C++ vector的操作

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文章目录

C++ vector的操作

版本C++11

删除符合条件的元素

常见但不推荐的方法

#include <vector>
#include <iostream>
#include <algorithm>
int main ()
{
    std::vector<int> vec;
    for(int i=0;i<10;i++)
    {
        vec.push_back(i);
    }

    std::vector<int>::iterator iter = vec.begin();
    while (iter != vec.end())
    {
        if ((*iter)%2==1)
        {

            iter = vec.erase(iter); //删除元素之后，iter指向已删除元素的下一个位置
        }
        else
            ++iter; //指向下一个元素位置
    }

    //输出结果 方式1 //0 2 4 6 8
    std::for_each(vec.begin(), vec.end(), [](int i){ std::cout << i << ' '; });

    //输出结果 方式2 //0 2 4 6 8
    for (auto &i : vec)
    {
        std::cout << i << ' ';
    }

}

remove_if与erase结合删除方式1

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

bool IsOdd (int i) { return ((i%2)==1); }
int main ()
{
    std::vector<int> vec;
    for(int i=0;i<10;i++)
    {
        vec.push_back(i);
    }

    vec.erase(std::remove_if(vec.begin(), vec.end(), IsOdd), vec.end());

    //输出结果 //0 2 4 6 8
    std::for_each(vec.begin(), vec.end(), [](int i){ std::cout << i << ' '; });

}

remove_if与erase结合删除方式2

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

int main ()
{
    std::vector<int> vec;
    for(int i=0;i<10;i++)
    {
        vec.push_back(i);
    }
    
    vec.erase(std::remove_if(vec.begin(), vec.end(), [](int i ){ return ((i%2)==1); }), vec.end());
    //输出结果 //0 2 4 6 8
    std::for_each(vec.begin(), vec.end(), [](int i){ std::cout << i << ' '; });
    
}

remove_if与erase结合删除方式3 类对象方式

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

class C
{ public:
    int m1;
    int m2;
};


int main ()
{
    std::vector<C> vec;
    for(int i=0;i<10;i++)
    {
        C t;
        t.m1=i;
        t.m2=i*2;
        vec.push_back(t);
    }


    vec.erase(std::remove_if(vec.begin(), vec.end(), [](C t ){ return ((t.m1%2)==1); }), vec.end());
    //输出结果 //0 2 4 6 8
    std::for_each(vec.begin(), vec.end(), [](C& t){ std::cout << t.m1 << ' '; });

}

remove_if与erase结合删除方式4

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

int main ()
{

    std::vector<int> vec;
    for(int i=0;i<10;i++)
    {
        vec.push_back(i);
    }


    auto IsOdd = [](int i) -> bool
    {
        { return ((i%2)==1); }
    };

    vec.erase(std::remove_if(vec.begin(), vec.end(), IsOdd), vec.end());
    //输出结果 //0 2 4 6 8
    std::for_each(vec.begin(), vec.end(), [](int i){ std::cout << i << ' '; });

}

为什么删除元素需要remove_if和erase配合呢

通过一个例子来说明

#include <iostream>
#include <algorithm>
#include <vector>
int main ()
{
std::vector<int> vec{ 1,1,2,3,4,5,6 };
auto removeNumbers = [&](int number) -> bool
{
    return number < 3;
};
auto iterator = std::remove_if(vec.begin(), vec.end(), removeNumbers);
vec.erase(iterator, vec.end());
std::for_each(vec.begin(), vec.end(), [](int i){ std::cout << i << ' '; });
}

在这里插入图片描述

因为remove_if只能把符合条件的元素，移动到vector的尾部，erase才是真正的删除

需要移动的元素被移到vector的尾部

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

int main ()
{



    std ::vector<int> vec{10, 20, 30, 10, 20, 30};
    //移动之前 10 20 30 10 20 30
    std::for_each(vec.begin(), vec.end(), [](int i){ std::cout << i << ' '; });
    std ::cout <<std::endl;


    std ::vector<int>::iterator  pend = std ::remove(vec.begin(), vec.end(), 20);
    //移动之后，结尾选择新的迭代器 10 30 10 30
    std::for_each(vec.begin(), pend, [](int i){ std::cout << i << ' '; });
    std ::cout <<std::endl;


}

删除重复的元素

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

int main()
{
    std::vector<int> v = { 1,1,3,3,2,2 };
    
    std::sort(v.begin(), v.end());
    
    std::vector<int>::iterator pos=std::unique(v.begin(), v.end());
    
    v.erase(pos, v.end());
    
    std::for_each(v.begin(), v.end(), [](int i){ std::cout << i << ' '; });
    
    //1 2 3
    return 0;
}

一个vector里的值赋值给另一个vector

#include <iostream>
#include <vector>
#include <algorithm>
int main ()
{

    std ::vector<int> vec{10, 20, 30, 10, 20, 30};
    std ::vector<int> new_vec{1,2,3};

    new_vec.assign (vec.begin(),vec.end());
    std::for_each(new_vec.begin(), new_vec.end(), [](int i){ std::cout << i << ' '; });
    //10 20 30 10 20 30
    return 0;
}

new_vec的值被新的值覆盖

一个vector的元素写入另一个vector的末尾

#include <iostream>
#include <vector>
#include <algorithm>
int main ()
{

    std ::vector<int> vec{10, 20, 30, 10, 20, 30};
    std ::vector<int> new_vec{1,2,3};

    new_vec.insert(new_vec.end(), vec.begin(), vec.end());
    std::for_each(new_vec.begin(), new_vec.end(), [](int i){ std::cout << i << ' '; });
    //1 2 3 10 20 30 10 20 30

    return 0;
}

两个vector拷贝到另一个vector中

#include <iostream>
#include <vector>
#include <algorithm>
int main ()
{

    std ::vector<int> vec1{10, 20, 30, 10, 20, 30};
    std ::vector<int> vec2{1,2,3};
    std ::vector<int> new_vec;

    std::copy(vec1.begin(), vec1.end(), std::back_inserter(new_vec));
    std::copy(vec2.begin(), vec2.end(), std::back_inserter(new_vec));

    std::for_each(new_vec.begin(), new_vec.end(), [](int i){ std::cout << i << ' '; });
    //10 20 30 10 20 30 1 2 3

    return 0;
}

复制符合条件的元素

#include <algorithm>
#include <iostream>
#include <vector>
#include <iterator>
#include <numeric>

int main()
{
    std::vector<int> from_vector(10);
    std::iota(from_vector.begin(), from_vector.end(), 0);
    //=  std ::vector<int> vec{1,2,3,4,5,6,7,8,9};
    std::for_each(from_vector.begin(), from_vector.end(), [](int i){ std::cout << i << ' '; });
    std ::cout <<std::endl;


    std::vector<int> to_vector;
    to_vector.clear();
    std::copy_if(from_vector.begin(), from_vector.end(),
                 std::back_inserter(to_vector),
                 [](int x) { return x % 2 == 0; });

    std::for_each(to_vector.begin(), to_vector.end(), [](int i){ std::cout << i << ' '; });
    //0 2 4 6 8
    std ::cout <<std::endl;


}

参考
https://en.cppreference.com/w/cpp/algorithm
https://www.cplusplus.com/reference/algorithm/remove_if/
https://studiofreya.com/cpp/std-remove_if-with-lambda/