How to Erase Duplicates and Sort a Vector in C++?

Last Updated : 29 Nov, 2024

In this article, we will learn how to remove duplicates and sort a vector in C++.

The simplest method to remove the duplicates and sort the vector is by using sort() and unique() functions. Let’s take a look at an example:

C++

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

int main() {
    vector<int> v = {1, 3, 2, 3, 1, 8};

    // Fort the vector
    sort(v.begin(), v.end());

    // Group unique elements together
    auto it = unique(v.begin(), v.end());

    // Erase duplicates
    v.erase(it, v.end());

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

Output

1 2 3 8

Explanation: In this method, first we sort the vector using sort() method. Then unique() method moves the unique elements to the front of vector and returns an iterator to the new end of vector. Finally, we call vector erase() with the range from returning iterator to the vector's end to remove all duplicates or extra elements.

There are also some other methods in C++ to remove the duplicates and sort vector. Some of them are as follows:

Table of Content

Using set
Using Counting Sort
Manually Using Loop

Using Set

Copy all its elements into a set which automatically stores unique elements in sorted order. Then assign the elements from the set back to the vector resulting in a sorted vector with only unique elements.

C++

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

int main() {
    vector<int> v = {1, 3, 2, 3, 1, 8};

    // Copy elements from vector to set
    set<int> s(v.begin(), v.end());

    // Assign elements back to vector from set
    v.assign(s.begin(), s.end());

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

Output

1 2 3 8

Using Counting Sort

Counting Sort is a non-comparison based sorting algorithm that can be used when the range of the elements is known and limited. This method can help remove duplicates efficiently by counting the occurrences of each element and storing the unique elements.

C++

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

vector<int> remDuplicate(vector<int>& v) {
  
  	// Counting sort logic
    int m = *max_element(v.begin(), v.end());
    vector<int> count(m + 1, 0);
    for (int i : v) {
        count[i]++;
    }

    // Collect unique elements in res
    vector<int> res;
    for (int i = 0; i <= m; ++i) {
        if (count[i] > 0) {
            res.push_back(i);
        }
    }

    return res;
}

int main() {
    vector<int> v = {1, 3, 2, 3, 1, 8};
    
    // Call the function to remove duplicates
    // and sort the vector
    v = remDuplicate(v);

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

Output

1 2 3 8

Manually Using Loop

In this method, first we sort the vector to group the same type of elements together. Then we iterate through vector using loop and delete duplicates using vector erase().

C++

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

int main() {
    vector<int> v = {1, 3, 2, 3, 1, 8};

    // Sort the vector
    sort(v.begin(), v.end());

    // Remove duplicates from vector
    int i = 1;
    while (i < v.size()) {
        while (i < v.size()) {
            if (v[i] == v[i - 1])
                v.erase(v.begin() + i);
            else
                break;
        }
        i++;
    }

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

Output

1 2 3 8

How to Store Duplicate Elements in Ordered Set in C++?

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How to Erase Duplicates and Sort a Vector in C++?

Using Set

Using Counting Sort

Manually Using Loop

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