Finding the Last Occurrence of an Element in a Container

To find the last occurrence of an element in a container using std::ranges::find() or std::find(), you can use the std::ranges::find_end() algorithm.

This algorithm searches for the last occurrence of a subsequence in a range, which can be adapted to find a single element.

Using std::ranges::find_end()

Here's how you can use std::ranges::find_end() to find the last occurrence of an element:

1#include <algorithm>
2#include <iostream>
3#include <ranges>
4#include <vector>
5
6int main() {
7  std::vector<int> numbers{1, 2, 3, 4, 2, 5, 2};
8
9  int target = 2;
10  auto result = std::ranges::find_end(
11    numbers, std::views::single(target));
12
13  if (result.begin() != numbers.end()) {
14    std::cout << "Last occurrence of "
15      << target << " found at position "
16      << std::distance(
17        numbers.begin(), result.begin());
18  } else {
19    std::cout << target << " not found";
20  }
21}

1Last occurrence of 2 found at position 6

Explanation

• std::ranges::find_end() is used to find the last occurrence of a subsequence in a range.
• In this case, the subsequence is a single element, represented by a pointer to the target and one past the target.

Alternative: Using Reverse Iterators

Another way to find the last occurrence is to use reverse iterators with std::find():

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 4, 2, 5, 2};
7
8  int target = 2;
9  auto rresult = std::find(
10    numbers.rbegin(), numbers.rend(), target);
11
12  if (rresult != numbers.rend()) {
13    auto position = std::distance(
14      numbers.begin(), rresult.base()) - 1;
15    std::cout << "Last occurrence of "
16      << target << " found at position "
17      << position;
18  } else {
19    std::cout << target << " not found";
20  }
21}

1Last occurrence of 2 found at position 6

Explanation

• std::find() is used with reverse iterators (rbegin() and rend()) to search the container from the end to the beginning.
• The position is calculated by converting the reverse iterator back to a normal iterator and adjusting the index.

Using either std::ranges::find_end() or reverse iterators with std::find(), you can efficiently find the last occurrence of an element in a container.