ch7¶
bytecounter/main.go¶
/gopl.io/ch7/bytecounter/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 173.
// Bytecounter demonstrates an implementation of io.Writer that counts bytes.
package main
import (
"fmt"
)
//!+bytecounter
type ByteCounter int
func (c *ByteCounter) Write(p []byte) (int, error) {
*c += ByteCounter(len(p)) // convert int to ByteCounter
return len(p), nil
}
//!-bytecounter
func main() {
//!+main
var c ByteCounter
c.Write([]byte("hello"))
fmt.Println(c) // "5", = len("hello")
c = 0 // reset the counter
var name = "Dolly"
fmt.Fprintf(&c, "hello, %s", name)
fmt.Println(c) // "12", = len("hello, Dolly")
//!-main
}
eval/ast.go¶
/gopl.io/ch7/eval/ast.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package eval
// An Expr is an arithmetic expression.
type Expr interface {
// Eval returns the value of this Expr in the environment env.
Eval(env Env) float64
// Check reports errors in this Expr and adds its Vars to the set.
Check(vars map[Var]bool) error
}
//!+ast
// A Var identifies a variable, e.g., x.
type Var string
// A literal is a numeric constant, e.g., 3.141.
type literal float64
// A unary represents a unary operator expression, e.g., -x.
type unary struct {
op rune // one of '+', '-'
x Expr
}
// A binary represents a binary operator expression, e.g., x+y.
type binary struct {
op rune // one of '+', '-', '*', '/'
x, y Expr
}
// A call represents a function call expression, e.g., sin(x).
type call struct {
fn string // one of "pow", "sin", "sqrt"
args []Expr
}
//!-ast
eval/check.go¶
/gopl.io/ch7/eval/check.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package eval
import (
"fmt"
"strings"
)
//!+Check
func (v Var) Check(vars map[Var]bool) error {
vars[v] = true
return nil
}
func (literal) Check(vars map[Var]bool) error {
return nil
}
func (u unary) Check(vars map[Var]bool) error {
if !strings.ContainsRune("+-", u.op) {
return fmt.Errorf("unexpected unary op %q", u.op)
}
return u.x.Check(vars)
}
func (b binary) Check(vars map[Var]bool) error {
if !strings.ContainsRune("+-*/", b.op) {
return fmt.Errorf("unexpected binary op %q", b.op)
}
if err := b.x.Check(vars); err != nil {
return err
}
return b.y.Check(vars)
}
func (c call) Check(vars map[Var]bool) error {
arity, ok := numParams[c.fn]
if !ok {
return fmt.Errorf("unknown function %q", c.fn)
}
if len(c.args) != arity {
return fmt.Errorf("call to %s has %d args, want %d",
c.fn, len(c.args), arity)
}
for _, arg := range c.args {
if err := arg.Check(vars); err != nil {
return err
}
}
return nil
}
var numParams = map[string]int{"pow": 2, "sin": 1, "sqrt": 1}
//!-Check
eval/coverage_test.go¶
/gopl.io/ch7/eval/coverage_test.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package eval
import (
"fmt"
"math"
"testing"
)
//!+TestCoverage
func TestCoverage(t *testing.T) {
var tests = []struct {
input string
env Env
want string // expected error from Parse/Check or result from Eval
}{
{"x % 2", nil, "unexpected '%'"},
{"!true", nil, "unexpected '!'"},
{"log(10)", nil, `unknown function "log"`},
{"sqrt(1, 2)", nil, "call to sqrt has 2 args, want 1"},
{"sqrt(A / pi)", Env{"A": 87616, "pi": math.Pi}, "167"},
{"pow(x, 3) + pow(y, 3)", Env{"x": 9, "y": 10}, "1729"},
{"5 / 9 * (F - 32)", Env{"F": -40}, "-40"},
}
for _, test := range tests {
expr, err := Parse(test.input)
if err == nil {
err = expr.Check(map[Var]bool{})
}
if err != nil {
if err.Error() != test.want {
t.Errorf("%s: got %q, want %q", test.input, err, test.want)
}
continue
}
got := fmt.Sprintf("%.6g", expr.Eval(test.env))
if got != test.want {
t.Errorf("%s: %v => %s, want %s",
test.input, test.env, got, test.want)
}
}
}
//!-TestCoverage
eval/eval.go¶
/gopl.io/ch7/eval/eval.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 198.
