[C++] STL container - vector dynamic array container ⑤ (vector container element access | at function | [] operator overloaded function | vector container head and tail element access)

1. Access to vector container elements

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1. The vector container accesses the element at the specified index - at function

The vector container accesses the element at the specified index by using at()functions and []operators;

The at function of the vector class can access the element at the specified index position. The function prototype is as follows:

const_reference at(size_type pos) const;

This function returns a constant reference to the element at the specified position in the container;

Special note: If the specified position exceeds the scope of the container, atthe function will throw std::out_of_rangean exception. Before using the at function, it is best to check whether the position is within the scope of the container;

It is recommended to use [0, vec.size() - 1] as the index value between closed intervals;

When traversing, it is recommended to use

    for (int i = 0; i < vec.size(); i++) {
    
    }

As a traversal condition;

Code example:

#include "iostream"
using namespace std;
#include "vector"

int main() {
    
    

    // 创建空的 vector 容器
    std::vector<int> vec{
    
    1, 2, 3};

    // 遍历打印 vector 容器的内容 
    for (int i = 0; i < vec.size(); i++) {
    
    
        std::cout << vec.at(i) << ' ';
    }
    std::cout << std::endl;

	
	// 控制台暂停 , 按任意键继续向后执行
	system("pause");

	return 0;
};

Results of the :

1 2 3
Press any key to continue . . .

2. The vector container accesses the element at the specified index - [] operator overloaded function

The vector container can use the [] operator to access its elements. The [] operator overloaded function is called. The function prototype is as follows:

reference operator[](size_type pos);  

This function returns a reference to the element at the specified position in the vector container;

The [] operator overloaded function is the same as the at function. If the positional parameter exceeds the scope of the container, the [] operator overloaded function will throw an exception;

Therefore, before using the [] operator overload, you should also check whether the position is within the scope of the container;

Code example:

#include "iostream"
using namespace std;
#include "vector"

int main() {
    
    

    // 创建空的 vector 容器
    std::vector<int> vec{
    
    1, 2, 3};

    // 遍历打印 vector 容器的内容 
    for (int i = 0; i < vec.size(); i++) {
    
    
        std::cout << vec[i] << ' ';
    }
    std::cout << std::endl;

	
	// 控制台暂停 , 按任意键继续向后执行
	system("pause");

	return 0;
};

2. Access to the first and last elements of the vector container

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1. Access function for the first and last elements of vector container

Vector container head and tail element access function:

• Access the first element of the vector container: The front() member function of the vector container class returns a constant reference, representing the first element in the container;

const_reference front() const noexcept;

• Access the tail element of the vector container: the back() member function of the vector container class returns a constant reference, representing the last element in the container;

const_reference back() const noexcept;

2. Code example - accessing the first and last elements of the vector container

Code example:

#include "iostream"
using namespace std;
#include "vector"

int main() {
    
    

    // 创建空的 vector 容器
    std::vector<int> vec{
    
    1, 2, 3};

    // 遍历打印 vector 容器的内容 
    for (int i = 0; i < vec.size(); i++) {
    
    
        std::cout << vec[i] << ' ';
    }
    std::cout << std::endl;

    std::cout << "首元素 : " << vec.front() << std::endl;
    std::cout << "尾元素 : " << vec.back() << std::endl;

	
	// 控制台暂停 , 按任意键继续向后执行
	system("pause");

	return 0;
};

Results of the :

1 2 3
first element: 1
last element: 3
Press any key to continue . . .