Search Algorithms

# Searching for Multiple Occurrences of a Value in a Container

## What is the best way to search for multiple occurrences of a value in a container using standard library algorithms?

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:

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
std::vector<int> numbers{1, 2, 3, 2, 4, 2, 5};

int target = 2;
auto it = numbers.begin();

while ((it = std::ranges::find(
it, numbers.end(), target)) != numbers.end()) {
std::cout << "Found " << target
<< " at position " << std::distance(
numbers.begin(), it) << '\n';
++it; // Move to the next element
}
}
Found 2 at position 1
Found 2 at position 3
Found 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():

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
std::vector<int> numbers{1, 2, 3, 2, 4, 2, 5};

auto it = numbers.begin();

while ((it = std::ranges::find_if(
it, numbers.end(), [](int n) {
return n == 2;
})) != numbers.end()) {
std::cout << "Found 2 at position "
<< std::distance(numbers.begin(), it)
<< "\n";
++it;  // Move to the next element
}
}
Found 2 at position 1
Found 2 at position 3
Found 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.

This Question is from the Lesson:

### Search Algorithms

An introduction to the 8 main searching algorithms in the C++ standard library, including find(), find_if(), find_if_not(), find_first_of(), adjacent_find(), search_n(), search(), and find_end().

Answers to questions are automatically generated and may not have been reviewed.

This Question is from the Lesson:

### Search Algorithms

An introduction to the 8 main searching algorithms in the C++ standard library, including find(), find_if(), find_if_not(), find_first_of(), adjacent_find(), search_n(), search(), and find_end().

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