C++ STL中的unordered_map reserve()

我们知道，桶是容器内部哈希表中的一个槽，所有元素都基于它们的键的哈希值分配到其中。桶从0到bucket_count编号。现在，由于一个桶保持可变数量的项。这个数值基于一个术语 负载因子 。当 负载因子(load_factor) 达到某个阈值时，容器增加桶的数量并重新哈希地图。但是当我们调用 rehash(n) 时，它直接将桶的数量设置为n，并触发整个哈希表的重建。但是，当我们调用 reserve(n) 时，它创建足够的桶来至少容纳n个项目。如果然后我们在映射中添加>n个项目，取决于负载因子，可能会触发重新哈希。通过使用预期的unordered_map容器大小来调用reserve，我们避免了容器大小增加可能产生的多次重新哈希，并优化了哈希表的大小。C++函数std::unordered_map::reserve()将容器(bucket_count)中的桶的数量设置为最适合包含至少n个元素。

语法:

unordered_map_name.reserve(N)

参数: 该函数接受一个强制性参数 N ，指定请求的元素数为最小容量。

返回值: 该函数不返回任何内容。

下面的程序说明以上功能：

程序1:

// C++ program to illustrate the
// unordered_map::reserve()
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
  
    // declaration
    unordered_map<int, int> sample1, sample2;
  
    // the sample1 size is reserved for
    // the bucket to contain a minimum of
    // one elements
    sample1.reserve(1);
  
    // inserts key and element
    // in sample1
    sample1.insert({ 10, 100 });
    sample1.insert({ 50, 500 });
  
    // inserts key and element
    // in sample1
  
    // the sample1 size is reserved for
    // the bucket to contain a minimum of
    // three elements
    sample2.reserve(3);
  
    sample2.insert({ 20, 200 });
    sample2.insert({ 30, 300 });
    sample2.insert({ 30, 150 });
  
    cout << "The size of Sample1 is: " << sample1.size();
  
    cout << "\nKey and Elements of Sample1 are:";
    for (auto it = sample1.begin(); it != sample1.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    cout << "\n\nThe size of Sample2 is: " << sample2.size();
  
    cout << "\nKey and Elements of Sample2 are:";
    for (auto it = sample2.begin(); it != sample2.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    return 0;
}

The size of Sample1 is: 2
Key and Elements of Sample1 are:{50,500} {10, 100} 

The size of Sample2 is: 2
Key and Elements of Sample2 are:{30, 300} {20, 200}

程序2:

// C++ program to demonstrate the
// use of unordered_map::reserve()
#include <bits/stdc++.h>
using namespace std;
 
int main()
{
    // Declaration of unordered_map
    unordered_map<string, string> umap;
 
    // Initialisation of unordered_map
    umap = { { "An", "Apple" }, { "Ball", "Cat" },
            { "Cat", "Dog" }, { "Elephant", "Fish" },
            { "Dog", "Goat" } };
 
    // displaying the original elements
    cout << "Original unordered_map::"
          << " \n Key \t Value \n";
    for (auto pair : umap) {
        cout << " " << pair.first
              << " \t " << pair.second
              << " \n ";
    }
 
    // clear unordered_map
    umap.clear();
 
    // reserve the bucket for at least 10 elements
    umap.reserve(10);
 
    // again checking size of unordered_map
    cout << "unordered_map is empty: "
          << (umap.empty() ? "True" : "False") << endl;
 
    return 0;
}

Original unordered_map:: 
 Key   Value 
 An   Apple 
 Ball   Cat 
 Cat   Dog 
 Dog   Goat 
 Elephant   Fish 

unordered_map is empty: True

// C++程序演示无序映射的reserve()
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
  
    // 声明
    unordered_map<char, char> sample1, sample2;
  
    // 将sample1的容量保留为至少1个元素的桶
    sample1.reserve(1);
  
    // 向sample1中插入键和元素
    sample1.insert({ 'a', 'A' });
    sample1.insert({ 'g', 'G' });
  
    // 向sample2中插入键和元素
  
    // 将sample2的容量保留为至少3个元素的桶
    sample2.reserve(3);
  
    sample2.insert({ 'b', 'B' });
    sample2.insert({ 'c', 'C' });
    sample2.insert({ 'd', 'D' });
  
    cout << "Sample1的大小是：" << sample1.size();
  
    cout << "\nSample1的键和元素为：";
    for (auto it = sample1.begin(); it != sample1.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    cout << "\n\nSample2的大小是：" << sample2.size();
  
    cout << "\nSample2的键和元素为：";
    for (auto it = sample2.begin(); it != sample2.end(); it++) {
        cout << "{" << it->first << ", " << it->second << "} ";
    }
  
    return 0;
}

Sample1的大小是：2
Sample1的键和元素为：{g, G} {a, A} 

Sample2的大小是：3
Sample2的键和元素为：{d, D} {c, C} {b, B}