《GO语言圣经》读书笔记 第一章 习题解答
2020-03-25 本文已影响0人
bocsoft
练习 1.1 ： 修改echo程序，使其能够打印os.Args[0]，即被执行命令本身的名字。
package main
import (
"fmt"
"strings"
"os"
)
func main() {
fmt.Println(strings.Join(os.Args[:]," "))
}
练习 1.2： 修改echo程序，使其打印每个参数的索引和值，每个一行。
package main
import (
"os"
"fmt"
)
func main() {
for index,arg := range os.Args[1:]{
fmt.Println(index+1,":",arg)
}
}
练习 1.3： 做实验测量潜在低效的版本和使用了strings.Join的版本的运行时间差异
package main
import (
"time"
"os"
"fmt"
"strings"
)
func main() {
joinEcho()
plusEcho()
}
func joinEcho(){
start := time.Now();
fmt.Println(strings.Join(os.Args[1:]," "))
fmt.Printf("echo2: %fs\n",time.Since(start).Seconds())
}
func plusEcho(){
start := time.Now()
s,sep := "",""
for _,arg := range os.Args[1:]{
s += sep + arg
sep = " "
}
fmt.Println(s)
fmt.Printf("plushEcho: %fs\n",time.Since(start).Seconds())
}
练习 1.4： 修改dup2，出现重复的行时打印文件名称。
package main
import (
"os"
"bufio"
"fmt"
)
func main() {
counts := make(map[string]int)
fileNames := make(map[string][]string)
files := os.Args[1:]
if len(files) == 0 {
countLines(os.Stdin,counts,fileNames)
}else{
for _,file := range files {
f, e := os.Open(file)
if e != nil {
fmt.Fprintf(os.Stderr,"dup : %v\n",e)
continue
}
countLines(f,counts,fileNames)
f.Close()
}
}
for line,n := range counts {
if n > 1 {
fmt.Printf("%d\t%s\t%s\n",n,line,fileNames[line])
}
}
}
func countLines(f *os.File,counts map[string]int,filenames map[string][]string){
input := bufio.NewScanner(f)
for input.Scan(){
line := input.Text()
counts[line]++
if !contains(filenames[line],f.Name()){
filenames[line] = append(filenames[line],f.Name())
}
}
}
func contains(names []string,target string) bool {
for _,v := range names {
if v == target {
return true
}
}
return false
}
练习 1.5： 修改前面的Lissajous程序里的调色板，由黑色改为绿色。我们可以用color.RGBA{0xRR, 0xGG, 0xBB, 0xff}来得到#RRGGBB这个色值，三个十六进制的字符串分别代表红、绿、蓝像素。
// Lissajous generates GIF animations of random Lissajous figures
package main
import (
"image/color"
"math/rand"
"time"
"io"
"image/gif"
"image"
"math"
"os"
"net/http"
"log"
)
var green = color.RGBA{0x00,0xff,0x00,0xff}
var palette = []color.Color{color.Black,green}
const(
blackIndex = 0 //first color in palette
greenIndex = 1 //next color in palette
)
func lissajous(out io.Writer){
const (
cycles = 5 // number of complete x oscillator revolutions
res = 0.001 // angular resolution
size = 100 // image canvas covers [-size...+size]
nframes = 64 // number of animation frames
delay = 8 // delay between frames in 10ms units
)
freq := rand.Float64() * 3.0 // relative frequency of y oscillator
anim := gif.GIF{LoopCount:nframes}
phase := 0.0 // phase defference
for i := 0 ; i < nframes; i++{
rect := image.Rect(0,0,2*size+1,2*size+1)
img := image.NewPaletted(rect,palette)
for t := 0.0; t < cycles*2*math.Pi; t += res {
x := math.Sin(t)
y := math.Sin(t*freq + phase)
img.SetColorIndex(size+int(x*size+0.5),size+int(y*size+0.5),greenIndex)
}
phase += 0.1
anim.Delay = append(anim.Delay,delay)
anim.Image = append(anim.Image,img)
}
gif.EncodeAll(out,&anim) // NOTE: ignoring encoding errors
}
func main() {
// The sequence of images if deterministic unless we seed
// the pseudo-random number generator using the current time.
rand.Seed(time.Now().UnixNano())
if len(os.Args) > 1 && os.Args[1] == "web" {
handler := func( w http.ResponseWriter,r *http.Request){
lissajous(w)
}
http.HandleFunc("/",handler)
log.Fatal(http.ListenAndServe("localhost:8000", nil))
return
}
lissajous(os.Stdout)
}
练习 1.6： 修改Lissajous程序，修改其调色板来生成更丰富的颜色，然后修改SetColorIndex的第三个参数，看看显示结果吧。
// Lissajous generates GIF animations of random Lissajous figures
package main
import (
"image/color"
"math/rand"
"time"
"io"
"image/gif"
"image"
"math"
"os"
"net/http"
"log"
)
var (
red = color.RGBA{0xff,0x00,0x00,0xff}
green = color.RGBA{0x00,0xff,0x00,0xff}
blue = color.RGBA{0x00,0x00,0xff,0xff}
cyan = color.RGBA{0x00,0xff,0xff,0xff}
magenta = color.RGBA{0xff,0x00,0xff,0xff}
yellow = color.RGBA{0xff,0xff,0x00,0xff}
)
var palette = []color.Color{color.Black, red, red, yellow, yellow, green, green, cyan, cyan, blue, blue, magenta, magenta}
const(
blackIndex = 0 //first color in palette
greenIndex = 1 //next color in palette
)
func lissajous(out io.Writer){
const (
cycles = 5 // number of complete x oscillator revolutions
res = 0.001 // angular resolution
size = 100 // image canvas covers [-size...+size]
nframes = 64 // number of animation frames
delay = 8 // delay between frames in 10ms units
)
freq := rand.Float64() * 3.0 // relative frequency of y oscillator
anim := gif.GIF{LoopCount:nframes}
phase := 0.0 // phase defference
for i := 0 ; i < nframes; i++{
rect := image.Rect(0,0,2*size+1,2*size+1)
img := image.NewPaletted(rect,palette)
for t := 0.0; t < cycles*2*math.Pi; t += res {
x := math.Sin(t)
y := math.Sin(t*freq + phase)
colorIndex := uint8(i%(len(palette)-1)+1)
img.SetColorIndex(size+int(x*size+0.5),size+int(y*size+0.5),colorIndex)
}
phase += 0.1
anim.Delay = append(anim.Delay,delay)
anim.Image = append(anim.Image,img)
}
gif.EncodeAll(out,&anim) // NOTE: ignoring encoding errors
}
func main() {
// The sequence of images if deterministic unless we seed
// the pseudo-random number generator using the current time.