// Package eval provides an expression evaluator.
package eval
import (
"fmt"
"math"
)
//!+env
type Env map[Var]float64
//!-env
//!+Eval1
func (v Var) Eval(env Env) float64 {
return env[v]
}
func (l literal) Eval(_ Env) float64 {
return float64(l)
}
//!-Eval1
//!+Eval2
func (u unary) Eval(env Env) float64 {
switch u.op {
case '+':
return +u.x.Eval(env)
case '-':
return -u.x.Eval(env)
}
panic(fmt.Sprintf("unsupported unary operator: %q", u.op))
}
func (b binary) Eval(env Env) float64 {
switch b.op {
case '+':
return b.x.Eval(env) + b.y.Eval(env)
case '-':
return b.x.Eval(env) - b.y.Eval(env)
case '*':
return b.x.Eval(env) * b.y.Eval(env)
case '/':
return b.x.Eval(env) / b.y.Eval(env)
}
panic(fmt.Sprintf("unsupported binary operator: %q", b.op))
}
func (c call) Eval(env Env) float64 {
switch c.fn {
case "pow":
return math.Pow(c.args[0].Eval(env), c.args[1].Eval(env))
case "sin":
return math.Sin(c.args[0].Eval(env))
case "sqrt":
return math.Sqrt(c.args[0].Eval(env))
}
panic(fmt.Sprintf("unsupported function call: %s", c.fn))
}
//!-Eval2
eval/eval_test.go¶
/gopl.io/ch7/eval/eval_test.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package eval
import (
"fmt"
"math"
"testing"
)
//!+Eval
func TestEval(t *testing.T) {
tests := []struct {
expr string
env Env
want string
}{
{"sqrt(A / pi)", Env{"A": 87616, "pi": math.Pi}, "167"},
{"pow(x, 3) + pow(y, 3)", Env{"x": 12, "y": 1}, "1729"},
{"pow(x, 3) + pow(y, 3)", Env{"x": 9, "y": 10}, "1729"},
{"5 / 9 * (F - 32)", Env{"F": -40}, "-40"},
{"5 / 9 * (F - 32)", Env{"F": 32}, "0"},
{"5 / 9 * (F - 32)", Env{"F": 212}, "100"},
//!-Eval
// additional tests that don't appear in the book
{"-1 + -x", Env{"x": 1}, "-2"},
{"-1 - x", Env{"x": 1}, "-2"},
//!+Eval
}
var prevExpr string
for _, test := range tests {
// Print expr only when it changes.
if test.expr != prevExpr {
fmt.Printf("\n%s\n", test.expr)
prevExpr = test.expr
}
expr, err := Parse(test.expr)
if err != nil {
t.Error(err) // parse error
continue
}
got := fmt.Sprintf("%.6g", expr.Eval(test.env))
fmt.Printf("\t%v => %s\n", test.env, got)
if got != test.want {
t.Errorf("%s.Eval() in %v = %q, want %q\n",
test.expr, test.env, got, test.want)
}
}
}
//!-Eval
/*
//!+output
sqrt(A / pi)
map[A:87616 pi:3.141592653589793] => 167
pow(x, 3) + pow(y, 3)
map[x:12 y:1] => 1729
map[x:9 y:10] => 1729
5 / 9 * (F - 32)
map[F:-40] => -40
map[F:32] => 0
map[F:212] => 100
//!-output
// Additional outputs that don't appear in the book.
