701 lines
13 KiB
Text
701 lines
13 KiB
Text
Базовые конструкции языка
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Лекция 2
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Фёдор Короткий
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* Имена
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25 ключевых слов.
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break default func interface select
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case defer go map struct
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chan else goto package switch
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const fallthrough if range type
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continue for import return var
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* Predeclated identifiers
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Константы
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true false iota nil
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Типы
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int int8 int16 int32 int64
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uint uint8 uint16 uint32 uint64 uintptr
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float32 float64 complex128 complex64
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bool byte rune string error
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Функции
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make len cap new append copy close delete
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complex real imag
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panic recover
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Идентификаторы можно переопределять по обычным правилам
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// just example, don't do this
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var true = false
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* Exported identifiers
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Регистр первой буквы определяет видимость имени за пределами пакета
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.play exported/example.go
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Пакеты всегда называют в нижнем регистре и без подчеркиваний
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fmt
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grpcmiddleware
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* Declaration
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- 4 типы объявлений `var`, `const`, `type`, `func`
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- Пакет состоит из множества `.go` файлов
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- Порядок объявлений в коде не важен
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* var
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Общая форма
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var name type = expression
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Примеры
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var i, j, k int // int, int, int
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var b, f, s = true, 2.3, "four" // bool, float64, string
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var f, err = os.Open(name) // os.Open returns a file and an error
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* short variable declaration
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i, j := 0, 1
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Существующим переменным присваиваются новые значения
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in, err := os.Open(infile)
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// ...
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out, err := os.Create(outfile)
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Но должна объявляться хотябы одна новая переменная
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f, err := os.Open(infile)
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// ...
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f, err := os.Create(outfile) // compile error: no new variables
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* pointers
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x := 1
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p := &x // p, of type *int, points to x
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fmt.Println(*p) // "1"
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*p = 2
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fmt.Println(x)
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* escape analysis
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var p = f()
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func f() *int {
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v := 1
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return &v
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}
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* flag
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.play flag/main.go
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* new
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`new(T)` создаёт новую переменную с типом `T`.
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func newInt() *int {
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return new(int)
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}
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func newInt() *int {
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var dummy int
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return &dummy
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}
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* zero size type
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p := new(int)
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q := new(int)
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fmt.Println(p == q) // "false"
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p := new(struct{})
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q := new(struct{})
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fmt.Println(p == q) // "true" or "false", depending on implementation
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* variable lifetime
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Память освобождается, после того как переменая становится недостижимой.
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Компилятор может переместить переменную со стека на кучу.
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var global *int
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func f() {
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var x int
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x = 1
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global = &x
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}
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И с кучи на стек.
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func g() {
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y := new(int)
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*y = 1
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}
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* type declaration
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type name underlying-type
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.play tempconv0/conv.go
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* packages
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- Файл `tempconv/types.go`
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.play tempconv/tempconv.go
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- Файл `tempconv/conv.go`
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.play tempconv/conv.go
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* packages
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fmt.Printf("Brrrr! %v\n", tempconv.AbsoluteZeroC)
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fmt.Println(tempconv.CToF(tempconv.BoilingC))
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* package initialization
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var a = b + c // a initialized third, to 3
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var b = f() // b initialized second, to 2, by calling f
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var c = 1 // c initialized first, to 1
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func f() int { return c + 1 }
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* package initialization
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.play popcount/popcount.go
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* scope
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.play scope/scope.go
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* scope
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.play scope/if.go
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* scope if
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- Неправильно
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if f, err := os.Open(fname); err != nil { // compile error: unused: f
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return err
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}
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f.ReadByte() // compile error: undefined f
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f.Close() // compile error: undefined f
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- Правильно
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f, err := os.Open(fname)
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if err != nil {
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return err
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}
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f.ReadByte()
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f.Close()
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* scope
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var cwd string
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func init() {
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cwd, err := os.Getwd() // NOTE: wrong!
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if err != nil {
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log.Fatalf("os.Getwd failed: %v", err)
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}
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log.Printf("Working directory = %s", cwd)
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}
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* strings
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`string` - неизменяемая последовательность байт.
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`s[i]` - обращается к i-тому байту (не символу).
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var s = "hello"
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var doc = `Go is a tool for managing Go source code.
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Usage:
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go command [arguments]
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...
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`
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* unicode
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Символы кодируются числами.
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type rune int32
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Кодировка utf8
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0xxxxxx runes 0−127
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11xxxxx 10xxxxxx 128−2047
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110xxxx 10xxxxxx 10xxxxxx 2048−65535
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1110xxx 10xxxxxx 10xxxxxx 10xxxxxx 65536−0x10ffff
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Разница между рунами и байтами
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import "unicode/utf8"
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func countRunes() {
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s := "Hello, 世界"
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fmt.Println(len(s)) // "13"
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fmt.Println(utf8.RuneCountInString(s)) // "9"
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}
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* utf8
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for i := 0; i < len(s); {
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r, size := utf8.DecodeRuneInString(s[i:])
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fmt.Printf("%d\t%c\n", i, r)
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i += size
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}
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Декодирование utf8 встроено в язык
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for i, r := range "Hello, 世界" {
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fmt.Printf("%d\t%q\t%d\n", i, r, r)
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}
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runes := []rune("Hello, 世界")
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s := string(runes)
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Некорректный байт превращается _unicode_replacement_character_ `'\uFFFD'`.
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- Может ли строка `string([]rune(s))` быть больше `s`?
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* stdlib
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- `strings` - HasSuffix, Split, Join, etc.
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- `bytes` - аналог `strings` для `[]byte`.
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- `unicode` - IsDigit, IsLetter.
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- `strconv` - конвертация между строкой и `int`, `float`.
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- `path` - работа с unix путями
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- `filepath` - работа с путями текущей платформы
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* []byte
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s := "abc"
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b := []byte(s)
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s2 := string(b)
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* bytes.Buffer
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func intsToString(values []int) string {
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var buf bytes.Buffer
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buf.WriteByte('[')
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for i, v := range values {
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if i > 0 {
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buf.WriteString(", ")
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}
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fmt.Fprintf(&buf, "%d", v)
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}
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buf.WriteByte(']')
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return buf.String()
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}
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* constants
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const (
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a = 1
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b
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c = 2
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d
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)
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fmt.Println(a, b, c, d) // "1 1 2 2"
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type Weekday int
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const (
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Sunday Weekday = iota
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Monday
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Tuesday
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)
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type Flags uint
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const (
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FlagUp Flags = 1 << iota // is up
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FlagBroadcast // supports broadcast access capability
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FlagLoopback // is a loopback interface
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FlagPointToPoint // belongs to a point-to-point link
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FlagMulticast // supports multicast access capability
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)
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* untyped constants
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const (
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_ = 1 << (10 * iota)
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KiB
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MiB
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GiB
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TiB // (exceeds 1 << 32)
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PiB
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EiB
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ZiB // (exceeds 1 << 64)
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YiB
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)
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- Компилятор гарантирует как минимум 256 бит точности
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fmt.Println(YiB/ZiB) // "1024"
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* arrays
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var a [3]int
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var q [3]int = [3]int{1, 2, 3}
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var r [3]int = [3]int{1, 2}
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d := [...]int{1, 2, 3}
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h := [...]int{99: -1}
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- arrays are values
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func zero(ptr *[32]byte) {
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*ptr = [32]byte{}
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}
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* slices
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- slice это тройка `data`, `len`, `cap`.
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s := make([]int, 10)
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s = s[:0]
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s = s[:10]
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* slices
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// reverse reverses a slice of ints in place.
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func reverse(s []int) {
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for i, j := 0, len(s)-1; i < j; i, j = i+1, j-1 {
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s[i], s[j] = s[j], s[i]
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}
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}
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* slices
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var a, b []string
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fmt.Println(a == b)
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var c []int
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d := []int{}
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fmt.Println(c != d)
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// c == nil && d != nil
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// len(c) == 0 && len(d) == 0
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* slices
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make([]T, len) // len == cap
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make([]T, len, cap)
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func append(s []T, elem ...T) []T
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var s []int
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s = append(s, 1)
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s = append(s, 2, 3)
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var a, b []int
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a = append(a, b...)
