Sets are STL containers that store unique elements of the same type in a sorted manner.
As the value of every element in a set is unique, the value itself acts as the key for identifying the element.
Set Properties
C++ set has the following properties.
1. Unique Elements
No two elements in a set can be equal.
2. Immutable
We can add or remove elements from a set, but we can't change the values of existing elements.
3. Sorted Order
Elements in a set are sorted following a specific strict weak ordering.
By default, C++ sets are sorted in ascending order, but we have the option to change that.
4. Associative
Elements of a set are referenced using their key, not by their position in the container.
This is unlike arrays, where elements are accessed using indexes.
Create a Set
To implement a set in C++, we must include the <set> header file in our program.
#include <set>
We can create a set in C++ using the following syntax:
// declare a set
std::set<data_type> set_name = {key1, key2, key3, ...};
Here,
std::set- declares an STL container of typeset.<data_type>- the data type of the values to be stored in the set.set_name- a unique name given to the set.key1, key2, key3, ...- key/value to be stored in the set.
For example,
// initialize set with elements
std::set<int> my_set1 = {5, 3, 8, 1, 3};
// create an empty set
std::set<int> my_set2;
Note: Moving forward, we will be using the std namespace so we can omit std:: from our syntax.
Example 1: Create a Set
#include <iostream>
#include <set>
using namespace std;
int main() {
set<int> my_set = {5, 3, 8, 1, 3};
for(int val : my_set) {
cout << val << " ";
}
return 0;
}
Output
1 3 5 8
Here, we have created a set of type int.
As you can see, the set returned the values in a sorted manner and ignored the repeated instance of 3.
Note: We cannot use the [] operator to add elements to a set. Doing so will result in an error.
// this is not allowed in a set
my_set[4] = 4
Sort a Set in Descending Order
To get the elements of a set sorted in descending order, we can modify our syntax as follows:
set<int, greater<int>> my_set;
For example,
#include <iostream>
#include <set>
using namespace std;
int main() {
set<int, greater<int>> my_set = {5, 3, 8, 1, 3};
for(int val : my_set) {
cout << val << " ";
}
return 0;
}
Output
8 5 3 1
As can be seen from the output, the set now returns the elements sorted in descending order.
C++ Set Methods
In C++, the set class provides various methods to perform various operations in a set.
| Operation | Description |
|---|---|
insert() |
Insert elements into a set. |
erase() |
Delete elements from a set. |
clear() |
Remove all the elements from a set. |
empty() |
Check if the set is empty. |
size() |
Returns the size of the set. |
Example 2: Add Values to a Set
#include <iostream>
#include <set>
using namespace std;
int main () {
set<int> my_set;
// add values to the set
my_set.insert(10);
my_set.insert(30);
my_set.insert(20);
my_set.insert(50);
my_set.insert(40);
my_set.insert(50);
// print the set elements
for (int i : my_set) {
cout << i << " ";
}
return 0;
}
Output
10 20 30 40 50
Here, we have created an empty set my_set of type int and added values to it using the insert() method.
Notice that the duplicate insertions are removed, and the set is sorted in ascending order despite the order of insertion being different.
Example 3: Check if an Element Exists in a Set
#include <iostream>
#include <set>
using namespace std;
int main () {
set<int> my_set;
// add values to the set
my_set.insert(10);
my_set.insert(30);
my_set.insert(20);
my_set.insert(50);
my_set.insert(40);
my_set.insert(50);
// check if 40 exists
int num = 40;
if(my_set.count(num) == 1) {
cout << num << " exists." << endl;
} else {
cout << num << " doesn't exist." << endl;
}
// check if 60 exists
num = 60;
if(my_set.count(num) == 1) {
cout << num << " exists." << endl;
} else {
cout << num << " doesn't exist." << endl;
}
return 0;
}
Output
40 exists. 60 doesn't exist.
Here, we have created an empty set my_set of type int and added values to it using the insert() method.
We checked if 40 and 60 exist in the set my_set by my_set.count(num) when num is 40 and 60.
my_set.count(num) returns 1 if num exists in my_set, otherwise it returns 0.
Example 4: Delete Elements From a Set
#include <iostream>
#include <set>
using namespace std;
int main () {
set<int> my_set = {10, 20, 30, 40};
// set before deletion
cout << "The set before deletion: ";
for (int i : my_set) {
cout << i << " ";
}
// delete values from the set
my_set.erase(10);
my_set.erase(20);
// set after deletion
cout << "\nThe set after deletion: ";
for (int i : my_set) {
cout << i << " ";
}
// delete all elements from the set
my_set.clear();
// set after clearing all elements
cout << "\nThe set after clearing all elements: ";
for (int i : my_set) {
cout << i << " ";
}
return 0;
}
Output
The set before deletion: 10 20 30 40 The set after deletion: 30 40 The set after clearing all elements:
Here, we have used the erase() method to delete individual set elements.
Then, we used the clear() method to delete all elements of the set.
Example 5: C++ Set empty() and size() Methods
In C++, the empty() and size() methods are called capacity methods.
The empty() method returns
- 1 (true) - if the set is empty
- 0 (false) - if the set is not empty
For example,
#include <iostream>
#include <set>
using namespace std;
int main () {
set<int> my_set = {10, 20, 30};
// set before clear
cout << "The set before clear: ";
for (int i : my_set) {
cout << i << " ";
}
// check if the set is empty
cout << "\nEmpty: " << my_set.empty() << endl;
// check the size of the set
cout << "Size: " << my_set.size() << endl;
// clear values from the set
my_set.clear();
// set after clear
cout << "\nThe set after clear: ";
for (int i : my_set) {
cout << i << " ";
}
// use the capacity methods again
cout << "\nEmpty: " << my_set.empty() << endl;
cout << "Size: " << my_set.size() << endl;
return 0;
}
Output
The set before clear: 10 20 30 Empty: 0 Size: 3 The set after clear: Empty: 1 Size: 0
Here, we have used the empty() and size() methods before and after clearing the contents of the set.
Before clearing the contents:
empty()returns 0, indicating that the set isn't emptysize()returns 3, indicating that there are three elements in the set
After clearing the contents:
empty()returns 1, indicating that the set is emptysize()returns 0, indicating that there are no elements in the set
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