-1 - x
map[x:1] => -2
-1 + -x
map[x:1] => -2
*/
func TestErrors(t *testing.T) {
for _, test := range []struct{ expr, wantErr string }{
{"x % 2", "unexpected '%'"},
{"math.Pi", "unexpected '.'"},
{"!true", "unexpected '!'"},
{`"hello"`, "unexpected '\"'"},
{"log(10)", `unknown function "log"`},
{"sqrt(1, 2)", "call to sqrt has 2 args, want 1"},
} {
expr, err := Parse(test.expr)
if err == nil {
vars := make(map[Var]bool)
err = expr.Check(vars)
if err == nil {
t.Errorf("unexpected success: %s", test.expr)
continue
}
}
fmt.Printf("%-20s%v\n", test.expr, err) // (for book)
if err.Error() != test.wantErr {
t.Errorf("got error %s, want %s", err, test.wantErr)
}
}
}
/*
//!+errors
x % 2 unexpected '%'
math.Pi unexpected '.'
!true unexpected '!'
"hello" unexpected '"'
log(10) unknown function "log"
sqrt(1, 2) call to sqrt has 2 args, want 1
//!-errors
*/
eval/parse.go¶
/gopl.io/ch7/eval/parse.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package eval
import (
"fmt"
"strconv"
"strings"
"text/scanner"
)
// ---- lexer ----
// This lexer is similar to the one described in Chapter 13.
type lexer struct {
scan scanner.Scanner
token rune // current lookahead token
}
func (lex *lexer) next() { lex.token = lex.scan.Scan() }
func (lex *lexer) text() string { return lex.scan.TokenText() }
type lexPanic string
// describe returns a string describing the current token, for use in errors.
func (lex *lexer) describe() string {
switch lex.token {
case scanner.EOF:
return "end of file"
case scanner.Ident:
return fmt.Sprintf("identifier %s", lex.text())
case scanner.Int, scanner.Float:
return fmt.Sprintf("number %s", lex.text())
}
return fmt.Sprintf("%q", rune(lex.token)) // any other rune
}
func precedence(op rune) int {
switch op {
case '*', '/':
return 2
case '+', '-':
return 1
}
return 0
}
// ---- parser ----
// Parse parses the input string as an arithmetic expression.
//
// expr = num a literal number, e.g., 3.14159
// | id a variable name, e.g., x
// | id '(' expr ',' ... ')' a function call
// | '-' expr a unary operator (+-)
// | expr '+' expr a binary operator (+-*/)
//
func Parse(input string) (_ Expr, err error) {
defer func() {
switch x := recover().(type) {
case nil:
// no panic
case lexPanic:
err = fmt.Errorf("%s", x)
default:
// unexpected panic: resume state of panic.
panic(x)
}
}()
lex := new(lexer)
lex.scan.Init(strings.NewReader(input))
lex.scan.Mode = scanner.ScanIdents | scanner.ScanInts | scanner.ScanFloats
lex.next() // initial lookahead
e := parseExpr(lex)
if lex.token != scanner.EOF {
return nil, fmt.Errorf("unexpected %s", lex.describe())
}
return e, nil
}
func parseExpr(lex *lexer) Expr { return parseBinary(lex, 1) }
// binary = unary ('+' binary)*
// parseBinary stops when it encounters an
// operator of lower precedence than prec1.
