std::copy、std_copy_if、std::copy_n

std::copy、std::copy_if、std::copy_n

【1】应用示例程序

#include <iostream>
#include <functional>
#include <vector>
#include <algorithm>

int main()
{
    auto printVector = [](const std::vector<int> & vec) {
        int index = 0;
        for (auto& item : vec)
        {
            std::cout << index++ << ": " << item << std::endl;
        }
        if (index != 0)
            std::cout << std::endl;
    };

    // 所有测试数据集
    std::vector<int> all = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
    std::cout << "全部测试数据：" << std::endl;
    printVector(all);
    
    // 全拷贝
    std::vector<int> dstVect;
    std::copy(all.begin(), all.end(), std::back_inserter(dstVect));
    std::cout << "拷贝全部测试数据：" << std::endl;
    printVector(dstVect);

    // 只拷贝偶数（即结果是偶数集）
    std::vector<int> even;
    std::copy_if(std::begin(all), std::end(all), std::back_inserter(even),
        [&](auto item)->bool { return (std::fmod(item, 2) == 0); });
    std::cout << "拷贝全部测试数据中所有偶数：" << std::endl;
    printVector(even);

    // 只拷贝前6个元素，使用std::back_inserter
    std::vector<int> frontSix;
    std::copy_n(std::begin(all), 6, std::back_inserter(frontSix));
    std::cout << "拷贝全部测试数据中前6个数据：" << std::endl;
    printVector(frontSix);

    // 使用resize + std::begin
    std::vector<int> backSix;
    backSix.resize(6);
    // 只拷贝后6个元素
    std::cout << "拷贝全部测试数据中后6个数据：" << std::endl;
    std::copy_n(std::rbegin(all), 6, backSix.begin());
    printVector(backSix);

    return 0;
}

/* run result:
全部测试数据：
0: 1
1: 2
2: 3
3: 4
4: 5
5: 6
6: 7
7: 8
8: 9
9: 10
10: 11
11: 12

拷贝全部测试数据：
0: 1
1: 2
2: 3
3: 4
4: 5
5: 6
6: 7
7: 8
8: 9
9: 10
10: 11
11: 12

拷贝全部测试数据中所有偶数：
0: 2
1: 4
2: 6
3: 8
4: 10
5: 12

拷贝全部测试数据中前6个数据：
0: 1
1: 2
2: 3
3: 4
4: 5
5: 6

拷贝全部测试数据中后6个数据：
0: 12
1: 11
2: 10
3: 9
4: 8
5: 7
*/