# Tour.golang exercise solutions

``````/* Exercise: Loops and Functions #43 */
package main

import (
"fmt"
"math"
)

func Sqrt(x float64) float64 {
z := float64(2.)
s := float64(0)
for {
z = z - (z*z - x)/(2*z)
if math.Abs(s-z) < 1e-15 {
break
}
s = z
}
return s
}

func main() {
fmt.Println(Sqrt(2))
fmt.Println(math.Sqrt(2))
}

/******************************************************************************************************/

/* Exercise: Maps #44 */
package main

import (
"tour/wc"
"strings"
)

func WordCount(s string) map[string]int {
ss := strings.Fields(s)
num := len(ss)
ret := make(map[string]int)
for i := 0; i < num; i++ {
(ret[ss[i]])++
}
return ret
}

func main() {
wc.Test(WordCount)
}

/******************************************************************************************************/

/* Exercise: Slices #45 */
package main

import "tour/pic"

func Pic(dx, dy int) [][]uint8 {
ret := make([][]uint8, dy)
for i := 0; i < dy; i++ {
ret[i] = make([]uint8, dx)
for j := 0; j < dx; j++ {
ret[i][j] = uint8(i^j+(i+j)/2)
}
}
return ret
}

func main() {
pic.Show(Pic)
}

/******************************************************************************************************/

/* Exercise: Fibonacci closure #46 */
package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
x := 0
y := 1
return func() int {
x,y = y,x+y
return x
}
}

func main() {
f := fibonacci()
for i := 0; i < 10; i++ {
fmt.Println(f())
}
}

/******************************************************************************************************/

/* Advanced Exercise: Complex cube roots #47 */
package main

import (
"fmt"
"math/cmplx"
)

func Cbrt(x complex128) complex128 {
z := complex128(2)
s := complex128(0)
for {
z = z - (cmplx.Pow(z,3) - x)/(3 * (z * z))
if cmplx.Abs(s-z) < 1e-17 {
break
}
s = z
}
return z
}

func main() {
fmt.Println(Cbrt(2))
}

/******************************************************************************************************/

/* Exercise: Errors #57 */
package main

import (
"fmt"
)

type ErrNegativeSqrt float64

func (e ErrNegativeSqrt) Error() string {
return fmt.Sprintf("cannot Sqrt negativ number: %g", float64(e))
}

func Sqrt(f float64) (float64, error) {
if f < 0 {
return 0, ErrNegativeSqrt(f)
}
return 0, nil
}

func main() {
fmt.Println(Sqrt(2))
fmt.Println(Sqrt(-2))
}

/******************************************************************************************************/

/* Exercise: Images #58 */
package main

import (
"image"
"tour/pic"
"image/color"
)

type Image struct{
Width, Height int
colr uint8
}

func (r *Image) Bounds() image.Rectangle {
return image.Rect(0, 0, r.Width, r.Height)
}

func (r *Image) ColorModel() color.Model {
return color.RGBAModel
}

func (r *Image) At(x, y int) color.Color {
return color.RGBA{r.colr+uint8(x), r.colr+uint8(y), 255, 255}
}

func main() {
m := Image{100, 100, 128}
pic.ShowImage(&m)
}

/******************************************************************************************************/
/* Exercise: Rot13 Reader #59: 'You cracked the code!' */
package main

import (
"io"
"os"
"strings"
)

}

for i := 0; i < len(p); i++ {
if (p[i] >= 'A' && p[i] < 'N') || (p[i] >='a' && p[i] < 'n') {
p[i] += 13
} else if (p[i] > 'M' && p[i] <= 'Z') || (p[i] > 'm' && p[i] <= 'z'){
p[i] -= 13
}
}
return
}

func main() {
"Lbh penpxrq gur pbqr!")
io.Copy(os.Stdout, &r)
}

/******************************************************************************************************/

/* Exercise: Equivalent Binary Trees #67 */
package main

import (
"tour/tree"
"fmt"
)

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int) {
_walk(t, ch)
close(ch)

}

func _walk(t *tree.Tree, ch chan int) {
if t != nil {
_walk(t.Left, ch)
ch <- t.Value
_walk(t.Right, ch)
}
}

// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool {
ch1 := make(chan int)
ch2 := make(chan int)
go Walk(t1, ch1)
go Walk(t2, ch2)
for i := range ch1 {
if i != <- ch2 {
return false
}
}
return true
}

func main() {
//tree.New(2)
ch := make(chan int)
go Walk(tree.New(1), ch)
for v := range ch {
fmt.Print(v)
}
fmt.Println(Same(tree.New(1), tree.New(1)))
fmt.Println(Same(tree.New(1), tree.New(2)))
}

/******************************************************************************************************/

/* Exercise: Web Crawler #69 */
package main

import (
"fmt"
)

type Fetcher interface {
// Fetch returns the body of URL and
// a slice of URLs found on that page.
Fetch(url string) (body string, urls []string, err error)
}

var store map[string]bool

func Krawl(url string, fetcher Fetcher, Urls chan []string) {
body, urls, err := fetcher.Fetch(url)
if err != nil {
fmt.Println(err)
} else {
fmt.Printf("found: %s %q\n", url, body)
}
Urls <- urls
}

func Crawl(url string, depth int, fetcher Fetcher) {
Urls := make(chan []string)
go Krawl(url, fetcher, Urls)
band := 1
store[url] = true // init for level 0 done
for i := 0; i < depth; i++ {
for band > 0 {
band--
next := <- Urls
for _, url := range next {
if _, done := store[url] ; !done {
store[url] = true
band++
go Krawl(url, fetcher, Urls)
}
}
}
}
return
}

func main() {
store = make(map[string]bool)
Crawl("http://golang.org/", 4, fetcher)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
body string
urls     []string
}

func (f *fakeFetcher) Fetch(url string) (string, []string, error) {
if res, ok := (*f)[url]; ok {
return res.body, res.urls, nil
}
}

// fetcher is a populated fakeFetcher.
var fetcher = &fakeFetcher{
"http://golang.org/": &fakeResult{
"The Go Programming Language",
[]string{
"http://golang.org/pkg/",
"http://golang.org/cmd/",
},
},
"http://golang.org/pkg/": &fakeResult{
"Packages",
[]string{
"http://golang.org/",
"http://golang.org/cmd/",
"http://golang.org/pkg/fmt/",
"http://golang.org/pkg/os/",
},
},
"http://golang.org/pkg/fmt/": &fakeResult{
"Package fmt",
[]string{
"http://golang.org/",
"http://golang.org/pkg/",
},
},
"http://golang.org/pkg/os/": &fakeResult{
"Package os",
[]string{
"http://golang.org/",
"http://golang.org/pkg/",
},
},
}
``````

Refer: zyxar