Searching for Multiple Occurrences of a Value in a Container

To search for multiple occurrences of a value in a container, you can use a combination of std::ranges::find() and a loop, or std::ranges::find_if() if the search criteria are more complex.

Using a Loop with std::ranges::find()

One way to find all occurrences is to use std::ranges::find() in a loop:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 2, 4, 2, 5};
7
8  int target = 2;
9  auto it = numbers.begin();
10
11  while ((it = std::ranges::find(
12    it, numbers.end(), target)) != numbers.end()) {
13    std::cout << "Found " << target
14      << " at position " << std::distance(
15        numbers.begin(), it) << '\n';
16    ++it; // Move to the next element
17  }
18}

1Found 2 at position 1
2Found 2 at position 3
3Found 2 at position 5

Explanation

• Use std::ranges::find() to find the first occurrence of the target value.
• Move the iterator one position forward after each find to continue the search.
• Loop until no more occurrences are found.

Using std::ranges::find_if()

If you need to find occurrences based on a condition, use std::ranges::find_if():

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{1, 2, 3, 2, 4, 2, 5};
7
8  auto it = numbers.begin();
9
10  while ((it = std::ranges::find_if(
11    it, numbers.end(), [](int n) {
12      return n == 2;
13    })) != numbers.end()) {
14    std::cout << "Found 2 at position "
15      << std::distance(numbers.begin(), it)
16      << "\n";
17    ++it;  // Move to the next element
18  }
19}

1Found 2 at position 1
2Found 2 at position 3
3Found 2 at position 5

Explanation

• Use std::ranges::find_if() with a lambda function to specify the search condition.
• Move the iterator forward after each find to continue searching.

Summary

Using a loop with std::ranges::find() or std::ranges::find_if(), you can effectively search for multiple occurrences of a value in a container.

This method allows you to handle simple equality checks or more complex conditions, making it a versatile approach for various search scenarios.