List in C++ - Some Useful Functions
Last Updated :
23 Mar, 2023
Lists are sequence containers that allow non-contiguous memory allocation. As compared to vector, the List has slow traversal, but once a position has been found, insertion and deletion is quick.
List Useful Functions:
1. emplace(position, value): This function is used to insert an element at the specified position.
2. emplace_back(value) :- This function adds value at end of list. It is different from push_back() by the fact that it directly creates elements at position, whereas push_back() first makes a temporary copy and copies from there. emplace_back() is faster in implementation than push_back() in most situations.
3. emplace_front(value): This function adds value at beginning of the List. It is different from push_front() by the fact that it directly creates elements at position, whereas push_front() first makes a temporary copy and copies from there. emplace_front() is faster in implementation than push_front() in most situations.
CPP
// C++ code to demonstrate the working of
// emplace(), emplace_front() and emplace_back()
#include <iostream>
#include <list> // for list functions
using namespace std;
// Driver Code
int main()
{
// Declaring a list
list<int> gqlist;
// Initialising list iterator
list<int>::iterator it = gqlist.begin();
// Entering list element using emplace_back()
for (int i = 1; i <= 5; i++)
gqlist.emplace_back(i);
// Displaying list elements
cout << "List after emplace_back operation is : ";
for (int& x : gqlist)
cout << x << " ";
cout << endl;
// Entering list element using emplace_front()
for (int i = 10; i <= 50; i += 10)
gqlist.emplace_front(i);
// Displaying list elements
cout << "List after emplace_front operation is : ";
for (int& x : gqlist)
cout << x << " ";
cout << endl;
// using advance() to advance iterator position
advance(it, 2);
// inserting element at 2nd position using emplace()
gqlist.emplace(it, 100);
// Displaying list elements
cout << "List after emplace operation is : ";
for (int& x : gqlist)
cout << x << " ";
cout << endl;
return 0;
}
OutputList after emplace_back operation is : 1 2 3 4 5
List after emplace_front operation is : 50 40 30 20 10 1 2 3 4 5
List after emplace operation is : 50 100 40 30 20 10 1 2 3 4 5
Time Complexity: O(1)
Auxiliary Space: O(1)
4. merge(list2): This function is used to merge list2 with list1. If both the lists are in sorted order, then the resulting list is also sorted.
5. remove_if(condition): This function removes the element from the List on the basis of the condition given in its argument.
CPP
// C++ code to demonstrate the working of
// merge() and remove_if()
#include <iostream>
#include <list> // for list functions
using namespace std;
// Driver Code
int main()
{
// Initializing list1
list<int> gqlist1 = { 1, 2, 3 };
// Initializing list2
list<int> gqlist2 = { 2, 4, 6 };
// using merge() to merge list1 with list2
gqlist1.merge(gqlist2);
// Displaying list elements
cout << "list1 after merge operation is : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl;
// using remove_if() to remove odd elements
// removes 1 and 3
gqlist1.remove_if([](int x) { return x % 2 != 0; });
// Displaying list elements
cout << "list1 after remove_if operation is : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl;
return 0;
}
Outputlist1 after merge operation is : 1 2 2 3 4 6
list1 after remove_if operation is : 2 2 4 6
Time Complexity: O(1)
Auxiliary Space: O(1)
6. unique(): This function is used to delete the repeated occurrences of the number. List has to be sorted for this function to get executed.
7. splice(position, list2): This function is used to transfer elements from one list into another.
CPP
// C++ code to demonstrate the working of
// unique() and splice()
#include <iostream>
#include <list> // for list functions
using namespace std;
// Driver Code
int main()
{
// Initializing list1
list<int> gqlist1 = { 1, 1, 1, 2, 2, 3, 3, 4 };
// Initializing list2
list<int> gqlist2 = { 2, 4, 6 };
// Initializing list1 iterator
list<int>::iterator it = gqlist1.begin();
// using advance() to increment iterator position
advance(it, 3);
// Displaying list elements
cout << "list1 before unique operation is : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl;
// using unique() to remove repeating elements
gqlist1.unique();
// Displaying list elements
cout << "list1 after unique operation is : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl << endl;
// using splice() to splice list2 in list1 at position
// it inserts list2 after 2nd position
gqlist1.splice(it, gqlist2);
// Displaying list elements
cout << "list1 after splice operation is : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl;
return 0;
}
Outputlist1 before unique operation is : 1 1 1 2 2 3 3 4
list1 after unique operation is : 1 2 3 4
list1 after splice operation is : 1 2 4 6 2 3 4
Time Complexity: O(1)
Auxiliary Space: O(1)
8. swap(list2): This function is used to swap one list element with other.
CPP
// C++ code to demonstrate the working of
// swap()
#include <iostream>
#include <list> // for list functions
using namespace std;
// Driver Code
int main()
{
// Initializing list1
list<int> gqlist1 = { 1, 2, 3, 4 };
// Initializing list1
list<int> gqlist2 = { 2, 4, 6 };
// Displaying list before swapping
cout << "The contents of 1st list "
"before swapping are : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl;
cout << "The contents of 2nd list "
"before swapping are : ";
for (int& x : gqlist2)
cout << x << " ";
cout << endl;
// Use of swap() to swap the list
gqlist1.swap(gqlist2);
// Displaying list after swapping
cout << "The contents of 1st list "
"after swapping are : ";
for (int& x : gqlist1)
cout << x << " ";
cout << endl;
cout << "The contents of 2nd list "
"after swapping are : ";
for (int& x : gqlist2)
cout << x << " ";
cout << endl;
return 0;
}
OutputThe contents of 1st list before swapping are : 1 2 3 4
The contents of 2nd list before swapping are : 2 4 6
The contents of 1st list after swapping are : 2 4 6
The contents of 2nd list after swapping are : 1 2 3 4
Time Complexity: O(1)
Auxiliary Space: O(1)
9. reverse(): This function is used to reverse the order of elements in a list.
C++
#include <iostream>
#include <list>
int main()
{
// Create a list of integers with 5 elements
std::list<int> mylist{ 1, 2, 3, 4, 5 };
// Print the original list
std::cout << "Original list: ";
for (auto it = mylist.begin(); it != mylist.end();
it++) {
std::cout << *it << " ";
}
std::cout << std::endl;
// Reverse the order of elements in the list
mylist.reverse();
// Print the reversed list
std::cout << "Reversed list: ";
for (auto it = mylist.begin(); it != mylist.end();
it++) {
std::cout << *it << " ";
}
std::cout << std::endl;
// Return 0 to indicate successful completion of the
// program
return 0;
}
OutputOriginal list: 1 2 3 4 5
Reversed list: 5 4 3 2 1
Time Complexity: O(1) where n is the number of elements in the list. This is because the function simply iterates over the list once and swaps the pointers for each element.
Auxiliary Space: O(1) where it does not use any extra memory beyond a few temporary variables used for swapping pointers.