diff --git a/eBook/04.4.md b/eBook/04.4.md index f3b489c..fc6a68c 100644 --- a/eBook/04.4.md +++ b/eBook/04.4.md @@ -1,5 +1,5 @@ ##啊哦，亲，你看得也太快了。。。还没翻译完呢 0 0 -要不等到 ***2013 年 4 月 22 日*** 再来看看吧~~ +要不等到 ***2013 年 4 月 23 日*** 再来看看吧~~ 这里还有一些其它的学习资源噢~ @@ -13,7 +13,115 @@ - [Golang中国](http://golang.tc) #4.4 变量 +#4.4.1 简介 +声明变量的一般形式是使用 `var` 关键字：`var identifier type`。 +需要注意的是，Go 和许多编程语言不同，它在声明变量时将变量的类型放在变量的名称之后。Go 为什么要选择这么做呢？ + +首先，它是为了避免像 C 语言中那样含糊不清的声明形式，例如：`int* a, b;`。在这个例子中，只有 a 是指针而 b 不是。如果你想要这两个变量都是指针，则需要将它们分开书写（你可以在该页面找到有关于这个话题的更多讨论：[http://blog.golang.org/2010/07/gos-declaration-syntax.html](http://blog.golang.org/2010/07/gos-declaration-syntax.html)）。 + +而在 Go 中，则可以和轻松地将它们都声明为指针类型：`var a, b *int`。 + +其次，这种语法能够按照从左至右的顺序阅读，使得代码更加容易理解。 + +示例： + + var a int + var b bool + var str string + +你也可以改写成这种形式： + + var ( + a int + b bool + str string + ) + +这种因式分解关键字的写法一般用于声明全局变量。 + +当一个变量被声明之后，系统自动赋予它该类型的零值：int 为 0，flost 为 0.0，bool 为 false，string 为空字符串，指针为 nil。记住，所有的内存在 Go 中都是经过初始化的。 + +变量的命名规则遵循骆驼命名法，即首个单词小写，每个新单词的首字母大写，例如：`numShips`, `startDate`。 + +但如果你的全局变量希望能够被外部包所使用，则需要将首个单词的首字母也大写（第 4.2 节：可见性规则）。 + +一个变量（常量、类型或函数）在程序中都有一定的作用范围，称之为作用域。如果一个变量在函数体外声明，则被认为是全局变量，可以在整个包甚至外部包（被导出后）使用，不管你声明在哪个源文件里或在哪个源文件里调用该变量。 + +在函数体内声明的变量称之为局部变量，它们的作用域只在函数体内，参数和返回值变量也是局部变量。在第 5 章，我们将会学习到像 if 和 for 这些控制结构，而在这些结构中声明的变量的作用域只在相应的代码块内。一般情况下，局部变量的作用域可以通过代码块（用大括号括起来的部分）判断。 + +尽管变量的标识符必须是唯一的，但你可以在某个代码块的内层代码块中使用相同名称的变量，则此时外部的同名变量将会暂时隐藏（结束内部代码块的执行后隐藏的外部同名变量又会出现，而内部同名变量则被释放），你任何的操作都只会影响内部代码块的局部变量。 + +变量可以编译期间就被赋值，赋值给变量使用运算符等号 `=`，当然你也可以在运行时对变量进行赋值操作。 + +示例： + + a = 15 + b = false + +一般情况下，只有类型相同的变量之间才可以相互赋值，例如：`a = b`。 + +声明与赋值（初始化）语句也可以组合起来。 + +示例： + + var identifier [type] = value + var a int = 15 + var i = 5 + var b bool = false + var str string = “Go says hello to the world!” + +但是 Go 编译器的智商已经高到可以根据变量的值来自动推断其类型，这有点像 Ruby 和 Python 这类动态语言，只不过它们是在运行时进行推断，而 Go 是在编译时就已经完成推断过程。因此，你还可以使用下面的这些形式来声明及初始化变量： + + var a = 15 + var b = false + var str = “Go says hello to the world!” + +或： + + var ( + a = 15 + b = false + str = “Go says hello to the world!” + numShips = 50 + city string + ) + +不过自动推断类型并不是任何时候都适用的，当你想要给变量的类型并不是自动推断出的某种类型时，你还是需要显式指定变量的类型，例如：`var n int64 = 2`。 + +然而，`var a` 这种语法是不正确的，因为编译器没有任何可以用于自动推断类型的依据。变量的类型也可以在运行时实现自动推断，例如： + + var ( + HOME = os.Getenv(“HOME”) + USER = os.Getenv(“USER”) + GOROOT = os.Getenv(“GOROOT”) + ) + +这种写法主要用于声明包级别的全局变量，当你在函数体内声明局部变量时，应使用简短声明语法 `:=`，例如：`a := 1`。 + +下面这个例子展示了如何在运行时获取所在的操作系统类型，它通过 `os` 包中的函数 `os.Getenv()` 来获取环境变量中的值，并保存到 string 类型的局部变量 `path` 中。 + +Example 4.5 [goos.go](examples/chapter_4/goos.go) + + package main + + import ( + “fmt” + “os” + ) + + func main() { + var goos string = os.Getenv(“GOOS”) + fmt.Printf(“The operating system is: %s\n”, goos) + path := os.Getenv(“PATH”) + fmt.Printf(“Path is %s\n”, path) + } + +如果你在 Windows 下运行这段代码，则会输出 `The operating system is: windows` 以及相应的环境变量的值；如果你在 Linux 下运行这段代码，则会输出 `The operating system is: linux` 以及相应的的环境变量的值。 + +这里用到了 `Printf` 的格式化输出的功能（第 4.4.3 节）。 + +#4.4.2 值类型和引用类型 ##链接 - [目录](directory.md) diff --git a/eBook/examples/chapter_4/alias.go b/eBook/examples/chapter_4/alias.go index 87f5605..ba30f40 100644 --- a/eBook/examples/chapter_4/alias.go +++ b/eBook/examples/chapter_4/alias.go @@ -1,6 +1,7 @@ package main -import fm "fmt" // alias3 -func main() { - fm.Println("hello, world") +import fm "fmt" // alias + +func main() { + fm.Println("hello, world") } diff --git a/eBook/examples/chapter_4/casting.go b/eBook/examples/chapter_4/casting.go new file mode 100644 index 0000000..35850d6 --- /dev/null +++ b/eBook/examples/chapter_4/casting.go @@ -0,0 +1,18 @@ +package main +import "fmt" + +func main() { + var n int16 = 34 + var m int32 + +// compiler error: cannot use n (type int16) as type int32 in assignment + //m = n + m = int32(n) + + fmt.Printf("32 bit int is: %d\n", m) + fmt.Printf("16 bit int is: %d\n", n) +} +/* Output: +32 bit int is: 34 +16 bit int is: 34 +*/ \ No newline at end of file diff --git a/eBook/examples/chapter_4/char.go b/eBook/examples/chapter_4/char.go new file mode 100644 index 0000000..67e5668 --- /dev/null +++ b/eBook/examples/chapter_4/char.go @@ -0,0 +1,22 @@ +// char.go +package main + +import ( + "fmt" +) + +func main() { + var ch int = '\u0041' + var ch2 int = '\u03B2' + var ch3 int = '\U00101234' + fmt.Printf("%d - %d - %d\n", ch, ch2, ch3) + fmt.Printf("%c - %c - %c\n", ch, ch2, ch3) + fmt.Printf("%X - %X - %X\n", ch, ch2, ch3) + fmt.Printf("%U - %U - %U", ch, ch2, ch3) +} +/* Ouput: +65 - 946 - 1053236 +A - β - 􁈴 +41 - 3B2 - 101234 +U+0041 - U+03B2 - U+101234 +*/ \ No newline at end of file diff --git a/eBook/examples/chapter_4/count_substring.go b/eBook/examples/chapter_4/count_substring.go new file mode 100644 index 0000000..e97712f --- /dev/null +++ b/eBook/examples/chapter_4/count_substring.go @@ -0,0 +1,17 @@ +package main + +import ( + "fmt" + "strings" +) + +func main() { + var str string = "Hello, how is it going, Hugo?" + var manyG = "gggggggggg" + + fmt.Printf("Number of H's in %s is: ", str) + fmt.Printf("%d\n", strings.Count(str, "H")) + + fmt.Printf("Number of double g's in %s is: ", manyG) + fmt.Printf("%d\n", strings.Count(manyG, "gg")) +} diff --git a/eBook/examples/chapter_4/goos.go b/eBook/examples/chapter_4/goos.go new file mode 100644 index 0000000..c7f0741 --- /dev/null +++ b/eBook/examples/chapter_4/goos.go @@ -0,0 +1,13 @@ +package main + +import ( + "fmt" + "os" +) + +func main() { + var goos string = os.Getenv("GOOS") + fmt.Printf("The operating system is: %s\n", goos) + path := os.Getenv("PATH") + fmt.Printf("Path is %s\n", path) +} diff --git a/eBook/examples/chapter_4/gotemplate.go b/eBook/examples/chapter_4/gotemplate.go index b51a2e0..68de9ca 100644 --- a/eBook/examples/chapter_4/gotemplate.go +++ b/eBook/examples/chapter_4/gotemplate.go @@ -1,7 +1,7 @@ package main import ( - "fmt" + "fmt" ) const c = "C" @@ -10,20 +10,21 @@ var v int = 5 type T struct{} -func init() { // initialization of package +func init() { + // initialization of package } func main() { - var a int - Func1() - // ... - fmt.Println(a) + var a int + Func1() + // ... + fmt.Println(a) } func (t T) Method1() { - //... + //... } func Func1() { // exported function Func1 - //... + //... } diff --git a/eBook/examples/chapter_4/hello_world.go b/eBook/examples/chapter_4/hello_world.go index 