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* stack
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stack = append(stack, v) // push v
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top := stack[len(stack)-1] // top of stack
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stack = stack[:len(stack)-1] // pop
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* copy
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func remove(slice []int, i int) []int {
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copy(slice[i:], slice[i+1:])
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return slice[:len(slice)-1]
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}
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* maps
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ages := make(map[string]int)
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ages := map[string]int{
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"alice": 31,
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"charlie": 34,
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}
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ages := make(map[string]int)
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ages["alice"] = 31
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ages["charlie"] = 34
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* maps
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ages["alice"] = 32
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fmt.Println(ages["alice"]) // "32"
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fmt.Println(ages["bob"]) // "0"
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delete(ages, "alice")
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ages["bob"] += 1
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ages["bob"]++
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_ = &ages["bob"] // compile error: cannot take address of map element
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* maps
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for name, age := range ages {
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fmt.Printf("%s\t%d\n", name, age)
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}
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var ages map[string]int
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fmt.Println(ages == nil) // "true"
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fmt.Println(len(ages) == 0) // "true"
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fmt.Println(ages["alice"]) // "0"
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ages["alice"] = 21 // panic
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* maps
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age, ok := ages["bob"]
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if !ok { /* "bob" is not a key in this map; age == 0. */ }
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if age, ok := ages["bob"]; !ok { /* ... */ }
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* set
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var s0 map[string]bool
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var s1 map[string]struct{}
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* struct
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type Employee struct {
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ID int
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Name string
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Address string
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Salary int
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}
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var dilbert Employee
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dilbert.Salary -= 5000 // demoted, for writing too few lines of code
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* struct
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type tree struct {
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value int
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left, right *tree
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}
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* struct
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type Point struct{ X, Y int }
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p := Point{1, 2}
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p := Point{X: 1, Y: 2}
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* comparing structs
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type Point struct{ X, Y int }
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p := Point{1, 2}
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q := Point{2, 1}
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fmt.Println(p.X == q.X && p.Y == q.Y) // "false"
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fmt.Println(p == q) // "false"
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Можно использовать структуры как ключи
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type address struct {
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hostname string
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port int
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}
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hits := make(map[address]int)
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hits[address{"golang.org", 443}]++
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* struct embedding
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type Point struct {
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X, Y int
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}
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type Circle struct {
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Point
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Radius int
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}
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c := Circle{
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Point: Point{X: 10, Y: 10},
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Radius: 1,
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}
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c.X = 0
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* json
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type Movie struct {
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Title string
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Year int `json:"year"`
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Color bool `json:"color,omitempty"`
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Actors []string
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}
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- marshal
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data, err := json.Marshal(movies)
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if err != nil {
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log.Fatalf("JSON marshaling failed: %s", err)
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}
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fmt.Printf("%s\n", data)
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data, err := json.MarshalIndent(movies, "", " ")
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if err != nil {
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log.Fatalf("JSON marshaling failed: %s", err)
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}
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fmt.Printf("%s\n", data)
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* unmarshal
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var movie Movie
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if err := json.Unmarshal(data, &movie); err != nil {
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log.Fatalf("JSON unmarshaling failed: %s", err)
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}
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fmt.Println(movie)
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* github
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.play github/github.go /func Search/,/^}/
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* functions
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func name(parameter-list) (result-list) {
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body
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}
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Примеры
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func hypot(x, y float64) float64 {
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return math.Sqrt(x*x + y*y)
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}
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fmt.Println(hypot(3, 4)) // "5"
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func f(i, j, k int, s, t string) { /* ... */ }
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func f(i int, j int, k int, s string, t string) { /* ... */ }
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* multiple return values
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func Get(url string) (*http.Responce, error) {
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// ...
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if err != nil {
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return nil, err
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}
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// ...
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}
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func logAndGet(url string) (*http.Responce, error) {
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log.Printf("logAndGet %s", url)
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return Get(url)
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}
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* named return values
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.play countwords/main.go /func Count/,/^}/
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* errors
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resp, err := http.Get(url)
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if err != nil {
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return nil, err
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}
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Дополнительный контекст
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doc, err := html.Parse(resp.Body)
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if err != nil {
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return nil, fmt.Errorf("parsing %s as HTML: %w", url, err)
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}
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Текст ошибки должен быть в lowercase.
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genesis: crashed: no parachute: G-switch failed: bad relay orientation
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* EOF
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package io
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import "errors"
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// EOF is the error returned by Read when no more input is available.
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var EOF = errors.New("EOF")
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|
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in := bufio.NewReader(os.Stdin)
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for {
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r, _, err := in.ReadRune()
|
||
if err == io.EOF {
|
||
break // finished reading
|
||
}
|
||
|
||
if err != nil {
|
||
return fmt.Errorf("read failed: %v", err)
|
||
}
|
||
// ...use r...
|
||
}
|
||
|
||
* variadic functions
|
||
|
||
func sum(vals ...int) int {
|
||
total := 0
|
||
for _, val := range vals {
|
||
total += val
|
||
}
|
||
return total
|
||
}
|
||
|
||
fmt.Println(sum()) // "0"
|
||
fmt.Println(sum(3)) // "3"
|
||
fmt.Println(sum(1, 2, 3, 4)) // "10"
|
||
|
||
values := []int{1, 2, 3, 4}
|
||
fmt.Println(sum(values...))
|
||
fmt.Println(sum(0, values...)) // compilation error
|
||
|
||
* function values
|
||
|
||
func Inc(i int) int { return i + 1 }
|
||
|
||
var f func(i int) int
|
||
if f != nil {
|
||
f = Inc
|
||
}
|
||
|
||
f = func(i int) int {
|
||
return i * 2
|
||
}
|
||
|
||
* recursion
|
||
|
||
type Node struct {
|
||
V int
|
||
L, R *Node
|
||
}
|
||
|
||
func PrintAll(w io.Writer, root *Node) {
|
||
var visit func(n *Node)
|
||
visit = func(n *Node) {
|
||
fmt.Fprintln(w, n.V)
|
||
if n.L != nil {
|
||
visit(n.L)
|
||
}
|
||
if n.R != nil {
|
||
visit(n.R)
|
||
}
|
||
}
|
||
|
||
visit(root)
|
||
}
|