How to access elements in C++ STL container?

How to access elements in a C++ STL container? There are several ways: Traverse the container: use an iterator range-based for loop to access specific elements: use an index (subscript operator []) use a key (std::map or std::unordered_map)

How to access elements in C++ STL containers

The C++ Standard Template Library (STL) provides various containers for efficient storage and management of data. Understanding how to access elements within these containers is critical to effectively utilizing the STL.

Traverse a container

The following methods are available to traverse a container and access its elements:

• Iterator: STL provides iterators, which can be used in sequence Access the elements in the container.

// 使用迭代器遍历 vector
vector<int> v = {1, 2, 3};
for (vector<int>::iterator it = v.begin(); it != v.end(); ++it) {
  cout << *it << endl;
}

• Range-based for loop: C++11 introduces the range-based for loop, making traversing the container more concise.

// 使用基于范围的 for 循环遍历 vector
vector<int> v = {1, 2, 3};
for (int& x : v) {
  cout << x << endl;
}

Accessing specific elements

In addition to traversing the container, you can also directly access specific elements through the index or key:

• Index : You can use the subscript operator ([]) to access elements in the container using indexes.

// 使用下标访问 vector 中的元素
vector<int> v = {1, 2, 3};
cout << v[0] << endl; // 输出 1

• Keys: If the container uses keys to store elements, you can use std::map or std::unordered_map [] operator or at() method in .

// 使用键访问 map 中的元素
map<string, int> m;
m["John"] = 30;
cout << m["John"] << endl; // 输出 30

Practical case

Suppose we have a std::vector that stores student grades:

vector<int> grades = {90, 85, 95, 88};

The following is how to use range-based The for loop accesses and modifies these elements:

// 使用基于范围的 for 循环遍历和修改 vector
for (int& grade : grades) {
  // 将每个成绩增加 5
  grade += 5;
}

Conclusion

Understanding how to access elements in C++ STL containers is critical to using these containers effectively. You can use iterators, range-based for loops, subscript operators, or keys, depending on the type of container you are using.

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