53bd87c..c3cb0ae 100644 --- a/eBook/examples/chapter_4/hello_world.go +++ b/eBook/examples/chapter_4/hello_world.go @@ -1,9 +1,7 @@ -// hello_world.go package main -import "fmt" +import "fmt" -func main() { +func main() { fmt.Println("hello, world") } - diff --git a/eBook/examples/chapter_4/hello_world2.go b/eBook/examples/chapter_4/hello_world2.go index cc6ff23..e8e85c4 100644 --- a/eBook/examples/chapter_4/hello_world2.go +++ b/eBook/examples/chapter_4/hello_world2.go @@ -3,6 +3,5 @@ package main import "fmt" // Package implementing formatted I/O. func main() { - fmt.Printf("Καλημέρα κόσμε; or こんにちは 世界\n") + fmt.Printf("Hello, world; or Καλημέρα κόσμε; or こんにちは 世界\n") } - diff --git a/eBook/examples/chapter_4/index_in_string.go b/eBook/examples/chapter_4/index_in_string.go new file mode 100644 index 0000000..2964bfa --- /dev/null +++ b/eBook/examples/chapter_4/index_in_string.go @@ -0,0 +1,21 @@ +package main + +import ( + "fmt" + "strings" +) + +func main() { + var str string = "Hi, I'm Marc, Hi." + + fmt.Printf("The position of \"Marc\" is: ") + fmt.Printf("%d\n", strings.Index(str, "Marc")) + + fmt.Printf("The position of the first instance of \"Hi\" is: ") + fmt.Printf("%d\n", strings.Index(str, "Hi")) + fmt.Printf("The position of the last instance of \"Hi\" is: ") + fmt.Printf("%d\n", strings.LastIndex(str, "Hi")) + + fmt.Printf("The position of \"Burger\" is: ") + fmt.Printf("%d\n", strings.Index(str, "Burger")) +} diff --git a/eBook/examples/chapter_4/init.go b/eBook/examples/chapter_4/init.go new file mode 100644 index 0000000..3555ce4 --- /dev/null +++ b/eBook/examples/chapter_4/init.go @@ -0,0 +1,9 @@ +package trans + +import "math" + +var Pi float64 + +func init() { + Pi = 4 * math.Atan(1) // init() function computes Pi +} diff --git a/eBook/examples/chapter_4/pointer.go b/eBook/examples/chapter_4/pointer.go new file mode 100644 index 0000000..ff230fd --- /dev/null +++ b/eBook/examples/chapter_4/pointer.go @@ -0,0 +1,12 @@ +package main + +import "fmt" + +func main() { + var i1 = 5 + fmt.Printf("An integer: %d, its location in memory: %p\n", i1, &i1) + + var intP *int + intP = &i1 + fmt.Printf("The value at memory location %p is %d\n", intP, *intP) +} diff --git a/eBook/examples/chapter_4/presuffix.go b/eBook/examples/chapter_4/presuffix.go new file mode 100644 index 0000000..c7484f6 --- /dev/null +++ b/eBook/examples/chapter_4/presuffix.go @@ -0,0 +1,14 @@ +package main + +import ( + "fmt" + "strings" +) + +func main() { + var str string = "This is an example of a string" + + fmt.Printf("T/F? Does the string \"%s\" have prefix %s? ", str, "Th") + fmt.Printf("%t\n", strings.HasPrefix(str, "Th")) +} +// Output: T/F? Does the string "This is an example of a string" have prefix Th? true \ No newline at end of file diff --git a/eBook/examples/chapter_4/random.go b/eBook/examples/chapter_4/random.go new file mode 100644 index 0000000..9dc8e61 --- /dev/null +++ b/eBook/examples/chapter_4/random.go @@ -0,0 +1,28 @@ +package main + +import ( + "fmt" + "math/rand" + "time" +) + +func main() { + for i := 0; i < 10; i++ { + a := rand.Int() + fmt.Printf("%d / ", a) + } + for i := 