std::remove in C++ STL

The std::remove() is a built-in algorithm of the C++ STL that is used to remove an element from the specified range of elements. This range can be the whole or a part of vectors, deque, arrays, lists and forward_lists only. std::remove is defined inside <algorithm> header file.

In this article, we will learn how to delete the given element from the container using std::remove() in C++.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1, 2, 3, 4, 3, 5};

    // Remove all occurrences of 3 from vector v
    auto ne = remove(v.begin(), v.end(), 3);
  	v.erase(ne, v.end());

    for (auto i: v)
        cout << i << " ";
    return 0;
}

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As you may have noticed, we still had to use the vector::erase() function to actually delete the elements. This is due to the fact that std::remove() function moves all the elements to remove at the end of the container and returns an iterator to the new end of the range that only contains the elements not to remove. It doesn't actually delete these elements from the container.

So, we still have to use the erase() function of the corresponding container to actually delete the elements.

std::remove Syntax

std::remove(first, last, val);

Parameters

• first: Iterator to the first element of the container.
• last: Iterator to the element after the last element of the container.
• val: Value to be removed.

Return Value

• Returns an iterator to the new end of the modified range that only contains the elements not be deleted.
• If there are no elements to be deleted, returns iterator to the container's end.

More Examples of std::remove()

std::remove() function can be used to remove the elements from any given container or an array as demonstrated by the below examples:

Example 1: Removing an Element from an Array

// C++ program to remove an element from an array
// using std::remove
#include <bits/stdc++.h>
using namespace std;

int main() {
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 3, 8};
    int s = sizeof(arr) / sizeof(arr[0]);

    // Use std::remove to shift all occurences of 3
  	// to the front
    int *ne = remove(arr, arr + s, 3);

    // Calculate the new size after removal
    int ns = ne - arr;
    for (int i = 0; i < ns; i++)
        cout << arr[i] << " ";
    return 0;
}

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Example 2: Using std::remove() without Erase Function

// C++ Program to illustrate how to remove the 
#include <bits/stdc++.h>
using namespace std;

int main() {
    list<int> l = {1, 2, 3, 4, 3, 5};
  	cout << "Initial Size: " << l.size()
      << endl;

    // Using only std::remove() to remove all
  	// occurrences of 3 from list
    auto ne = remove(l.begin(), l.end(), 3);
	
  	cout << "Size After Removing 3: "
      << l.size();
    return 0;
}

Initial Size: 6
Size After Removing 3: 6

Explanation: This program verifies that std::remove() algorithm does not actually remove the elements from the container, just shifts them. Due to this, there is no effect on the size of the container.

Example 3: Trying to Remove an Element not Present in the Vector

// C++ program to remove an element not present in
// a vector using std::remove()
#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<int> v = {1, 2, 4, 5, 6};

    // Try to remove 3 that is not present
    auto ne = remove(v.begin(), v.end(), 3);

    // Erase the 'removed' part which does nothing
  	// since 3 isn't in the vector
    v.erase(ne, v.end());

    for (int i : v)
        cout << i << " ";
    return 0;
}

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