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2023-06-26 17:30:52 +00:00
< ol class = "chapter" > < li class = "chapter-item expanded affix " > < a href = "../index.html" > Go语言圣经< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface-zh.html" > 译者序< / a > < / li > < li class = "chapter-item expanded affix " > < a href = "../preface.html" > 前言< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1.html" > < strong aria-hidden = "true" > 1.< / strong > 入门< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-01.html" > < strong aria-hidden = "true" > 1.1.< / strong > Hello, World< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-02.html" > < strong aria-hidden = "true" > 1.2.< / strong > 命令行参数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-03.html" > < strong aria-hidden = "true" > 1.3.< / strong > 查找重复的行< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-04.html" > < strong aria-hidden = "true" > 1.4.< / strong > GIF动画< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-05.html" > < strong aria-hidden = "true" > 1.5.< / strong > 获取URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-06.html" > < strong aria-hidden = "true" > 1.6.< / strong > 并发获取多个URL< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-07.html" > < strong aria-hidden = "true" > 1.7.< / strong > Web服务< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch1/ch1-08.html" > < strong aria-hidden = "true" > 1.8.< / strong > 本章要点< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2.html" > < strong aria-hidden = "true" > 2.< / strong > 程序结构< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-01.html" > < strong aria-hidden = "true" > 2.1.< / strong > 命名< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-02.html" > < strong aria-hidden = "true" > 2.2.< / strong > 声明< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-03.html" > < strong aria-hidden = "true" > 2.3.< / strong > 变量< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-04.html" > < strong aria-hidden = "true" > 2.4.< / strong > 赋值< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-05.html" > < strong aria-hidden = "true" > 2.5.< / strong > 类型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-06.html" > < strong aria-hidden = "true" > 2.6.< / strong > 包和文件< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch2/ch2-07.html" > < strong aria-hidden = "true" > 2.7.< / strong > 作用域< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3.html" > < strong aria-hidden = "true" > 3.< / strong > 基础数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-01.html" > < strong aria-hidden = "true" > 3.1.< / strong > 整型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-02.html" > < strong aria-hidden = "true" > 3.2.< / strong > 浮点数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-03.html" > < strong aria-hidden = "true" > 3.3.< / strong > 复数< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-04.html" > < strong aria-hidden = "true" > 3.4.< / strong > 布尔型< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-05.html" > < strong aria-hidden = "true" > 3.5.< / strong > 字符串< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch3/ch3-06.html" > < strong aria-hidden = "true" > 3.6.< / strong > 常量< / a > < / li > < / ol > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4.html" > < strong aria-hidden = "true" > 4.< / strong > 复合数据类型< / a > < / li > < li > < ol class = "section" > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-01.html" > < strong aria-hidden = "true" > 4.1.< / strong > 数组< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-02.html" > < strong aria-hidden = "true" > 4.2.< / strong > Slice< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-03.html" > < strong aria-hidden = "true" > 4.3.< / strong > Map< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-04.html" > < strong aria-hidden = "true" > 4.4.< / strong > 结构体< / a > < / li > < li class = "chapter-item expanded " > < a href = "../ch4/ch4-05.html" > < strong aria-hidden = "true" > 4.5.< / strong > JSON< /
2022-08-04 06:59:33 +00:00
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2024-06-12 07:20:07 +00:00
< li > < em > Go语言圣经读书笔记(挖坑中...): < a href = "https://github.com/chai2010/gopl-notes-zh" > https://github.com/chai2010/gopl-notes-zh< / a > < / em > < / li >
< li > < em > 凹语言(Go实现, 面向WASM设计): < a href = "https://github.com/wa-lang/wa" > https://github.com/wa-lang/wa< / a > < / em > < / li >
2023-06-26 17:46:27 +00:00
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2022-08-04 06:59:33 +00:00
< hr >
< h2 id = "65-示例-bit数组" > < a class = "header" href = "#65-示例-bit数组" > 6.5. 示例: Bit数组< / a > < / h2 >
< p > Go语言里的集合一般会用map[T]bool这种形式来表示, T代表元素类型。集合用map类型来表示虽然非常灵活, 但我们可以以一种更好的形式来表示它。例如在数据流分析领域, 集合元素通常是一个非负整数, 集合会包含很多元素, 并且集合会经常进行并集、交集操作, 这种情况下, bit数组会比map表现更加理想。( 译注: 这里再补充一个例子, 比如我们执行一个http下载任务, 把文件按照16kb一块划分为很多块, 需要有一个全局变量来标识哪些块下载完成了, 这种时候也需要用到bit数组。) < / p >
< p > 一个bit数组通常会用一个无符号数或者称之为“字”的slice来表示, 每一个元素的每一位都表示集合里的一个值。当集合的第i位被设置时, 我们才说这个集合包含元素i。下面的这个程序展示了一个简单的bit数组类型, 并且实现了三个函数来对这个bit数组来进行操作: < / p >
< p > < u > < i > gopl.io/ch6/intset< / i > < / u > < / p >
< pre > < code class = "language-go" > // An IntSet is a set of small non-negative integers.
// Its zero value represents the empty set.
type IntSet struct {
words []uint64
}
// Has reports whether the set contains the non-negative value x.
func (s *IntSet) Has(x int) bool {
word, bit := x/64, uint(x%64)
return word < len(s.words) & & s.words[word]& (1< < bit) != 0
}
// Add adds the non-negative value x to the set.
func (s *IntSet) Add(x int) {
word, bit := x/64, uint(x%64)
for word > = len(s.words) {
s.words = append(s.words, 0)
}
s.words[word] |= 1 < < bit
}
// UnionWith sets s to the union of s and t.
func (s *IntSet) UnionWith(t *IntSet) {
for i, tword := range t.words {
if i < len(s.words) {
s.words[i] |= tword
} else {
s.words = append(s.words, tword)
}
}
}
< / code > < / pre >
< p > 因为每一个字都有64个二进制位, 所以为了定位x的bit位, 我们用了x/64的商作为字的下标, 并且用x%64得到的值作为这个字内的bit的所在位置。UnionWith这个方法里用到了bit位的“或”逻辑操作符号|来一次完成64个元素的或计算。( 在练习6.5中我们还会有程序用到这个64位字的例子。) < / p >
< p > 当前这个实现还缺少了很多必要的特性, 我们把其中一些作为练习题列在本小节之后。但是有一个方法如果缺失的话我们的bit数组可能会比较难混: 将IntSet作为一个字符串来打印。这里我们来实现它, 让我们来给上面的例子添加一个String方法, 类似2.5节中做的那样:< / p >
< pre > < code class = "language-go" > // String returns the set as a string of the form " {1 2 3}" .
func (s *IntSet) String() string {
var buf bytes.Buffer
buf.WriteByte('{')
for i, word := range s.words {
if word == 0 {
continue
}
for j := 0; j < 64; j++ {
if word& (1< < uint(j)) != 0 {
if buf.Len() > len(" {" ) {
buf.WriteByte(' ')
}
fmt.Fprintf(& buf, " %d" , 64*i+j)
}
}
}
buf.WriteByte('}')
return buf.String()
}
< / code > < / pre >
< p > 这里留意一下String方法, 是不是和3.5.4节中的intsToString方法很相似; bytes.Buffer在String方法里经常这么用。当你为一个复杂的类型定义了一个String方法时, fmt包就会特殊对待这种类型的值, 这样可以让这些类型在打印的时候看起来更加友好, 而不是直接打印其原始的值。fmt会直接调用用户定义的String方法。这种机制依赖于接口和类型断言, 在第7章中我们会详细介绍。< / p >
< p > 现在我们就可以在实战中直接用上面定义好的IntSet了: < / p >
< pre > < code class = "language-go" > var x, y IntSet
x.Add(1)
x.Add(144)
x.Add(9)
fmt.Println(x.String()) // " {1 9 144}"
y.Add(9)
y.Add(42)
fmt.Println(y.String()) // " {9 42}"
x.UnionWith(& y)
fmt.Println(x.String()) // " {1 9 42 144}"
fmt.Println(x.Has(9), x.Has(123)) // " true false"
< / code > < / pre >
< p > 这里要注意: 我们声明的String和Has两个方法都是以指针类型< code > *IntSet< / code > 来作为接收器的, 但实际上对于这两个类型来说, 把接收器声明为指针类型也没什么必要。不过另外两个函数就不是这样了, 因为另外两个函数操作的是s.words对象, 如果你不把接收器声明为指针对象, 那么实际操作的是拷贝对象, 而不是原来的那个对象。因此, 因为我们的String方法定义在IntSet指针上, 所以当我们的变量是IntSet类型而不是IntSet指针时, 可能会有下面这样让人意外的情况: < / p >
< pre > < code class = "language-go" > fmt.Println(& x) // " {1 9 42 144}"
fmt.Println(x.String()) // " {1 9 42 144}"
fmt.Println(x) // " {[4398046511618 0 65536]}"
< / code > < / pre >
< p > 在第一个Println中, 我们打印一个< code > *IntSet< / code > 的指针, 这个类型的指针确实有自定义的String方法。第二Println, 我们直接调用了x变量的String()方法; 这种情况下编译器会隐式地在x前插入& 操作符, 这样相当于我们还是调用的IntSet指针的String方法。在第三个Println中, 因为IntSet类型没有String方法, 所以Println方法会直接以原始的方式理解并打印。所以在这种情况下& 符号是不能忘的。在我们这种场景下, 你把String方法绑定到IntSet对象上, 而不是IntSet指针上可能会更合适一些, 不过这也需要具体问题具体分析。< / p >
< p > < strong > 练习6.1:< / strong > 为bit数组实现下面这些方法< / p >
< pre > < code class = "language-go" > func (*IntSet) Len() int // return the number of elements
func (*IntSet) Remove(x int) // remove x from the set
func (*IntSet) Clear() // remove all elements from the set
func (*IntSet) Copy() *IntSet // return a copy of the set
< / code > < / pre >
< p > < strong > 练习 6.2: < / strong > 定义一个变参方法(*IntSet).AddAll(...int), 这个方法可以添加一组IntSet, 比如s.AddAll(1,2,3)。< / p >
< p > < strong > 练习 6.3: < / strong > (*IntSet).UnionWith会用< code > |< / code > 操作符计算两个集合的并集, 我们再为IntSet实现另外的几个函数IntersectWith( 交集: 元素在A集合B集合均出现) , DifferenceWith( 差集: 元素出现在A集合, 未出现在B集合) , SymmetricDifference( 并差集: 元素出现在A但没有出现在B, 或者出现在B没有出现在A) 。< / p >
2022-09-07 05:22:06 +00:00
< p > < strong > 练习 6.4: < / strong > 实现一个Elems方法, 返回集合中的所有元素, 用于做一些range之类的遍历操作。< / p >
2022-08-04 06:59:33 +00:00
< p > < strong > 练习 6.5: < / strong > 我们这章定义的IntSet里的每个字都是用的uint64类型, 但是64位的数值可能在32位的平台上不高效。修改程序, 使其使用uint类型, 这种类型对于32位平台来说更合适。当然了, 这里我们可以不用简单粗暴地除64, 可以定义一个常量来决定是用32还是64, 这里你可能会用到平台的自动判断的一个智能表达式: 32 < < (^uint(0) > > 63)< / p >
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