rand.Seed(time.Now().UnixNano())
if len(os.Args) > 1 && os.Args[1] == "web" {
handler := func( w http.ResponseWriter,r *http.Request){
lissajous(w)
}
http.HandleFunc("/",handler)
log.Fatal(http.ListenAndServe("localhost:8000", nil))
return
}
lissajous(os.Stdout)
}
练习 1.7： 函数调用io.Copy(dst, src)会从src中读取内容，并将读到的结果写入到dst中，使用这个函数替代掉例子中的ioutil.ReadAll来拷贝响应结构体到os.Stdout，避免申请一个缓冲区（例子中的b）来存储。记得处理io.Copy返回结果中的错误。
package main
import (
"os"
"net/http"
"fmt"
"io"
)
func main() {
for _,url := range os.Args[1:]{
resp, err := http.Get(url)
if err != nil {
fmt.Fprintf(os.Stderr,"fetch: %v\n",err)
os.Exit(1)
}
_, err = io.Copy(os.Stdout, resp.Body)
resp.Body.Close()
if err != nil {
fmt.Fprintf(os.Stderr,"fetch: reading %s : %v ",url,err)
os.Exit(1)
}
}
}
练习 1.8： 修改fetch这个范例，如果输入的url参数没有 http:// 前缀的话，为这个url加上该前缀。你可能会用到strings.HasPrefix这个函数。
package main
import (
"os"
"net/http"
"fmt"
"io"
"strings"
)
func main() {
const prefix string = "http://"
for _,url := range os.Args[1:]{
if !strings.HasPrefix(url,"http://"){
url = prefix + url
}
resp, err := http.Get(url)
if err != nil {
fmt.Fprintf(os.Stderr,"fetch: %v\n",err)
os.Exit(1)
}
_, err = io.Copy(os.Stdout, resp.Body)
resp.Body.Close()
if err != nil {
fmt.Fprintf(os.Stderr,"fetch: reading %s : %v ",url,err)
os.Exit(1)
}
}
}
练习 1.9： 修改fetch打印出HTTP协议的状态码，可以从resp.Status变量得到该状态码。
package main
import (
"os"
"net/http"
"fmt"
"io/ioutil"
)
func main() {
for _,url := range os.Args[1:]{
resp, err := http.Get(url)
if err != nil {
fmt.Fprintf(os.Stderr,"fetch: %v\n",err)
os.Exit(1)
}
bytes, err := ioutil.ReadAll(resp.Body)//申请一个缓存区 bytes
resp.Body.Close()
if err != nil {
fmt.Fprintf(os.Stderr,"fetch: reading %s: %v\n",url,err)
os.Exit(1)
}
fmt.Printf("%s\n%s",resp.Status,bytes)
}
}
练习 1.10： 找一个数据量比较大的网站，用本小节中的程序调研网站的缓存策略，对每个URL执行两遍请求，查看两次时间是否有较大的差别，并且每次获取到的响应内容是否一致，修改本节中的程序，将响应结果输出，以便于进行对比。
//Fetchall fetches URLs in parallel and reports their times and sizes.
package main
import (
"time"
"net/http"
"fmt"
"io"
"os"
"strings"
"strconv"
)
func main() {
start := time.Now()
ch := make(chan string)
for _,url := range os.Args[1:]{
go fetch(url,ch) // start a goroutine
}
for range os.Args[1:]{
fmt.Println(<-ch) // receive from channel ch
}
fmt.Printf("%.2fs elapsed\n",time.Since(start).Seconds())
}
func fetch(url string,ch chan <- string){
start := time.Now()
resp, err := http.Get(url)
if err != nil {
ch <- fmt.Sprint(err) // send to channel ch
return
}
filename := getFileName(url)
file, err := os.Create(filename)
if err != nil {
ch <- fmt.Sprint(err)
return
}
defer file.Close()
nbytes, err := io.Copy(file, resp.Body)
resp.Body.Close()
if err != nil {
ch <- fmt.Sprintf("while reading %s: %v",url,err)
return
}
secs := time.Since(start).Seconds()
ch <- fmt.Sprintf("%.2fs %7d %s \t-> %s",secs,nbytes,url,filename)
}
func getFileName(url string) string {
basename := url
index := strings.Index(basename,"://") + len("://")
if index > 0 {
basename = basename[index:len(basename)]
}
filename := basename
for i := 1; i <= 10; i++ {
if !exists(filename) {
return filename
}
filename = basename + "." + strconv.Itoa(i)
}
return basename
}
func exists(filename string ) bool {
_, err := os.Stat(filename)
return err == nil
}
练习 1.11： 在fetchall中尝试使用长一些的参数列表，比如使用在alexa.com的上百万网站里排名靠前的。如果一个网站没有回应，程序将采取怎样的行为？（Section8.9 描述了在这种情况下的应对机制）。