0; i < 5; i++ { + r := rand.Intn(8) + fmt.Printf("%d / ", r) + } + fmt.Println() + timens := int64(time.Now().Nanosecond()) + rand.Seed(timens) + for i := 0; i < 10; i++ { + fmt.Printf("%2.2f / ", 100*rand.Float32()) + } +} +/* Output: +134020434 / 1597969999 / 1721070109 / 2068675587 / 1237770961 / 220031192 / 2031484958 / 583324308 / 958990240 / 413002649 / 6 / 7 / 2 / 1 / 0 / +22.84 / 10.12 / 44.32 / 58.58 / 15.49 / 12.23 / 30.16 / 88.48 / 34.26 / 27.18 / +*/ diff --git a/eBook/examples/chapter_4/repeat_string.go b/eBook/examples/chapter_4/repeat_string.go new file mode 100644 index 0000000..043ee14 --- /dev/null +++ b/eBook/examples/chapter_4/repeat_string.go @@ -0,0 +1,14 @@ +package main + +import ( + "fmt" + "strings" +) + +func main() { + var origS string = "Hi there! " + var newS string + + newS = strings.Repeat(origS, 3) + fmt.Printf("The new repeated string is: %s\n", newS) +} diff --git a/eBook/examples/chapter_4/string_conversion.go b/eBook/examples/chapter_4/string_conversion.go new file mode 100644 index 0000000..4506023 --- /dev/null +++ b/eBook/examples/chapter_4/string_conversion.go @@ -0,0 +1,20 @@ +package main + +import ( + "fmt" + "strconv" +) + +func main() { + var orig string = "666" + var an int + var newS string + + fmt.Printf("The size of ints is: %d\n", strconv.IntSize) + + an, _ = strconv.Atoi(orig) + fmt.Printf("The integer is: %d\n", an) + an = an + 5 + newS = strconv.Itoa(an) + fmt.Printf("The new string is: %s\n", newS) +} diff --git a/eBook/examples/chapter_4/string_pointer.go b/eBook/examples/chapter_4/string_pointer.go new file mode 100644 index 0000000..abd62bd --- /dev/null +++ b/eBook/examples/chapter_4/string_pointer.go @@ -0,0 +1,13 @@ +package main + +import "fmt" + +func main() { + s := "good bye" + var p *string = &s + *p = "ciao" + + fmt.Printf("Here is the pointer p: %p\n", p) // prints address + fmt.Printf("Here is the string *p: %s\n", *p) // prints string + fmt.Printf("Here is the string s: %s\n", s) // prints same string +} diff --git a/eBook/examples/chapter_4/strings_splitjoin.go b/eBook/examples/chapter_4/strings_splitjoin.go new file mode 100644 index 0000000..8d64620 --- /dev/null +++ b/eBook/examples/chapter_4/strings_splitjoin.go @@ -0,0 +1,33 @@ +// strings.go +package main + +import ( + "fmt" + "strings" +) + +func main() { + str := "The quick brown fox jumps over the lazy dog" + sl := strings.Fields(str) + fmt.Printf("Splitted in slice: %v\n", sl) + for _, val := range sl { + fmt.Printf("%s - ", val) + } + fmt.Println() + str2 := "GO1|The ABC of Go|25" + sl2 := strings.Split(str2, "|") + fmt.Printf("Splitted in slice: %v\n", sl2) + for _, val := range sl2 { + fmt.Printf("%s - ", val) + } + fmt.Println() + str3 := strings.Join(sl2,";") + fmt.Printf("sl2 joined by ;: %s\n", str3) +} +/* Output: +Splitted in slice: [The quick brown fox jumps over the lazy dog] +The - quick - brown - fox - jumps - over - the - lazy - dog - +Splitted in slice: [GO1 The ABC of Go 25] +GO1 - The ABC of Go - 25 - +sl2 joined by ;: GO1;The ABC of Go;25 +*/ \ No newline at end of file diff --git a/eBook/examples/chapter_4/testcrash.go b/eBook/examples/chapter_4/testcrash.go