Errata

Creating a Multishape like described:

multiShape := MultiShape {
shapes : [] Shape {
Circle { 0 , 0 , 5 },
Rectangle { 0 , 0 , 10 , 10 },
},
}

spits a compile error:
cannot use Circle literal (type Circle) as type Shape in array or slice literal:
Circle does not implement Shape (area method has pointer receiver)
cannot use Rectangle literal (type Rectangle) as type Shape in array or slice literal:
Rectangle does not implement Shape (area method has pointer receiver)

Note from the Author or Editor:
Apologies, the area method for Circle and Rectangle should be non-pointer types:

func (c Circle) area() float64 {
return math.Pi * c.r*c.r
}

func (r Rectangle) area() float64 {
l := distance(r.x1, r.y1, r.x1, r.y2)
w := distance(r.x1, r.y1, r.x2, r.y1)
return l * w
}

Or the literals should be constructed like this:

multiShape := MultiShape{
shapes: []Shape{
&Circle { 0 , 0 , 5 },
&Rectangle { 0 , 0 , 10 , 10 },
},
}

The import is missing one package: "io"

On the next page, page 74, there is a line of code :
_, err := io.Copy(h, f)

This will get error if there is no "io" package in the import statement.

The import should be:

import (
"fmt"
"hash/crc32"
"io/ioutil"
"io"
)

Note from the Author or Editor:
Yes this is a mistake. "io/ioutil" should be replaced with "io":

package main

import (
"fmt"
"hash/crc32"
"io"
"os"
)

func getHash(filename string) (uint32, error) {
// open the file
f, err := os.Open(filename)
if err != nil {
return 0, err
}
// remember to always close opened files
defer f.Close()

// create a hasher
h := crc32.NewIEEE()
// copy the file into the hasher
// - copy takes (dst, src) and returns (bytesWritten, error)
_, err = io.Copy(h, f)
// we don't care about how many bytes were written, but we do want to
// handle the error
if err != nil {
return 0, err
}
return h.Sum32(), nil
}

func main() {
h1, err := getHash("test1.txt")
if err != nil {
return
}
h2, err := getHash("test2.txt")
if err != nil {
return
}
fmt.Println(h1, h2, h1 == h2)
}

As printed (in the Safari Online Book) the second program listed - the one with the getHash function has *several* compile errors with go version go1.5.3 darwin/amd64.

dhassler% go run crc32.go
# command-line-arguments
./crc32.go:6: imported and not used: "io/ioutil"
./crc32.go:11: undefined: os in os.Open
./crc32.go:22: no new variables on left side of :=
./crc32.go:22: undefined: io in io.Copy

I have resolved each to get the example to run but would be interested in the author's solutions - what is the correct version of this example?

Have the examples in the book been tested???

Note from the Author or Editor:
The example is indeed broken. A corrected version is here: https://play.golang.org/p/ofZVaifgJG

I made the following changes:

* The imports are:

import (
"fmt"
"io"
"os"

"hash/crc32"
)

* Since `err` was already defined I changed `:=` to `=` for:

_, err = io.Copy(h, f)