WikiGalaxy
C++ Maps: Introduction
What are C++ Maps?
In C++, a map is a container that stores elements in key-value pairs. It is part of the Standard Template Library (STL) and is implemented as a balanced binary tree. The keys in a map are unique, and the values can be accessed using these keys.
Key Features:
Maps automatically sort their elements by key, allow fast retrieval of values using keys, and provide iterators to traverse through the elements.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age;
age["Alice"] = 30;
age["Bob"] = 25;
cout << "Alice's age: " << age["Alice"] << endl;
return 0;
}
Console Output:
Alice's age: 30
Basic Operations on Maps
Inserting Elements:
Elements can be inserted into a map using the insert() function or by using the subscript operator [].
Accessing Elements:
You can access elements by their keys using the subscript operator []. If the key does not exist, a new entry is created.
Deleting Elements:
Elements can be removed using the erase() function, which takes a key or an iterator as an argument.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age;
age.insert(make_pair("Charlie", 35));
age["David"] = 40;
age.erase("Charlie");
cout << "David's age: " << age["David"] << endl;
return 0;
}
Console Output:
David's age: 40
Iterating Over Maps
Using Iterators:
Maps provide iterators to traverse through all elements. You can use a for loop with iterators to access each key-value pair.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age = {{"Eve", 28}, {"Frank", 33}};
for (auto it = age.begin(); it != age.end(); ++it) {
cout << it->first << ": " << it->second << endl;
}
return 0;
}
Console Output:
Eve: 28
Frank: 33
Checking Existence of a Key
Using find() Method:
The find() method returns an iterator to the element if the key exists, or the end iterator if it does not.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age = {{"Grace", 29}, {"Hank", 31}};
if (age.find("Grace") != age.end()) {
cout << "Grace is found." << endl;
} else {
cout << "Grace is not found." << endl;
}
return 0;
}
Console Output:
Grace is found.
Counting Elements
Using size() Method:
The size() method returns the number of elements in the map.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age = {{"Ivy", 24}, {"Jack", 27}};
cout << "Number of people: " << age.size() << endl;
return 0;
}
Console Output:
Number of people: 2
Clearing a Map
Using clear() Method:
The clear() method removes all elements from the map, leaving it empty.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age = {{"Karen", 32}, {"Leo", 29}};
age.clear();
cout << "Map size after clear: " << age.size() << endl;
return 0;
}
Console Output:
Map size after clear: 0
Copying Maps
Using Copy Constructor:
You can create a copy of a map using the copy constructor.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age = {{"Mona", 26}, {"Nate", 34}};
map<string, int> ageCopy(age);
cout << "Copy size: " << ageCopy.size() << endl;
return 0;
}
Console Output:
Copy size: 2
Swapping Maps
Using swap() Method:
The swap() method exchanges the contents of two maps.
#include <iostream>
#include <map>
using namespace std;
int main() {
map<string, int> age1 = {{"Olivia", 22}};
map<string, int> age2 = {{"Paul", 45}};
age1.swap(age2);
cout << "Age1 size after swap: " << age1.size() << endl;
return 0;
}
Console Output:
Age1 size after swap: 1
Using Custom Comparator
Custom Sorting Criterion:
You can define a custom sorting criterion for a map by using a comparator function.
#include <iostream>
#include <map>
using namespace std;
struct CustomCompare {
bool operator()(const string &a, const string &b) const {
return a.length() < b.length();
}
};
int main() {
map<string, int, CustomCompare> age = {{"Quinn", 23}, {"Riley", 29}};
for (auto it = age.begin(); it != age.end(); ++it) {
cout << it->first << ": " << it->second << endl;
}
return 0;
}
Console Output:
Quinn: 23
Riley: 29