func parseBinary(lex *lexer, prec1 int) Expr {
lhs := parseUnary(lex)
for prec := precedence(lex.token); prec >= prec1; prec-- {
for precedence(lex.token) == prec {
op := lex.token
lex.next() // consume operator
rhs := parseBinary(lex, prec+1)
lhs = binary{op, lhs, rhs}
}
}
return lhs
}
// unary = '+' expr | primary
func parseUnary(lex *lexer) Expr {
if lex.token == '+' || lex.token == '-' {
op := lex.token
lex.next() // consume '+' or '-'
return unary{op, parseUnary(lex)}
}
return parsePrimary(lex)
}
// primary = id
// | id '(' expr ',' ... ',' expr ')'
// | num
// | '(' expr ')'
func parsePrimary(lex *lexer) Expr {
switch lex.token {
case scanner.Ident:
id := lex.text()
lex.next() // consume Ident
if lex.token != '(' {
return Var(id)
}
lex.next() // consume '('
var args []Expr
if lex.token != ')' {
for {
args = append(args, parseExpr(lex))
if lex.token != ',' {
break
}
lex.next() // consume ','
}
if lex.token != ')' {
msg := fmt.Sprintf("got %q, want ')'", lex.token)
panic(lexPanic(msg))
}
}
lex.next() // consume ')'
return call{id, args}
case scanner.Int, scanner.Float:
f, err := strconv.ParseFloat(lex.text(), 64)
if err != nil {
panic(lexPanic(err.Error()))
}
lex.next() // consume number
return literal(f)
case '(':
lex.next() // consume '('
e := parseExpr(lex)
if lex.token != ')' {
msg := fmt.Sprintf("got %s, want ')'", lex.describe())
panic(lexPanic(msg))
}
lex.next() // consume ')'
return e
}
msg := fmt.Sprintf("unexpected %s", lex.describe())
panic(lexPanic(msg))
}
eval/print.go¶
/gopl.io/ch7/eval/print.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
package eval
import (
"bytes"
"fmt"
)
// Format formats an expression as a string.
// It does not attempt to remove unnecessary parens.
func Format(e Expr) string {
var buf bytes.Buffer
write(&buf, e)
return buf.String()
}
func write(buf *bytes.Buffer, e Expr) {
switch e := e.(type) {
case literal:
fmt.Fprintf(buf, "%g", e)
case Var:
fmt.Fprintf(buf, "%s", e)
case unary:
fmt.Fprintf(buf, "(%c", e.op)
write(buf, e.x)
buf.WriteByte(')')
case binary:
buf.WriteByte('(')
write(buf, e.x)
fmt.Fprintf(buf, " %c ", e.op)
write(buf, e.y)
buf.WriteByte(')')
case call:
fmt.Fprintf(buf, "%s(", e.fn)
for i, arg := range e.args {
if i > 0 {
buf.WriteString(", ")
}
write(buf, arg)
}
buf.WriteByte(')')
default:
panic(fmt.Sprintf("unknown Expr: %T", e))
}
}
http1/main.go¶
/gopl.io/ch7/http1/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 191.
// Http1 is a rudimentary e-commerce server.
package main
import (
"fmt"
"log"
"net/http"
)
//!+main
func main() {
db := database{"shoes": 50, "socks": 5}
log.Fatal(http.ListenAndServe("localhost:8000", db))
}
type dollars float32
func (d dollars) String() string { return fmt.Sprintf("$%.2f", d) }
type database map[string]dollars
func (db database) ServeHTTP(w http.ResponseWriter, req *http.Request) {
for item, price := range db {
fmt.Fprintf(w, "%s: %s\n", item, price)
}
}
//!-main
/*
//!+handler
package http
type Handler interface {
ServeHTTP(w ResponseWriter, r *Request)
}
func ListenAndServe(address string, h Handler) error
//!-handler
*/
http2/main.go¶
/gopl.io/ch7/http2/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 192.
// Http2 is an e-commerce server with /list and /price endpoints.
package main
import (
"fmt"
"log"
"net/http"
)
func main() {
db := database{"shoes": 50, "socks": 5}
log.Fatal(http.ListenAndServe("localhost:8000", db))
}
type dollars float32
func (d dollars) String() string { return fmt.Sprintf("$%.2f", d) }
type database map[string]dollars
//!+handler
func (db database) ServeHTTP(w http.ResponseWriter, req *http.Request) {
switch req.URL.Path {
case "/list":
for item, price := range db {
fmt.Fprintf(w, "%s: %s\n", item, price)
}
case "/price":
item := req.URL.Query().Get("item")
price, ok := db[item]
if !ok {
w.WriteHeader(http.StatusNotFound) // 404
fmt.Fprintf(w, "no such item: %q\n", item)
return
}
fmt.Fprintf(w, "%s\n", price)
default:
w.WriteHeader(http.StatusNotFound) // 404
fmt.Fprintf(w, "no such page: %s\n", req.URL)
}
}
//!-handler
http3/main.go¶
/gopl.io/ch7/http3/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 194.
