How to Remove Elements from a List based on the given Condition

Lists are sequence containers that allow for the allocation of non-contiguous memory. List traversal is slower than vector traversal, but once a position is found, insertion and deletion are fast. Normally, when we talk about a List, we mean a doubly linked list. We use a forward list to implement a singly linked list.

Given a list, the task is to remove all the elements from the list which satisfies the given condition

Examples:

Input:

intlist({ 1, 3, 5, 6, 7, 5, 2, 3, 5, 9, 8 }) 
element=5

Output:

1 3 6 7 2 3 9 8

Remove Elements from a List based on the given Condition

The member functions std::list::remove and std::list::remove_if are used to remove elements based on their value.

Method #1:Using list::remove

void remove (const value_type& value);

It removes all items from a list matching the passed element value.

Let us remove all the occurrences of element ‘5’ from the list.

Below is the implementation:

#include <bits/stdc++.h>
using namespace std;
int main()
{
    // creating a integer list and initialize with some
    // elements
    list<int> intlist({ 1, 3, 5, 6, 7, 5, 2, 3, 5, 9, 8 });
    // removing '5' element from the given intlist
    intlist.remove(5);
    // print the values of the list
    for (int value : intlist)
        cout << value << " ";

    return 0;
}

Output:

1 3 6 7 2 3 9 8

Method #2:Using remove_if

We deleted all items from a list that match the passed values in the preceding example. However, this may not always be the requirement. Some elements must be removed from a list many times based on certain criteria Like,

Remove from the list all items greater than 1 but less than 6 .

In order to delete every element in the list with a criterion, we use another members function of std::list

template <class Predicate>
void remove_if (Predicate predCallback);

As an argument, std::list::remove if accepts a Callback inform of a Predicate. Then it iterates through the list of elements, calling our Predicate on each one. Elements in the list that have a Predicate that returns true will be removed.

This Predicate is a Callback in this case, which can be,

Function Object
Function Lambda
Function Object

Let’s look at how to use remove if and lambda to delete all elements from a list whose value is greater than or equal to 1 but less than 6.

Below is the implementation:

#include <bits/stdc++.h>
using namespace std;
int main()
{
    // creating a integer list and initialize with some
    // elements
    list<int> intlist({ 1, 3, 5, 6, 7, 5, 2, 3, 5, 9, 8 });

    // removing all the elements which are greater than 1
    // and less than 6
    intlist.remove_if([](const int& value) {
        if (value > 1 && value < 6)
            return true;
        else
            return false;
    });

    // print the values of the list
    for (int value : intlist)
        cout << value << " ";

    return 0;
}

Output: