Efficiently Remove Duplicates

To efficiently remove duplicate elements from a container using std::ranges::remove(), you need to first ensure the container is sorted.

This is because std::ranges::remove() works on contiguous elements and duplicates must be adjacent for this to work effectively.

Here's a step-by-step guide:

1. Sort the container to bring duplicate elements next to each other.
2. Use std::ranges::remove() to shift the duplicates to the end.
3. Use the erase() method to remove the surplus elements.

Here's an example using a std::vector:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> Source{3, 1, 2, 3, 2, 1, 4, 5};
7
8  // Step 1: Sort the container
9  std::sort(Source.begin(), Source.end());
10
11  // Step 2: Apply std::ranges::remove to shift duplicates
12  auto NewEnd = std::ranges::unique(Source);
13
14  // Step 3: Erase surplus elements
15  Source.erase(NewEnd.begin(), NewEnd.end());
16
17  // Display the result
18  std::cout << "Unique elements in Source: ";
19  for (auto Num : Source) {
20    std::cout << Num << ", ";
21  }
22}

1Unique elements in Source: 1, 2, 3, 4, 5,

In this example:

• We first sort the Source vector to arrange the elements in ascending order.
• We then use std::ranges::unique() to move the duplicate elements to the end. Note that std::ranges::unique() is more suitable for this task than std::ranges::remove(), as it directly deals with adjacent duplicates.
• Finally, we erase the surplus elements to resize the container.

Using std::ranges::remove_if() for Complex Conditions

If you need to remove duplicates based on a more complex condition (e.g., user-defined types), you can use std::ranges::remove_if() in combination with sorting:

1#include <algorithm>
2#include <iostream>
3#include <string>
4#include <vector>
5
6struct Player {
7  std::string Name;
8  int Score;
9
10  bool operator<(const Player& other) const {
11    return Score < other.Score;  // Sort by Score
12  }
13};
14
15int main() {
16  std::vector<Player> Players{
17    {"Alice", 10}, {"Bob", 20},
18    {"Charlie", 10}, {"Diana", 20},
19    {"Eve", 30}
20  };
21
22  // Step 1: Sort the container
23  std::sort(Players.begin(), Players.end());
24
25  // Step 2: Apply std::ranges::unique to shift duplicates
26  auto NewEnd = std::ranges::unique(
27    Players,
28    [](const Player& a, const Player& b) {
29      return a.Score == b.Score;
30    }
31  );
32
33  // Step 3: Erase surplus elements
34  Players.erase(NewEnd.begin(), NewEnd.end());
35
36  // Display the result
37  std::cout << "Unique players by score:\n";
38  for (const auto& player : Players) {
39    std::cout << player.Name << " (Score "
40      << player.Score << ")\n";
41  }
42}

1Unique players by score:
2Alice (Score 10)
3Bob (Score 20)
4Eve (Score 30)

In this example:

• We define a custom comparison operator for sorting Player objects by their Score.
• We use std::ranges::unique() with a custom predicate to handle duplicates based on the Score.
• The erase() method removes the surplus elements, resulting in a container with unique scores.

By sorting the container and using std::ranges::unique(), you can efficiently remove duplicates and maintain a clean, unique set of elements.