// Http3 is an e-commerce server that registers the /list and /price
// endpoints by calling (*http.ServeMux).Handle.
package main
import (
"fmt"
"log"
"net/http"
)
type dollars float32
func (d dollars) String() string { return fmt.Sprintf("$%.2f", d) }
//!+main
func main() {
db := database{"shoes": 50, "socks": 5}
mux := http.NewServeMux()
mux.Handle("/list", http.HandlerFunc(db.list))
mux.Handle("/price", http.HandlerFunc(db.price))
log.Fatal(http.ListenAndServe("localhost:8000", mux))
}
type database map[string]dollars
func (db database) list(w http.ResponseWriter, req *http.Request) {
for item, price := range db {
fmt.Fprintf(w, "%s: %s\n", item, price)
}
}
func (db database) price(w http.ResponseWriter, req *http.Request) {
item := req.URL.Query().Get("item")
price, ok := db[item]
if !ok {
w.WriteHeader(http.StatusNotFound) // 404
fmt.Fprintf(w, "no such item: %q\n", item)
return
}
fmt.Fprintf(w, "%s\n", price)
}
//!-main
/*
//!+handlerfunc
package http
type HandlerFunc func(w ResponseWriter, r *Request)
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
f(w, r)
}
//!-handlerfunc
*/
http3a/main.go¶
/gopl.io/ch7/http3a/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 195.
// Http3a is an e-commerce server that registers the /list and /price
// endpoints by calling (*http.ServeMux).HandleFunc.
package main
import (
"fmt"
"log"
"net/http"
)
func main() {
db := database{"shoes": 50, "socks": 5}
mux := http.NewServeMux()
//!+main
mux.HandleFunc("/list", db.list)
mux.HandleFunc("/price", db.price)
//!-main
log.Fatal(http.ListenAndServe("localhost:8000", mux))
}
type database map[string]int
func (db database) list(w http.ResponseWriter, req *http.Request) {
for item, price := range db {
fmt.Fprintf(w, "%s: $%d\n", item, price)
}
}
func (db database) price(w http.ResponseWriter, req *http.Request) {
item := req.URL.Query().Get("item")
if price, ok := db[item]; ok {
fmt.Fprintf(w, "$%d\n", price)
} else {
w.WriteHeader(http.StatusNotFound) // 404
fmt.Fprintf(w, "no such item: %q\n", item)
}
}
/*
//!+handlerfunc
package http
type HandlerFunc func(w ResponseWriter, r *Request)
func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
f(w, r)
}
//!-handlerfunc
*/
http4/main.go¶
/gopl.io/ch7/http4/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 195.
// Http4 is an e-commerce server that registers the /list and /price
// endpoint by calling http.HandleFunc.
package main
import (
"fmt"
"log"
"net/http"
)
//!+main
func main() {
db := database{"shoes": 50, "socks": 5}
http.HandleFunc("/list", db.list)
http.HandleFunc("/price", db.price)
log.Fatal(http.ListenAndServe("localhost:8000", nil))
}
//!-main
type dollars float32
func (d dollars) String() string { return fmt.Sprintf("$%.2f", d) }
type database map[string]dollars
func (db database) list(w http.ResponseWriter, req *http.Request) {
for item, price := range db {
fmt.Fprintf(w, "%s: %s\n", item, price)
}
}
func (db database) price(w http.ResponseWriter, req *http.Request) {
item := req.URL.Query().Get("item")
if price, ok := db[item]; ok {
fmt.Fprintf(w, "%s\n", price)
} else {
w.WriteHeader(http.StatusNotFound) // 404
fmt.Fprintf(w, "no such item: %q\n", item)
}
}
sleep/sleep.go¶
/gopl.io/ch7/sleep/sleep.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 179.
// The sleep program sleeps for a specified period of time.
package main
import (
"flag"
"fmt"
"time"
)
//!+sleep
var period = flag.Duration("period", 1*time.Second, "sleep period")
func main() {
flag.Parse()
fmt.Printf("Sleeping for %v...", *period)
time.Sleep(*period)
fmt.Println()
}
//!-sleep
sorting/main.go¶
/gopl.io/ch7/sorting/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 187.
