C++ List

C++ List is a STL container that stores elements randomly in unrelated locations. To maintain sequential ordering, every list element includes two links:

• one that points to the previous element
• another that points to the next element

In C++, the STL list implements the doubly-linked list data structure. As a result, we can iterate both forward and backward.

Create C++ STL List

To create a list, we need to include the list header file in our program.

#include<list>

Once we import the header file, we can now declare a list using the following syntax:

std::list<Type> list_name = {value1, value2, ...};

Here,

• std::list - declares a STL container of type list
• <Type> - the data type of the values to be stored in the list
• list_name - a unique name given to the list
• value1, value2, ... - values to be stored in the list

Let's see an example,

// create a list of integer type
std::list<int> numbers = {1, 2, 3, 4, 5};

// create a list of character type
std::list<char> vowels = {'a', 'e', 'i', 'o', 'u'};

std::list<int> {1, 2, 3, 4, 5};

Example: C++ STL List

#include <iostream>
#include <list>

using namespace std;

int main() {

    // create the list
    list<int> numbers {1, 2, 3, 4};
  
    // display the elements of the list
    cout << "List Elements: ";
    for(int number : numbers) {
        cout << number <<", ";
    }
    
    return 0;

}

Output

List Elements: 1, 2, 3, 4,

In the above example, we have created a list named numbers with elements: 1, 2, 3, 4. We then used a ranged for loop to print the list elements.

Note: We have used list instead of std::list because we have already defined std namespace using using namespace std;.

Basic operations on List

C++ STL provides various functions that we can use to perform different operations on lists. Let's look at some commonly used list functions to perform the following operations:

1. Add elements
2. Access elements
3. Remove elements

1. Add Elements to a List in C++

We can add values in a list using the following functions:

• push_front() - inserts an element to the beginning of the list
• push_back() - adds an element to the end of the list

Let's see an example,

#include <iostream>
#include <list>

using namespace std;

int main() {
    
    // create a list
    list<int> numbers = {1, 2, 3};
  
    // display the original list 
    cout << "Initial List: ";
    for(int number: numbers) {
        cout << number << ", ";
    }
  
    // add element at the beginning
    numbers.push_front(0);

    // add element at the end
    numbers.push_back(4);

    // display the modified list
    cout << endl << "Final List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }

  return 0;

}

Output

Initial List: 1, 2, 3, 
Final List: 0, 1, 2, 3, 4,

2. Access List Elements

We can access list elements using the following functions:

• front() - returns the first element of the list
• back() - returns the last element of the list

Let's see an example,

#include <iostream>
#include <list>

using namespace std;

int main() {

    // create a list
    list<int> numbers = {1, 2, 3, 4, 5};
  
    // display the first element
    cout << "First Element: " << numbers.front() << endl;
  
    // display the last element
    cout << "Last Element: " << numbers.back();
  
    return 0;
    
}

Output

First Element: 1
Last Element: 5

3. Delete List Elements

We can delete list elements using the following functions:

• pop_front() - removes the element at the beginning of the list
• pop_back() - removes the element at the end of the list

Here's an example,

#include <iostream>
#include <list>

using namespace std;

int main() {
    // create a list
    list<int> numbers = {1, 2, 3, 4, 5};
 
    // display the original list 
    cout << "Inital List: ";
    for(int number : numbers) {
        cout << number << ",  ";
    }

    // remove the first element of the list
    numbers.pop_front();

    // remove the last element of the list
    numbers.pop_back();

    // display the modified list 
    cout << endl << "Final List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }

    return 0;
}

Output

Inital List: 1,  2,  3,  4,  5,  
Final List: 2, 3, 4, 

Other List Functions in C++

While there are many functions that can be used with lists, we will only look at some of the functions in the table below:

Access elements using an iterator

We can use iterators to access a list element at a specified position. For example,

#include <iostream>
#include <list>

using namespace std;

int main() {

     // create a list
    list<int> numbers = {1, 2, 3, 4, 5};

    // create an iterator to point to the first element of the list
    list<int>::iterator itr = numbers.begin();
  
    // increment itr to point to the 2nd element
    ++itr;
    
    //display the 2nd element
    cout << "Second Element: " << *itr << endl;
  
    // increment itr to point to the 4th element
    ++itr;
    ++itr;

    // display the 4th element
    cout << "Fourth Element: " << *itr;
  
    return 0;
}

Output

Second Element: 2
Fourth Element: 4

In the above example,

• list<int>::iterator - defines an iterator for a list of int type
• numbers.begin() - sets the iterator to point to the beginning of the list

Notice that we have used ++itr; repeatedly instead of adding an integer to itr like itr+3;.

This is because iterators are not simple numeric values like regular integers. They point to specific memory locations in the container.

Incrementing an iterator with the ++ operator makes it point to the next element in the container.

To learn more about iterators, visit C++ STL Iterators.

list_name.insert(iterator, value);

#include <iostream>
#include <list>

using namespace std;

int main() {
    
    // create a list
    list<int> numbers {1, 2, 3};
    
    // display the original list  
    cout<<"Initial List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }

    // create an iterator to point to the 1st position
    list<int>::iterator itr = numbers.begin();

    // increment the iterator to point to the 3rd position
    ++itr; 
    ++itr;  

    // insert 0 at the 3rd position of list
    numbers.insert(itr, 0);

    // display the modified list  
    cout<<endl<<"Final List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }
    
    return 0;

}

Initial List: 1, 2, 3, 
Final List: 1, 2, 0, 3,

list_name.remove(element);

#include <iostream>
#include <list>

using namespace std;

int main() {

    // create a list
    list<int> numbers {1, 2, 1, 3, 4, 1};
  
    // display the original list  
    cout << "Initial List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }
  
    // remove all the elements with value 1
    numbers.remove(1);

    // display the modified list
    cout << endl << "Final List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }

    return 0;

}

Initial List: 1, 2, 1, 3, 4, 1, 
Final List: 2, 3, 4, 

#include <iostream>
#include <list>

using namespace std;

int main() {
    
    // create a list and an iterator
    list<int> numbers {0, 1, 2, 3, 4, 5, 3};
    list<int>::iterator itr = numbers.begin();
  
    // display the original list
    cout<<"Initial List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }

    // point iterator to the 4th element
    for (int i = 0; i < 3; i++){
        ++itr;
    }

    // remove the 4th element
    numbers.remove(*itr);

    // display the modified list
    cout << endl << "Final List: ";
    for(int number : numbers) {
        cout << number << ", ";
    }

  return 0;
  
}

Initial List: 0, 1, 2, 3, 4, 5, 3, 
Final List: 0, 1, 2, 4, 5,