Use C's std::map to store key-value pairs to master its automatic sorting and efficient search characteristics, initialize such as std::map<:string int> student_scores; insert the available insert or [] operator, 1. insert does not replace the existing key, but 2. [] will overwrite; iterate through the available iterator or range for loop, 3. It is recommended to use find to obtain the iterator; modify it directly to assign the value to delete the available erase key or iterator; note that map is arranged in ascending order by default, and the performance is O(log n). If there is no need for order, you can consider std::unordered_map.
Using C's std::map to store key-value pairs is actually quite intuitive. The key is to understand its automatic sorting and efficient features. As long as you master the basic operations, it will be easy to use in daily development.
Initialize and insert data
Declaring a map is very simple. For example, if you want to save the correspondence between student names and grades, you can write it like this:
std::map<std::string, int> student_scores;
There are several ways to insert data:
Use
insertmethod:
student_scores.insert(std::make_pair("Alice", 90));Or use the
[]operator directly:student_scores["Bob"] = 85;
Note that if the key already exists, [] will overwrite the old value; insert will not replace it.
Traversal and find elements
To traverse maps, it is generally used to use iterators or range for loops:
for (const auto& pair : student_scores) {
std::cout << pair.first << ": " << pair.second << std::endl;
} To find out whether a key exists, you can use find :
if (student_scores.find("Charlie") != student_scores.end()) {
// Found} else {
// Not found} This method is more recommended than count , because the returned iterator can be used directly to access the value.
Modify and delete elements
Modifying the value is very simple, just assign the value directly through [] :
student_scores["Alice"] = 95;
Use erase to delete a specific element:
student_scores.erase("Bob");You can also pass in an iterator to delete elements at a certain location:
auto it = student_scores.find("Alice");
if (it != student_scores.end()) {
student_scores.erase(it);
}Common precautions
- map is an ordered structure , and the default is arranged in ascending order.
- The insertion and search efficiency are relatively high, and it is O(log n).
- If you don't care about order and only focus on performance, consider
std::unordered_map.
Basically that's it. If you use it too much, you will find that it is particularly convenient when dealing with configuration, caching, statistics and other scenarios.
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