// Sorting sorts a music playlist into a variety of orders.
package main
import (
"fmt"
"os"
"sort"
"text/tabwriter"
"time"
)
//!+main
type Track struct {
Title string
Artist string
Album string
Year int
Length time.Duration
}
var tracks = []*Track{
{"Go", "Delilah", "From the Roots Up", 2012, length("3m38s")},
{"Go", "Moby", "Moby", 1992, length("3m37s")},
{"Go Ahead", "Alicia Keys", "As I Am", 2007, length("4m36s")},
{"Ready 2 Go", "Martin Solveig", "Smash", 2011, length("4m24s")},
}
func length(s string) time.Duration {
d, err := time.ParseDuration(s)
if err != nil {
panic(s)
}
return d
}
//!-main
//!+printTracks
func printTracks(tracks []*Track) {
const format = "%v\t%v\t%v\t%v\t%v\t\n"
tw := new(tabwriter.Writer).Init(os.Stdout, 0, 8, 2, ' ', 0)
fmt.Fprintf(tw, format, "Title", "Artist", "Album", "Year", "Length")
fmt.Fprintf(tw, format, "-----", "------", "-----", "----", "------")
for _, t := range tracks {
fmt.Fprintf(tw, format, t.Title, t.Artist, t.Album, t.Year, t.Length)
}
tw.Flush() // calculate column widths and print table
}
//!-printTracks
//!+artistcode
type byArtist []*Track
func (x byArtist) Len() int { return len(x) }
func (x byArtist) Less(i, j int) bool { return x[i].Artist < x[j].Artist }
func (x byArtist) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
//!-artistcode
//!+yearcode
type byYear []*Track
func (x byYear) Len() int { return len(x) }
func (x byYear) Less(i, j int) bool { return x[i].Year < x[j].Year }
func (x byYear) Swap(i, j int) { x[i], x[j] = x[j], x[i] }
//!-yearcode
func main() {
fmt.Println("byArtist:")
sort.Sort(byArtist(tracks))
printTracks(tracks)
fmt.Println("\nReverse(byArtist):")
sort.Sort(sort.Reverse(byArtist(tracks)))
printTracks(tracks)
fmt.Println("\nbyYear:")
sort.Sort(byYear(tracks))
printTracks(tracks)
fmt.Println("\nCustom:")
//!+customcall
sort.Sort(customSort{tracks, func(x, y *Track) bool {
if x.Title != y.Title {
return x.Title < y.Title
}
if x.Year != y.Year {
return x.Year < y.Year
}
if x.Length != y.Length {
return x.Length < y.Length
}
return false
}})
//!-customcall
printTracks(tracks)
}
/*
//!+artistoutput
Title Artist Album Year Length
----- ------ ----- ---- ------
Go Ahead Alicia Keys As I Am 2007 4m36s
Go Delilah From the Roots Up 2012 3m38s
Ready 2 Go Martin Solveig Smash 2011 4m24s
Go Moby Moby 1992 3m37s
//!-artistoutput
//!+artistrevoutput
Title Artist Album Year Length
----- ------ ----- ---- ------
Go Moby Moby 1992 3m37s
Ready 2 Go Martin Solveig Smash 2011 4m24s
Go Delilah From the Roots Up 2012 3m38s
Go Ahead Alicia Keys As I Am 2007 4m36s
//!-artistrevoutput
//!+yearoutput
Title Artist Album Year Length
----- ------ ----- ---- ------
Go Moby Moby 1992 3m37s
Go Ahead Alicia Keys As I Am 2007 4m36s
Ready 2 Go Martin Solveig Smash 2011 4m24s
Go Delilah From the Roots Up 2012 3m38s
//!-yearoutput
//!+customout
Title Artist Album Year Length
----- ------ ----- ---- ------
Go Moby Moby 1992 3m37s
Go Delilah From the Roots Up 2012 3m38s
Go Ahead Alicia Keys As I Am 2007 4m36s
Ready 2 Go Martin Solveig Smash 2011 4m24s
//!-customout
*/
//!+customcode
type customSort struct {
t []*Track
less func(x, y *Track) bool
}
func (x customSort) Len() int { return len(x.t) }
func (x customSort) Less(i, j int) bool { return x.less(x.t[i], x.t[j]) }
func (x customSort) Swap(i, j int) { x.t[i], x.t[j] = x.t[j], x.t[i] }
//!-customcode
func init() {
//!+ints
values := []int{3, 1, 4, 1}
fmt.Println(sort.IntsAreSorted(values)) // "false"
sort.Ints(values)
fmt.Println(values) // "[1 1 3 4]"
fmt.Println(sort.IntsAreSorted(values)) // "true"
sort.Sort(sort.Reverse(sort.IntSlice(values)))
fmt.Println(values) // "[4 3 1 1]"
fmt.Println(sort.IntsAreSorted(values)) // "false"
//!-ints
}
surface/surface.go¶
/gopl.io/ch7/surface/surface.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 203.
// The surface program plots the 3-D surface of a user-provided function.
package main
import (
"fmt"
"io"
"log"
"math"
"net/http"
)
//!+parseAndCheck
import "gopl.io/ch7/eval"
//!-parseAndCheck
// -- copied from gopl.io/ch3/surface --
const (
width, height = 600, 320 // canvas size in pixels
cells = 100 // number of grid cells
xyrange = 30.0 // x, y axis range (-xyrange..+xyrange)
xyscale = width / 2 / xyrange // pixels per x or y unit
zscale = height * 0.4 // pixels per z unit
)
var sin30, cos30 = 0.5, math.Sqrt(3.0 / 4.0) // sin(30°), cos(30°)
func corner(f func(x, y float64) float64, i, j int) (float64, float64) {
// find point (x,y) at corner of cell (i,j)
x := xyrange * (float64(i)/cells - 0.5)
y := xyrange * (float64(j)/cells - 0.5)
z := f(x, y) // compute surface height z
// project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy)
sx := width/2 + (x-y)*cos30*xyscale
sy := height/2 + (x+y)*sin30*xyscale - z*zscale
return sx, sy
}
func surface(w io.Writer, f func(x, y float64) float64) {
fmt.Fprintf(w, "<svg xmlns='http://www.w3.org/2000/svg' "+
"style='stroke: grey; fill: white; stroke-width: 0.7' "+
"width='%d' height='%d'>", width, height)
for i := 0; i < cells; i++ {
for j := 0; j < cells; j++ {
ax, ay := corner(f, i+1, j)
bx, by := corner(f, i, j)
cx, cy := corner(f, i, j+1)
dx, dy := corner(f, i+1, j+1)
fmt.Fprintf(w, "<polygon points='%g,%g %g,%g %g,%g %g,%g'/>\n",
ax, ay, bx, by, cx, cy, dx, dy)
}
}
fmt.Fprintln(w, "</svg>")
}
// -- main code for gopl.io/ch7/surface --
//!+parseAndCheck
func parseAndCheck(s string) (eval.Expr, error) {
if s == "" {
return nil, fmt.Errorf("empty expression")
}
expr, err := eval.Parse(s)
if err != nil {
return nil, err
}
vars := make(map[eval.Var]bool)
if err := expr.Check(vars); err != nil {
return nil, err
}
for v := range vars {
if v != "x" && v != "y" && v != "r" {
return nil, fmt.Errorf("undefined variable: %s", v)
}
}
return expr, nil
}
//!-parseAndCheck
//!+plot
func plot(w http.ResponseWriter, r *http.Request) {
r.ParseForm()
expr, err := parseAndCheck(r.Form.Get("expr"))
if err != nil {
http.Error(w, "bad expr: "+err.Error(), http.StatusBadRequest)
return
}
w.Header().Set("Content-Type", "image/svg+xml")
surface(w, func(x, y float64) float64 {
r := math.Hypot(x, y) // distance from (0,0)
return expr.Eval(eval.Env{"x": x, "y": y, "r": r})
})
}
//!-plot
//!+main
func main() {
http.HandleFunc("/plot", plot)
log.Fatal(http.ListenAndServe("localhost:8000", nil))
}
//!-main
tempconv/tempconv.go¶
/gopl.io/ch7/tempconv/tempconv.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 180.
