vector：C++ STL中的resize()

向量称为动态数组，可以在插入或删除元素时自动更改其大小。此存储由容器维护。

向量::: resize()

该函数在容器中插入或删除元素来实际更改容器的内容。碰巧是这样

• 如果给定的n值小于当前的大小，则将删除多余的元素。
• 如果n大于容器的当前大小，则即将出现的元素将附加到向量的末尾。

句法：

vectorname.resize(int n, int val)

Parameters:

n – it is new container size, expressed in number of elements.

val – if this parameter is specified then new elements are initialized with this value.

Return value:

This function do not returns anything.

Exception:

The only exception if it so happens is Bad_alloc thrown, if reallocation fails.

下面的程序说明了该函数的工作

1. 矢量容器的尺寸减小了。

   // resizing of the vector
   #include 
   #include 
     
   using namespace std;
     
   int main()
   {
       vector vec;
     
       // 5 elements are inserted
       // in the vector
       vec.push_back(1);
       vec.push_back(2);
       vec.push_back(3);
       vec.push_back(4);
       vec.push_back(5);
     
       cout << "Contents of vector before resizing:" 
            << endl;
         
       // displaying the contents of the
       // vector before resizing
       for (int i = 0; i < vec.size(); i++)
           cout << vec[i] << " ";
     
       cout << endl;
     
       // vector is resized
       vec.resize(4);
     
       cout << "Contents of vector after resizing:" 
            << endl;
         
       // displaying the contents of the
       // vector after resizing
       for (int i = 0; i < vec.size(); i++)
           cout << vec[i] << " ";
     
       return 0;
   }
   

   输出：

   Contents of the vector before resizing:
   1 2 3 4 5 
   Contents of the vector after resizing:
   1 2 3 4 
   

2. 向量容器的尺寸增加。

   // resizing of the vector
   #include 
   #include 
     
   using namespace std;
     
   int main()
   {
       vector vec;
     
       // 5 elements are inserted 
       // in the vector
       vec.push_back(1);
       vec.push_back(2);
       vec.push_back(3);
       vec.push_back(4);
       vec.push_back(5);
     
       cout << "Contents of vector before resizing:" 
            << endl;
         
       // displaying the contents of the
       // vector before resizing
       for (int i = 0; i < vec.size(); i++)
           cout << vec[i] << " ";
     
       cout << endl;
     
       // vector is resized
       vec.resize(8);
     
       cout << "Contents of vector after resizing:" 
            << endl;
     
       // displaying the contents of 
       // the vector after resizing
       for (int i = 0; i < vec.size(); i++)
           cout << vec[i] << " ";
     
       return 0;
   }
   

   输出：

   Contents of the vector before resizing:
   1 2 3 4 5 
   Contents of the vector after resizing:
   1 2 3 4 5 0 0 0 
   

3. 向量容器的大小会增加，并使用指定的值初始化新元素。

   // resizing of the vector
   #include 
   #include 
     
   using namespace std;
     
   int main()
   {
       vector vec;
     
       // 5 elements are inserted
       // in the vector
       vec.push_back(1);
       vec.push_back(2);
       vec.push_back(3);
       vec.push_back(4);
       vec.push_back(5);
     
       cout << "Contents of vector before resizing:"
            << endl;
     
       // displaying the contents of 
       // the vector before resizing
       for (int i = 0; i < vec.size(); i++)
           cout << vec[i] << " ";
     
       cout << endl;
     
       // vector is resized
       vec.resize(12, 9);
     
       cout << "Contents of vector after resizing:" 
            << endl;
         
       // displaying the contents 
       // of the vector after resizing
       for (int i = 0; i < vec.size(); i++)
           cout << vec[i] << " ";
     
       return 0;
   }
   

   输出：

   Contents of the vector before resizing:
   1 2 3 4 5 
   Contents of the vector after resizing:
   1 2 3 4 5 9 9 9 9 9 9 9 
   

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