pc[i] = pc[i/2] + byte(i&1)
}
}
// PopCount returns the population count (number of set bits) of x.
func PopCount(x uint64) int {
return int(pc[byte(x>>(0*8))] +
pc[byte(x>>(1*8))] +
pc[byte(x>>(2*8))] +
pc[byte(x>>(3*8))] +
pc[byte(x>>(4*8))] +
pc[byte(x>>(5*8))] +
pc[byte(x>>(6*8))] +
pc[byte(x>>(7*8))])
}
func PopCountByBitClear(x uint64) int {
n := 0
for x != 0 {
x = x & (x - 1)
n++
}
return n
}
package popcount
import (
"reflect"
"testing"
)
func assert(t *testing.T, expected, actual interface{}) {
if !reflect.DeepEqual(expected, actual) {
t.Errorf("(expected, actual) = (%v, %v)\n", expected, actual)
}
}
func TestPopCount(t *testing.T) {
assert(t, 32, PopCount(0x1234567890ABCDEF))
}
func TestPopCountByBitClear(t *testing.T) {
assert(t, 32, PopCountByBitClear(0x1234567890ABCDEF))
}
func BenchmarkPopCount(b *testing.B) {
for i := 0; i < b.N; i++ {
PopCount(0x1234567890ABCDEF)
}
}
func BenchmarkPopCountByBitClear(b *testing.B) {
for i := 0; i < b.N; i++ {
PopCountByBitClear(0x1234567890ABCDEF)
}
}