// Package tempconv performs Celsius and Fahrenheit temperature computations.
package tempconv
import (
"flag"
"fmt"
)
type Celsius float64
type Fahrenheit float64
func CToF(c Celsius) Fahrenheit { return Fahrenheit(c*9.0/5.0 + 32.0) }
func FToC(f Fahrenheit) Celsius { return Celsius((f - 32.0) * 5.0 / 9.0) }
func (c Celsius) String() string { return fmt.Sprintf("%0.2f°C", c) }
func (f Fahrenheit) String() string { return fmt.Sprintf("%0.2f°F", f) }
/*
//!+flagvalue
package flag
// Value is the interface to the value stored in a flag.
type Value interface {
String() string
Set(string) error
}
//!-flagvalue
*/
//!+celsiusFlag
// *celsiusFlag satisfies the flag.Value interface.
type celsiusFlag struct{ Celsius }
func (f *celsiusFlag) Set(s string) error {
var unit string
var value float64
fmt.Sscanf(s, "%f%s", &value, &unit) // no error check needed
switch unit {
case "C", "°C":
f.Celsius = Celsius(value)
return nil
case "F", "°F":
f.Celsius = FToC(Fahrenheit(value))
return nil
}
return fmt.Errorf("invalid temperature %q", s)
}
//!-celsiusFlag
//!+CelsiusFlag
// CelsiusFlag defines a Celsius flag with the specified name,
// default value, and usage, and returns the address of the flag variable.
// The flag argument must have a quantity and a unit, e.g., "100C".
func CelsiusFlag(name string, value Celsius, usage string) *Celsius {
f := celsiusFlag{value} //定义celsiusFlag类型、celsiusFlag实现了flag.Value 接口(String、Set)
flag.CommandLine.Var(&f, name, usage)
return &f.Celsius
}
//!-CelsiusFlag
tempflag/tempflag.go¶
/gopl.io/ch7/tempflag/tempflag.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 181.
// Tempflag prints the value of its -temp (temperature) flag.
package main
import (
"flag"
"fmt"
"gopl.io/ch7/tempconv"
)
//!+
var temp = tempconv.CelsiusFlag("temp", 20.0, "the temperature")
func main() {
flag.Parse()
fmt.Printf("%s, %s\n",*temp, tempconv.CToF(*temp) )
fmt.Printf("%v",flag.CommandLine.NArg())
}
//!-
xmlselect/main.go¶
/gopl.io/ch7/xmlselect/main.go¶
// Copyright © 2016 Alan A. A. Donovan & Brian W. Kernighan.
// License: https://creativecommons.org/licenses/by-nc-sa/4.0/
// See page 214.
//!+
// Xmlselect prints the text of selected elements of an XML document.
package main
import (
"encoding/xml"
"fmt"
"io"
"os"
"strings"
)
func main() {
dec := xml.NewDecoder(os.Stdin)
var stack []string // stack of element names
for {
tok, err := dec.Token()
if err == io.EOF {
break
} else if err != nil {
fmt.Fprintf(os.Stderr, "xmlselect: %v\n", err)
os.Exit(1)
}
switch tok := tok.(type) {
case xml.StartElement:
stack = append(stack, tok.Name.Local) // push
case xml.EndElement:
stack = stack[:len(stack)-1] // pop
case xml.CharData:
if containsAll(stack, os.Args[1:]) {
fmt.Printf("%s: %s\n", strings.Join(stack, " "), tok)
}
}
}
}
// containsAll reports whether x contains the elements of y, in order.
func containsAll(x, y []string) bool {
for len(y) <= len(x) {
if len(y) == 0 {
return true
}
if x[0] == y[0] {
y = y[1:]
}
x = x[1:]
}
return false
}
//!-