我們知道,存儲桶是容器內部哈希表中的一個插槽,所有元素均基於其鍵的哈希值分配到該插槽。存儲桶的編號從0到bucket_count。現在,作為存儲項的可變數量的存儲桶。這個數字是基於術語Load Factor的,當Load Factor(load_factor)達到某個閾值時,容器會增加存儲桶的數量並重新映射Map,但是當我們調用rehash(n)時,它將直接設置存儲桶的數量到n並觸發整個哈希表的重建。但是當我們調用reserve(n)時,它將創建足夠的Bucket以容納至少n個項目。如果然後我們向Map添加> n個項目,則可能會觸發重新哈希操作,具體取決於負載係數。通過使用unordered_map容器我們期望的大小來調用reserve,我們避免了容器大小增加可能產生的多次重複,並優化了哈希表的大小。 C++函數std::unordered_map::reserve()將容器中的存儲桶數(bucket_count)設置為最合適,以包含至少n個元素。
用法:
unordered_map_name.reserve(N)
參數:該函數接受單個強製性參數N,該參數將請求的元素數指定為最小容量。
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:
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;
}
輸出:
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 illustrate the
// unordered_map::reserve()
#include <bits/stdc++.h>
using namespace std;
int main()
{
// declaration
unordered_map<char, char> 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({ '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.reserve(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}
相關用法
- C++ unordered_multiset reserve()用法及代碼示例
- C++ unordered_multimap reserve()用法及代碼示例
- C++ unordered_set reserve()用法及代碼示例
注:本文由純淨天空篩選整理自YashRaj5大神的英文原創作品 unordered_map reserve() 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。