std::unordered_map::begin, std::unordered_map::cbegin

来自cppreference.com
 
 
 
 
iterator begin() noexcept;
(C++11 起)
const_iterator begin() const noexcept;
(C++11 起)
const_iterator cbegin() const noexcept;
(C++11 起)

返回指向容器首元素的迭代器。

若容器为空,则返回的迭代将等于 end()

range-begin-end.svg

目录

[编辑] 参数

(无)

[编辑] 返回值

指向首元素的迭代器

[编辑] 复杂度

常数

[编辑] 示例

#include <cmath>
#include <iostream>
#include <unordered_map>
 
struct Node { double x, y; };
 
int main() {
    Node nodes[3] = { {1, 0}, {2, 0}, {3, 0} };
 
    // mag 是将 Node 的地址映射到其在平面中的模的映射
    std::unordered_map<Node *, double> mag = {
        { nodes,     1 },
        { nodes + 1, 2 },
        { nodes + 2, 3 }
    };
 
    // 将每个 y 坐标从 0 更改到模
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
        auto cur = iter->first; // 指向 Node 的指针
        cur->y = mag[cur]; // 可以也使用 cur->y = iter->second;
    }
 
    // 更新并打印每个结点的模
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
        auto cur = iter->first;
        mag[cur] = std::hypot(cur->x, cur->y);
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
        std::cout << iter->second << '\n';
    }
 
    // 以基于范围的 for 循环重复上述者
    for(auto i : mag) {
        auto cur = i.first;
        cur->y = i.second;
        mag[cur] = std::hypot(cur->x, cur->y);
        std::cout << "The magnitude of (" << cur->x << ", " << cur->y << ") is ";
        std::cout << mag[cur] << '\n';
        // 注意与 std::cout << iter->second << '\n'; 相反,上述的 
        // std::cout << i.second << '\n'; 不会打印更新的模
    }
}

可能的输出:

The magnitude of (3, 3) is 4.24264
The magnitude of (1, 1) is 1.41421
The magnitude of (2, 2) is 2.82843
The magnitude of (3, 4.24264) is 5.19615
The magnitude of (1, 1.41421) is 1.73205
The magnitude of (2, 2.82843) is 3.4641

[编辑] 参阅

返回指向容器尾端的迭代器
(公开成员函数) [编辑]