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zyxar/exercise.tour.go

Last active April 17, 2026 15:45
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tour.golang exercise solutions
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exercise.tour.go
/* 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 {
var x, y = 0, 1
return func() (z int) {
z, x, y = x, y, x+y
return
}
}
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"
)
type rot13Reader struct {
r io.Reader
}
func (rot *rot13Reader) Read(p []byte) (n int, err error) {
n,err = rot.r.Read(p)
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() {
s := strings.NewReader(
"Lbh penpxrq gur pbqr!")
r := rot13Reader{s}
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
}
return "", nil, fmt.Errorf("not found: %s", url)
}
// 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/",
},
},
}
@fenix-hub
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fenix-hub commented Apr 17, 2026 •
edited
Loading

@apo1967

@Aternus

Reader Solution:

package tour

import (
	"fmt"
)

type MyReader struct{}

func (r MyReader) Read(b []byte) (int, error) {
	bufferLength := len(b)
	if bufferLength <= 0 {
		return 0, fmt.Errorf("buffer length must be greater than 0")
	}
	buffer := make([]byte, bufferLength)
	for k := range buffer {
		buffer[k] = uint8('A')
	}
	n := copy(b, buffer)
	return n, nil
}

Is there a reason why you just don't set the 'A' directly in b? Why creating an internal slice and then copying to b? Just curious...

Just did the same as you...

func (r MyReader) Read(b []byte) (int, error) {
	l := len(b)
	for i := range l {
		b[i] = byte('A')
	}
	return l, nil
}

Can't find a specific reason, other than explicit exercise constraints which are not included in this case, since b will already be a copy of whatever variable the Validate function will use as an argument.

@fenix-hub
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fenix-hub commented Apr 17, 2026

My solution for rot13:

package main

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

type rot13Reader struct {
	r io.Reader
}

func (reader *rot13Reader) Read(b []byte) (int, error) {
	n, err := reader.r.Read(b)
	for i := range n {
		if (b[i] >= 65 && b[i] <= 77 || b[i] >= 97 && b[i] <= 109) {
			b[i] += 13
		} else if (b[i] >= 78 && b[i] <= 90 || b[i] >= 110 && b[i] <= 122) {
			b[i] -= 13
		}
	}
	return n, err
}

func main() {
	s := strings.NewReader("Lbh penpxrq gur pbqr!")
	r := rot13Reader{s}
	io.Copy(os.Stdout, &r)
}

I've not seen this solution already, which I propose as an efficient/smart alternative: instead of relying on explicit characters, ASCII characters can be just handled by their int value (through the standard Ascii Table here )

@fenix-hub
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fenix-hub commented Apr 17, 2026

My solution for Crawler.
Honestly this seemed the hardest so far: why doesn't the TourGo website instructs the user about the WaitGroup before this exercise? Without it this exercise was almost impossible without changing most of the function definitions.

package main

import (
	"fmt"
	"sync"
)


type Fetcher interface {
	Fetch(url string) (string, []string, error)
}

type FetchResult struct { 
	body string
	urls []string
	err error 
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
	defer wg.Done() // gets executed at the end of Crawl
	
	if depth <= 0 {
		return
	}
	if cache.Check(url) {
		fmt.Printf("Hit! %s\n", url)
		return
	}
	
	body, urls, err := fetcher.Fetch(url)
	cache.Set(url)
	
	if err != nil {
		fmt.Println(err)
		return
	}
	
	fmt.Printf("found: %s %q\n", url, body)
	for _, u := range urls {
		wg.Add(1)
		go Crawl(u, depth-1, fetcher)
	}
	return
}


type Cache struct {
	v map[string]bool // Url -> Body
	m sync.Mutex
}

func (c *Cache) Check(key string) bool {
	c.m.Lock()
	defer c.m.Unlock()
	return c.v[key]  
}

func (c *Cache) Set(key string) {
	c.m.Lock()
	c.v[key] = true
	c.m.Unlock()
}

// 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
	}
	return "", nil, fmt.Errorf("not found: %s", url)
}

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

var cache Cache = Cache{ v: make(map[string]bool) }
var wg sync.WaitGroup     // required to let all launched goroutines finish before main() exits

func main() {
	wg.Add(1)
	go Crawl("https://golang.org/", 4, fetcher)
	wg.Wait()
}

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