小朋友学C++（14）：两数交换

清疚 2022-05-22 01:28 163阅读 0赞

之前学C语言的时候，咱们直接在main函数中使用“异或”位运算符，很容易实现了两数交换。  
本节课将在此基础上，把交换两个数的算法，封装到swap函数中。这样不管是哪个地方想要交换两个数，调用swap函数就可以了。

程序1：

#include <iostream>
    using namespace std;
    
    void swap(int m, int n)
    {
        cout << "Memory address of m: " << &m << endl;
        cout << "Memory address of n: " << &n << endl;
    
        cout << "\nBefore Swap:" << endl;
        cout << "m = " << m << endl;
        cout << "n = " << n << endl;
    
        m ^= n;
        n ^= m;
        m ^= n;
    
        cout << "\nAfter Swap:" << endl;
        cout << "m = " << m << endl;
        cout << "n = " << n << endl;    
    }
    
    int main(int argc, char** argv) 
    {
        int a = 1;
        int b = 2;
    
        cout << "Memory address of a: " << &a << endl;
        cout << "Memory address of b: " << &b << endl;  
    
        swap(a, b);
    
        cout << "a = " << a << endl;
        cout << "b = " << b << endl;
    
        return 0;
    }

运行结果：

Memory address of a: 0x7fff90371cd8
    Memory address of b: 0x7fff90371cdc
    Memory address of m: 0x7fff90371c9c
    Memory address of n: 0x7fff90371c98
    
    Before Swap:
    m = 1
    n = 2
    
    After Swap:
    m = 2
    n = 1
    a = 1
    b = 2

分析：  
从结果可以看出，a和b调用swap之后，值并没对换过来。  
其原因在于，形参m和n的作用域只是在swap函数内。在swap内，m = a = 1, n = b = 2，交换后m = 2, n = 1。但是，m和n的值并不会传回给a和b，导致a和b的值没有被对换。

解决方案，使用上节课讲过的引用。  
程序2：

#include <iostream>
    using namespace std;
    
    void swap(int &m, int &n)
    {
        cout << "Memory address of m: " << &m << endl;
        cout << "Memory address of n: " << &n << endl;
    
        cout << "\nBefore Swap:" << endl;
        cout << "m = " << m << endl;
        cout << "n = " << n << endl;
    
        m ^= n;
        n ^= m;
        m ^= n;
    
        cout << "\nAfter Swap:" << endl;
        cout << "m = " << m << endl;
        cout << "n = " << n << endl;    
    }
    
    int main(int argc, char** argv) 
    {
        int a = 1;
        int b = 2;
    
        cout << "Memory address of a: " << &a << endl;
        cout << "Memory address of b: " << &b << endl;  
    
        swap(a, b);
    
        cout << "a = " << a << endl;
        cout << "b = " << b << endl;
    
        return 0;
    }

运行结果：

Memory address of a: 0x7fffd009de98
    Memory address of b: 0x7fffd009de9c
    Memory address of m: 0x7fffd009de98
    Memory address of n: 0x7fffd009de9c
    
    Before Swap:
    m = 1
    n = 2
    
    After Swap:
    m = 2
    n = 1
    a = 2
    b = 1

分析：  
使用引用后，达到了交换a和b的目的。这是因为形参m和n是实参a和b的引用，m和a是一回事，n和b是一回事。  
交换m和n的值，就是交换a和b的值。

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