My answser for `A Tour of Go` Part II

error.go

package main

import "fmt"

type ErrNegativeSqrt float64

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

func Sqrt(f float64) (float64, error) {
	if f < 0 {
		return 0.0,ErrNegativeSqrt(f)
	}
	z := f / 2
	for i := 0; i < 10; i++ {
		z = z - (z * z - f) / (2 * z)
	}
	return z, nil
}

func main() {
	fmt.Println(Sqrt(2))
	fmt.Println(Sqrt(-2.2))
}

images.go

package main

import "image"
import "image/color"

import "tour/pic"

type Image struct{}

func (img *Image) Bounds() image.Rectangle{
	return image.Rect(0, 0, 200, 200)
}

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

func (img *Image) At(x,y int) color.Color {
	return color.RGBA{2, 2, 255, 255}
}

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

rot13.go

package main

import "io"
import "os"
import "strings"

type rot13Reader struct {
	r io.Reader
}

func (r *rot13Reader) Read(b []byte) (int, error) {
	cr,a := r.r.Read(b)
	for i := range b {
		if((b[i] >= 'A' && b[i] <= 'M') || (b[i] >= 'a' && b[i] <= 'm')) {
			b[i] = b[i] + 13
		}else if((b[i] >= 'N' && b[i] <= 'Z') || (b[i] >= 'n' && b[i] <= 'z')) {
			b[i] = b[i] - 13
		}
	}
	return cr,a
}

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