C++ unordered_multimap rehash()用法及代碼示例

unordered_multimap::rehash(N)是C++ STL中的內置函數，該函數將容器中的存儲桶數設置為N或更大。如果N大於容器中的當前存儲桶數(bucket_count)，則會強製進行重新哈希處理。
新的存儲區計數可以等於或大於N。如果n小於容器中當前存儲區的數量(bucket_count)，該函數可能對存儲區計數沒有影響，也可能不會強製重新哈希。
重新哈希是哈希表的重建：容器中的所有元素根據其哈希值重新排列到新的存儲桶集中。盡管可以保留具有等效鍵的元素的相對順序，但是這可能會更改容器內元素的迭代順序。每當容器重新進行修補時，容器都會自動執行負載係數在操作中將超越其max_load_factor。通過調用rehash在哈希表中保留一定數量的最小存儲桶，我們避免了容器擴展可能引起的多次哈希。

用法:

unordered_multimap_name.rehash(N)

參數：該函數接受單個強製性參數N，該參數指定容器哈希表的最小存儲桶數。

返回值：該函數不返回任何內容。

以下示例程序旨在說明上述函數：

示例1:

// C++ program to illustrate the 
// unordered_multimap::rehash() 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
  
    // declaration 
    unordered_multimap<int, int> sample1, sample2; 
  
    // the sample1 size is reserved for 
    // the bucket to contain a minimum of 
    // one elements 
    sample1.rehash(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.rehash(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; 
}

輸出：

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

The size of Sample2 is: 3
Key and Elements of Sample2 are:{30, 150} {30, 300} {20, 200}

示例2:

// C++ program to illustrate the 
// unordered_multimap::rehash() 
#include <bits/stdc++.h> 
using namespace std; 
  
int main() 
{ 
  
    // declaration 
    unordered_multimap<char, char> sample1, sample2; 
  
    // the sample1 size is reserved for 
    // the bucket to contain a minimum of 
    // one elements 
    sample1.rehash(1); 
  
    // inserts key and element 
    // in sample1 
    sample1.insert({ 'a', 'A' }); 
    sample1.insert({ 'g', 'G' }); 
  
    // inserts key and element 
    // in sample1 
  
    // the sample1 size is reserved for 
    // the bucket to contain a minimum of 
    // three elements 
    sample2.rehash(3); 
  
    sample2.insert({ 'b', 'B' }); 
    sample2.insert({ 'c', 'C' }); 
    sample2.insert({ 'd', 'D' }); 
  
    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; 
}

輸出：

The size of Sample1 is: 2
Key and Elements of Sample1 are:{g, G} {a, A} 

The size of Sample2 is: 3
Key and Elements of Sample2 are:{d, D} {c, C} {b, B}

參考：http://www.cplusplus.com/reference/unordered_map/unordered_multimap/rehash/

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注：本文由純淨天空篩選整理自gopaldave大神的英文原創作品 unordered_multimap rehash() function in C++ STL。非經特殊聲明，原始代碼版權歸原作者所有，本譯文未經允許或授權，請勿轉載或複製。