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@unknwon
unknwon committed Jul 16, 2015
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Expand Up @@ -10,9 +10,9 @@

for 初始化语句; 条件语句; 修饰语句 {}

Listing 5.6 [for1.go](examples/chapter_5/for1.go)：
示例 5.6 [for1.go](examples/chapter_5/for1.go)：

```
```go
package main

import "fmt"
Expand All @@ -26,13 +26,11 @@ func main() {

输出：

```
This is the 0 iteration
This is the 1 iteration
This is the 2 iteration
This is the 3 iteration
This is the 4 iteration
```
This is the 0 iteration
This is the 1 iteration
This is the 2 iteration
This is the 3 iteration
This is the 4 iteration

由花括号括起来的代码块会被重复执行已知次数，该次数是根据计数器（此例为 i）决定的。循环开始前，会执行且仅会执行一次初始化语句 `i := 0;`；这比在循环之前声明更为简短。紧接着的是条件语句 `i < 5;`，在每次循环开始前都会进行判断，一旦判断结果为 false，则退出循环体。最后一部分为修饰语句 `i++`，一般用于增加或减少计数器。

Expand All @@ -44,13 +42,15 @@ This is the 4 iteration

您还可以在循环中同时使用多个计数器：

for i, j := 0, N; i < j; i, j = i+1, j-1 {}
```go
for i, j := 0, N; i < j; i, j = i+1, j-1 {}
```

这得益于 Go 语言具有的平行赋值的特性（可以查看第 7 章 string_reverse.go 中反转数组的示例）。

您可以将两个 for 循环嵌套起来：

```
```go
for i:=0; i<5; i++ {
for j:=0; j<10; j++ {
println(j)
Expand All @@ -60,9 +60,9 @@ for i:=0; i<5; i++ {

如果您使用 for 循环迭代一个 Unicode 编码的字符串，会发生什么？

Listing 5.7 [for_string.go](examples/chapter_5/for_string.go)：
示例 5.7 [for_string.go](examples/chapter_5/for_string.go)：

```
```go
package main

import "fmt"
Expand All @@ -83,46 +83,44 @@ func main() {

输出：

```
The length of str is: 27
Character on position 0 is: G
Character on position 1 is: o
Character on position 2 is:
Character on position 3 is: i
Character on position 4 is: s
Character on position 5 is:
Character on position 6 is: a
Character on position 7 is:
Character on position 8 is: b
Character on position 9 is: e
Character on position 10 is: a
Character on position 11 is: u
Character on position 12 is: t
Character on position 13 is: i
Character on position 14 is: f
Character on position 15 is: u
Character on position 16 is: l
Character on position 17 is:
Character on position 18 is: l
Character on position 19 is: a
Character on position 20 is: n
Character on position 21 is: g
Character on position 22 is: u
Character on position 23 is: a
Character on position 24 is: g
Character on position 25 is: e
Character on position 26 is: !
The length of str2 is: 9
Character on position 0 is: æ
Character on position 1 is: —
Character on position 2 is: ¥
Character on position 3 is: æ
Character on position 4 is: œ
Character on position 5 is: ¬
Character on position 6 is: è
Character on position 7 is: ª
Character on position 8 is: ž
```
The length of str is: 27
Character on position 0 is: G
Character on position 1 is: o
Character on position 2 is:
Character on position 3 is: i
Character on position 4 is: s
Character on position 5 is:
Character on position 6 is: a
Character on position 7 is:
Character on position 8 is: b
Character on position 9 is: e
Character on position 10 is: a
Character on position 11 is: u
Character on position 12 is: t
Character on position 13 is: i
Character on position 14 is: f
Character on position 15 is: u
Character on position 16 is: l
Character on position 17 is:
Character on position 18 is: l
Character on position 19 is: a
Character on position 20 is: n
Character on position 21 is: g
Character on position 22 is: u
Character on position 23 is: a
Character on position 24 is: g
Character on position 25 is: e
Character on position 26 is: !
The length of str2 is: 9
Character on position 0 is: æ
Character on position 1 is: —
Character on position 2 is: ¥
Character on position 3 is: æ
Character on position 4 is: œ
Character on position 5 is: ¬
Character on position 6 is: è
Character on position 7 is: ª
Character on position 8 is: ž

如果我们打印 str 和 str2 的长度，会分别得到 27 和 9。

Expand All @@ -139,14 +137,12 @@ Character on position 8 is: ž

创建一个程序，要求能够打印类似下面的结果（直到每行 25 个字符时为止）：

```
G
GG
GGG
GGGG
GGGGG
GGGGGG
```
G
GG
GGG
GGGG
GGGGG
GGGGGG

1. 使用 2 层嵌套 for 循环。
2. 使用一层 for 循环以及字符串截断。
Expand All @@ -171,7 +167,7 @@ for 结构的第二种形式是没有头部的条件判断迭代（类似其它

Listing 5.8 [for2.go](examples/chapter_5/for2.go)：

```
```go
package main

import "fmt"
Expand All @@ -188,14 +184,12 @@ func main() {

输出：

```
The variable i is now: 4
The variable i is now: 3
The variable i is now: 2
The variable i is now: 1
The variable i is now: 0
The variable i is now: -1
```
The variable i is now: 4
The variable i is now: 3
The variable i is now: 2
The variable i is now: 1
The variable i is now: 0
The variable i is now: -1

## 5.4.3 无限循环

Expand All @@ -209,7 +203,7 @@ The variable i is now: -1

无限循环的经典应用是服务器，用于不断等待和接受新的请求。

```
```go
for t, err = p.Token(); err == nil; t, err = p.Token() {
...
}
Expand All @@ -221,17 +215,17 @@ for t, err = p.Token(); err == nil; t, err = p.Token() {

要注意的是，`val` 始终为集合中对应索引的值拷贝，因此它一般只具有只读性质，对它所做的任何修改都不会影响到集合中原有的值（**译者注：如果 `val` 为指针，则会产生指针的拷贝，依旧可以修改集合中的原值**）。一个字符串是 Unicode 编码的字符（或称之为 `rune`）集合，因此您也可以用它迭代字符串：

```
```go
for pos, char := range str {
...
}
```

每个 rune 字符和索引在 for-range 循环中是一一对应的。它能够自动根据 UTF-8 规则识别 Unicode 编码的字符。

Listing 5.9 [range_string.go](examples/chapter_5/range_string.go)：
示例 5.9 [range_string.go](examples/chapter_5/range_string.go)：

```
```go
package main

import "fmt"
Expand Down Expand Up @@ -323,7 +317,7 @@ index int(rune) rune char bytes

**练习 5.9** 以下程序的输出结果是什么？

```
```go
for i := 0; i < 5; i++ {
var v int
fmt.Printf("%d ", v)
Expand All @@ -335,23 +329,23 @@ for i := 0; i < 5; i++ {

1.

```
```go
for i := 0; ; i++ {
fmt.Println("Value of i is now:", i)
}
```

2.

```
```go
for i := 0; i < 3; {
fmt.Println("Value of i:", i)
}
```

3.

```
```go
s := ""
for ; s != "aaaaa"; {
fmt.Println("Value of s:", s)
Expand All @@ -361,7 +355,7 @@ for ; s != "aaaaa"; {

4.

```
```go
for i, j, s := 0, 5, "a"; i < 3 && j < 100 && s != "aaaaa"; i, j,
s = i+1, j+1, s + "a" {
fmt.Println("Value of i, j, s:", i, j, s)
Expand Down

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