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std::map::begin, std::map::cbegin

提供: cppreference.com
< cpp‎ | container‎ | map
 
 
 
 
iterator begin();
(C++11未満)
iterator begin() noexcept;
(C++11以上)
const_iterator begin() const;
(C++11未満)
const_iterator begin() const noexcept;
(C++11以上)
const_iterator cbegin() const noexcept;
(C++11以上)

コンテナの最初の要素を指すイテレータを返します。

コンテナが空の場合は、返されたイテレータは end() と等しくなります。

range-begin-end.svg

目次

[編集] 引数

(なし)

[編集] 戻り値

最初の要素を指すイテレータ。

[編集] 計算量

一定。


[編集]

#include <iostream>
#include <map>
 
int main() {
  std::map<int, float> num_map;
  num_map[4] = 4.13;
  num_map[9] = 9.24;
  num_map[1] = 1.09;
  // calls a_map.begin() and a_map.end()
  for (auto it = num_map.begin(); it != num_map.end(); ++it) {
    std::cout << it->first << ", " << it->second << '\n';
  }
}

出力:

1, 1.09
4, 4.13
9, 9.24

カスタム比較関数を使った例

#include <cmath>
#include <iostream>
#include <map>
 
struct Point { double x, y; };
 
typedef Point * PointPtr;
//Compare the x-coordinates of two Point pointers
struct PointCmp {
    bool operator()(const PointPtr &lhs, const PointPtr &rhs) const { 
        return lhs->x < rhs->x; 
    }
};
 
int main() {
    //Note that although the x-coordinates are out of order, the
    // map will be iterated through by increasing x-coordinates
    Point points[3] = { {2, 0}, {1, 0}, {3, 0} };
 
    //mag is a map sending the address of node to its magnitude in the x-y plane
    //Although the keys are pointers-to-Point, we want to order the map by the
    // x-coordinates of the points and NOT by the addresses of the Points. This
    // is done by using the PointCmp class's comparison method.
    std::map<Point *, double, PointCmp> mag({
        { points,     2 },
        { points + 1, 1 },
        { points + 2, 3 }
    });
 
    //Change each y-coordinate from 0 to the magnitude
    for(auto iter = mag.begin(); iter != mag.end(); ++iter){
        auto cur = iter->first; // pointer to Node
        cur->y = mag[cur]; // could also have used  cur->y = iter->second;
    }
 
    //Update and print the magnitude of each node
    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';
    }
 
    //Repeat the above with the range-based for loop
    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';
        //Note that in contrast to std::cout << iter->second << '\n'; above, 
        // std::cout << i.second << '\n'; will NOT print the updated magnitude
    }
}

出力:

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

[編集] 関連項目

終端を指すイテレータを返します
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