package main
import (
"fmt"
"io"
"io/ioutil"
"net/http"
"os"
"time"
)
func main() {
start := time.Now()
ch := make(chan string)
for _, url := range os.Args[1:] {
go fetch(url, ch) // start a goroutine
}
for range os.Args[1:] {
fmt.Println(<-ch) // receive from channel ch
}
fmt.Printf("%.2fs elapsed\n", time.Since(start).Seconds())
}
func fetch(url string, ch chan<- string) {
start := time.Now()
resp, err := http.Get(url)
if err != nil {
ch <- fmt.Sprint(err) // send to channel ch
return
}
nbytes, err := io.Copy(ioutil.Discard, resp.Body)
resp.Body.Close() // don't leak resources
if err != nil {
ch <- fmt.Sprintf("while reading %s: %v", url, err)
return
}
secs := time.Since(start).Seconds()
ch <- fmt.Sprintf("%.2fs %7d %s", secs, nbytes, url)
}
练习 1.12： 修改Lissajour服务，从URL读取变量，比如你可以访问 http://localhost:8000/?cycles=20 这个URL，这样访问可以将程序里的cycles默认的5修改为20。字符串转换为数字可以调用strconv.Atoi函数。你可以在godoc里查看strconv.Atoi的详细说明。
// Lissajous generates GIF animations of random Lissajous figures
package main
import (
"image/color"
"math/rand"
"time"
"io"
"image/gif"
"image"
"math"
"os"
"net/http"
"log"
"strconv"
)
var (
red = color.RGBA{0xff,0x00,0x00,0xff}
green = color.RGBA{0x00,0xff,0x00,0xff}
blue = color.RGBA{0x00,0x00,0xff,0xff}
cyan = color.RGBA{0x00,0xff,0xff,0xff}
magenta = color.RGBA{0xff,0x00,0xff,0xff}
yellow = color.RGBA{0xff,0xff,0x00,0xff}
)
var palette = []color.Color{color.Black, red, red, yellow, yellow, green, green, cyan, cyan, blue, blue, magenta, magenta}
const(
blackIndex = 0 //first color in palette
greenIndex = 1 //next color in palette
)
var (
cycles = 5 // number of complete x oscillator revolutions
res = 0.001 // angular resolution
size = 100 // image canvas covers [-size...+size]
nframes = 64 // number of animation frames
delay = 8 // delay between frames in 10ms units
)
func lissajous(out io.Writer){
freq := rand.Float64() * 3.0 // relative frequency of y oscillator
anim := gif.GIF{LoopCount:nframes}
phase := 0.0 // phase difference
for i := 0 ; i < nframes; i++{
rect := image.Rect(0,0,2*size+1,2*size+1)
img := image.NewPaletted(rect,palette)
for t := 0.0; t < float64(cycles)*2*math.Pi; t += res {
x := math.Sin(t)
y := math.Sin(t*freq + phase)
colorIndex := uint8(i%(len(palette)-1)+1)
img.SetColorIndex(size+int(x*float64(size)+0.5),size+int(y*float64(size)+0.5),colorIndex)
}
phase += 0.1
anim.Delay = append(anim.Delay,delay)
anim.Image = append(anim.Image,img)
}
gif.EncodeAll(out,&anim) // NOTE: ignoring encoding errors
}
func main() {
// The sequence of images if deterministic unless we seed
// the pseudo-random number generator using the current time.
rand.Seed(time.Now().UnixNano())
if len(os.Args) > 1 && os.Args[1] == "web" {
handler := func( w http.ResponseWriter,r *http.Request){
if err := r.ParseForm(); err != nil {
log.Print(err)
}
for k,v := range r.Form {
switch k {
case "cycles":
cycles,_ = strconv.Atoi(v[0])
case "res":
res,_ = strconv.ParseFloat(v[0],64)
case "size":
size,_ = strconv.Atoi(v[0])
case "nframes":
nframes,_ = strconv.Atoi(v[0])
case "delay":
delay,_= strconv.Atoi(v[0])
}
}
lissajous(w)
}
http.HandleFunc("/",handler)
log.Fatal(http.ListenAndServe("localhost:8000", nil))
return
}
lissajous(os.Stdout)
}
