GO语言圣经 第一章习题
1.2 命令行参数
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"
"os"
)
func main() {
var s, sep string
for i := 0; i < len(os.Args); i++ {
s += sep + os.Args[i]
sep = " "
}
fmt.Println(s)
}
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 (
"fmt"
"os"
)
func main() {
for index, arg := range os.Args[1:] {
fmt.Printf("%d %s\n", index, arg)
}
}
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。
1.3 查找重复的行
修改 dup2，出现重复的行时打印文件名称。
参考博客，需要注意分情况，map中该键没有对应的值时，需要创建新的结构体。
package main
import (
"bufio"
"fmt"
"os"
)
type Line struct {
count int
filename []string
}
func main() {
counts := make(map[string]*Line)
files := os.Args[1:]
if len(files) == 0 {
countLines(os.Stdin, counts)
} else {
for _, arg := range files {
f, err := os.Open(arg)
if err != nil {
fmt.Fprintf(os.Stderr, "dup2: %v\n", err)
continue
}
countLines(f, counts)
f.Close()
}
}
for k, v := range counts {
if v.count > 1 {
fmt.Printf("\n%d %s %v\n", v.count, k, v.filename)
}
}
}
func countLines(f *os.File, counts map[string]*Line) {
input := bufio.NewScanner(f)
for input.Scan() {
key := input.Text()
if _, ok := counts[key]; ok {
counts[key].count++
counts[key].filename = append(counts[key].filename, f.Name())
} else {
counts[key] = &Line{
1,
make([]string, 1),
}
counts[key].filename[0] = f.Name()
}
}
}
1.4 GIF动画
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。
package main
import (
"image"
"image/color"
"image/gif"
"io"
"math"
"math/rand"
"os"
)
//!-main
// Packages not needed by version in book.
import (
"log"
"net/http"
"time"
)
//!+main
var palette = []color.Color{color.White, color.Black, color.RGBA{0, 255, 0, 255}, color.RGBA{255, 0, 0, 255}, color.RGBA{0, 0, 255, 255}, color.RGBA{255, 255, 0, 255}, color.RGBA{0, 255, 255, 255}, color.RGBA{255, 0, 255, 255}}
const (
whiteIndex = 0 // first color in palette
blackIndex = 1 // next color in palette
)
func main() {
//!-main
// The sequence of images is deterministic unless we seed
// the pseudo-random number generator using the current time.
// Thanks to Randall McPherson for pointing out the omission.
rand.Seed(time.Now().UTC().UnixNano())
if len(os.Args) > 1 && os.Args[1] == "web" {
//!+http
handler := func(w http.ResponseWriter, r *http.Request) {
lissajous(w)
}
http.HandleFunc("/", handler)
//!-http
log.Fatal(http.ListenAndServe("localhost:8000", nil))
return
}
//!+main
lissajous(os.Stdout)
}
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 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 < 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),
uint8(rand.Intn(7)))
}
phase += 0.1
anim.Delay = append(anim.Delay, delay)
anim.Image = append(anim.Image, img)
}
gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
}
1.5 获取URL
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 (
"fmt"
"io"
"net/http"
"os"
)
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\n", url, err)
os.Exit(1)
}
}
}
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 (
"fmt"
"io"
"net/http"
"os"
"strings"
)
func main() {
for _, url := range os.Args[1:] {
if !strings.HasPrefix(url, "http://") {
url = "http://" + 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\n", url, err)
os.Exit(1)
}
}
}
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 (
"fmt"
"io"
"net/http"
"os"
"strings"
)
func main() {
for _, url := range os.Args[1:] {
if !strings.HasPrefix(url, "http://") {
url = "http://" + 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\n", url, err)
os.Exit(1)
}
fmt.Printf("status code:%d", resp.StatusCode)
}
}
1.6 并发获取多个URL
修改本节中的程序，将响应结果输出到文件，以便于进行对比。
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 17.
//!+
// Fetchall fetches URLs in parallel and reports their times and sizes.
package main
import (
"fmt"
"io"
"io/ioutil"
"net/http"
"os"
"time"
)
func main() {
testRetFile, _ := os.OpenFile("res.txt", os.O_WRONLY|os.O_CREATE, 0666)
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:] {
testRetFile.WriteString(<-ch)
}
fmt.Fprintf(testRetFile, "%.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)
}
//!-
1.7 Web服务
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。
package main
import (
"fmt"
"image"
"image/color"
"image/gif"
"io"
"log"
"math"
"math/rand"
"net/http"
"strconv"
)
var palette = []color.Color{color.White, color.Black, color.RGBA{0, 255, 0, 255}, color.RGBA{255, 0, 0, 255}, color.RGBA{0, 0, 255, 255}, color.RGBA{255, 255, 0, 255}, color.RGBA{0, 255, 255, 255}, color.RGBA{255, 0, 255, 255}}
func main() {
http.HandleFunc("/", gifhandler) // each request calls handler
log.Fatal(http.ListenAndServe("localhost:8000", nil))
}
func gifhandler(w http.ResponseWriter, r *http.Request) {
params := r.URL.Query()
fmt.Println(params)
cStr := params.Get("cycles")
cInt, _ := strconv.Atoi(cStr)
lissajous(w, float64(cInt))
}
func lissajous(out io.Writer, c float64) {
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 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 < c*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),
uint8(rand.Intn(7)))
}
phase += 0.1
anim.Delay = append(anim.Delay, delay)
anim.Image = append(anim.Image, img)
}
gif.EncodeAll(out, &anim) // NOTE: ignoring encoding errors
}