（7）STL算法之交换赋值

3、互换

STL的大部分容器都提供了成员函数swap()，用于两个不同容器对象之间的元素交换，同时，STL算法也提供了算法swap()。个人认为尽量使用容器自己的swap()。

（1）swap()

// 声明及定义
template <class T> 
void swap (T& a, T& b);

template <class T> 
void swap (T& a, T& b)
{
    T c(std::move(a)); 
    a=std::move(b); 
    b=std::move(c);
}

template <class T, size_t N> 
void swap (T (&a)[N], T (&b)[N])
{
    for (size_t i = 0; i<N; ++i) 
        swap (a[i],b[i]);
}

用法示例

#include <iostream>    
#include <algorithm>    
#include <vector>  
using namespace std;

int main() {

    int x = 10, y = 20;                    // x=10   y=20
    swap(x, y);                            // x=20   y=10
    vector<int> v1(4, x), v2(6, y);        // 4个20  6个10
    swap(v1, v2);                          // 6个10  4个20
    cout << "vector v1:";
    for (auto it = v1.begin(); it != v1.end(); ++it)
        cout << *it << " ";
    cout << endl;
    cout << "vector v2:";
    for (int val : v2)
        cout << val << " ";
    cout << endl;
    return 0;
}

(2) swap_ranges()

这个函数可以交换两个容器中的部分元素

template <class ForwardIterator1, class ForwardIterator2>
ForwardIterator2 swap_ranges (ForwardIterator1 first1, ForwardIterator1 last1,ForwardIterator2 first2);

template<class ForwardIterator1, class ForwardIterator2>
ForwardIterator2 swap_ranges (ForwardIterator1 first1, ForwardIterator1 last1,ForwardIterator2 first2)
{
    while (first1!=last1) 
    {
        swap (*first1, *first2);
        ++first1; 
        ++first2;
    }
    return first2;
}

用法示例：

#include <iostream>  
#include <algorithm>    
#include <vector> 
using namespace std;

int main() {
    vector<int> v1(5, 10);      
    vector<int> v2(3, 33);        
    // 用容器一中四个元素交换 容器二从 begin开始的四个元素，
    //如果容器二中没有这么大空间，会出问题，比如将上边改为 vector<int> v2(3, 33); 
    swap_ranges(v1.begin(), v1.begin() + 4, v2.begin());
    cout << "vector v1:";
    for (auto it = v1.begin(); it != v1.end(); ++it)
        cout << *it << " ";
    cout << '\n';
    cout << "vector v2:";
    for (auto it = v2.begin(); it != v2.end(); ++it)
        cout << *it << " ";
    cout << '\n';
    return 0;
}

4、赋值

可以使用 fill()、fill_n()、generate()、generate_n() 4个函数来给容器赋值。

fill()、fill_n()用于给指定区间每个元素赋予相同的数值，

（1）fill()

// fill() 声明及定义
template <class ForwardIterator, class T>
void fill (ForwardIterator first, ForwardIterator last, const T& val);

template <class ForwardIterator, class T>
void fill (ForwardIterator first, ForwardIterator last, const T& val)
{
    while (first != last) 
    {
        *first = val;
        ++first;
    }
}

用法示例：

#include <iostream>   
#include <algorithm>    
#include <vector>   
using namespace std;

int main() {
    vector<int> v(8);                   // v: 0 0 0 0 0 0 0 0
    fill(v.begin(), v.begin() + 4, 5);  // v: 5 5 5 5 0 0 0 0
    fill(v.begin() + 3, v.end() - 2, 8);// v: 5 5 5 8 8 8 0 0
    cout << "vector v:";
    for (auto it = v.begin(); it != v.end(); ++it)
        cout << ' ' << *it;
    cout << '\n';
    return 0;
}

（2）fill_n()

template <class OutputIterator, class Size, class T>
OutputIterator fill_n (OutputIterator first, Size n, const T& val);

template <class OutputIterator, class Size, class T>
OutputIterator fill_n (OutputIterator first, Size n, const T& val)
{
    while (n>0) 
    {
        *first = val;
        ++first; 
        --n;
    }
    return first;    
}

用法示例：

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

int main() {
    std::vector<int> v(8, 10);         // v: 10 10 10 10 10 10 10 10
    std::fill_n(v.begin(), 4, 20);     // v: 20 20 20 20 10 10 10 10
    std::fill_n(v.begin() + 3, 3, 33); // v: 20 20 20 33 33 33 10 10
    return 0;
}

（3）generate()、generate_n()

template <class ForwardIterator, class Generator>
void generate (ForwardIterator first, ForwardIterator last, Generator gen);

template <class ForwardIterator, class Generator>
void generate ( ForwardIterator first, ForwardIterator last, Generator gen )
{
    while (first != last) 
    {
        *first = gen();
        ++first;
    }
}

template <class OutputIterator, class Size, class Generator>
OutputIterator generate_n (OutputIterator first, Size n, Generator gen);

template <class OutputIterator, class Size, class Generator>
void generate_n ( OutputIterator first, Size n, Generator gen )
{
    while (n>0) 
    {
        *first = gen();
        ++first;
        --n;
    }
}

用法示例：

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
using namespace std;
int Fibonacci(void)
{
    static int r;
    static int f1 = 0;
    static int f2 = 1;
    r = f1 + f2;
    f1 = f2;
    f2 = r;
    return f1;
}
void main()
{
    vector<int> v2(5, 0), v3(6, 0); 
    generate(v2.begin(), v2.end(), rand);
    copy(v2.begin(), v2.end(), ostream_iterator<int>(cout, ", ")); //打印
    cout << endl;
    
    generate_n(v2.begin(), 3, rand);  //从 begin() 赋值 3个随机数
    copy(v2.begin(), v2.end(), ostream_iterator<int>(cout, ", "));
    cout << endl;
    // 自定义规则
    generate_n(v3.begin(), 6, Fibonacci);
    copy(v3.begin(), v3.end(), ostream_iterator<int>(cout, ", "));
    cout << endl;
}