new file mode 100644 index 0000000..e2891c1 --- /dev/null +++ b/eBook/examples/chapter_4/testcrash.go @@ -0,0 +1,11 @@ +// testcrash.go +// compiles , but crashes +package main + +func main() { + var p *int = nil + *p = 0 + +} +// in Windows: stops only with: +// runtime error: invalid memory address or nil pointer dereference diff --git a/eBook/examples/chapter_4/time.go b/eBook/examples/chapter_4/time.go new file mode 100644 index 0000000..1228c20 --- /dev/null +++ b/eBook/examples/chapter_4/time.go @@ -0,0 +1,27 @@ +package main + +import ( + "fmt" + "time" +) + +var week time.Duration + +func main() { + t := time.Now() + fmt.Println(t) // Wed Dec 21 09:52:14 +0100 RST 2011 + fmt.Printf("%02d.%02d.%4d\n", t.Day(), t.Month(), t.Year()) // 21.12.2011 + t = time.Now().UTC() + fmt.Println(t) // Wed Dec 21 08:52:14 +0000 UTC 2011 + fmt.Println(time.Now()) // Wed Dec 21 09:52:14 +0100 RST 2011 + // calculating times: + week = 60 * 60 * 24 * 7 * 1e9 // must be in nanosec + week_from_now := t.Add(week) + fmt.Println(week_from_now) // Wed Dec 28 08:52:14 +0000 UTC 2011 + // formatting times: + fmt.Println(t.Format(time.RFC822)) // 21 Dec 11 0852 UTC + fmt.Println(t.Format(time.ANSIC)) // Wed Dec 21 08:56:34 2011 + fmt.Println(t.Format("02 Jan 2006 15:04")) // 21 Dec 2011 08:52 + s := t.Format("20060102") + fmt.Println(t, "=>", s) // Wed Dec 21 08:52:14 +0000 UTC 2011 => 20111221 +} diff --git a/eBook/examples/chapter_4/toupper_lower.go b/eBook/examples/chapter_4/toupper_lower.go new file mode 100644 index 0000000..a170831 --- /dev/null +++ b/eBook/examples/chapter_4/toupper_lower.go @@ -0,0 +1,18 @@ +package main + +import ( + "fmt" + "strings" +) + +func main() { + var orig string = "Hey, how are you George?" + var lower string + var upper string + + fmt.Printf("The original string is: %s\n", orig) + lower = strings.ToLower(orig) + fmt.Printf("The lowercase string is: %s\n", lower) + upper = strings.ToUpper(orig) + fmt.Printf("The uppercase string is: %s\n", upper) +} diff --git a/eBook/examples/chapter_4/type.go b/eBook/examples/chapter_4/type.go new file mode 100644 index 0000000..5f1d539 --- /dev/null +++ b/eBook/examples/chapter_4/type.go @@ -0,0 +1,11 @@ +package main + +import "fmt" + +type TZ int + +func main() { + var a, b TZ = 3, 4 + c := a + b + fmt.Printf("c has the value: %d", c) +} \ No newline at end of file diff --git a/eBook/examples/chapter_4/type_mixing.go b/eBook/examples/chapter_4/type_mixing.go new file mode 100644 index 0000000..d59eee8 --- /dev/null +++ b/eBook/examples/chapter_4/type_mixing.go @@ -0,0 +1,9 @@ +package main + +func main() { + var a int + var b int32 + a = 15 + b = a + a // compiler error + b = b + 5 // ok: 5 is a constant +} \ No newline at end of file diff --git a/eBook/examples/chapter_4/use_init.go b/eBook/examples/chapter_4/use_init.go new file mode 100644 index 0000000..9cdf255 --- /dev/null +++ b/eBook/examples/chapter_4/use_init.go @@ -0,0 +1,12 @@ +package main + +import ( + "fmt" + "./trans" +) + +var twoPi = 2 * trans.Pi // decl computes twoPi + +func main() { + fmt.Printf("2*Pi = %g\n", twoPi) // 2*Pi = 6.283185307179586 +}