Learn Predicates in STL | Working with Predicates and Algorithms

Predicates are a fundamental concept in the Standard Template Library (STL) and play a crucial role in making algorithms flexible and reusable. A predicate is any callable entity—such as a function, function pointer, function object, or lambda expression—that takes one or more arguments and returns a boolean value. In the context of STL algorithms, predicates allow you to specify custom conditions for searching, filtering, counting, or transforming elements in a collection.

The importance of predicates lies in their ability to encapsulate logic that determines whether an element satisfies a certain condition. This enables algorithms like std::count_if, std::find_if, and std::remove_if to operate on a wide variety of problems without being tied to a specific criterion. By passing different predicates, you can reuse the same algorithm for different tasks, making your code more modular and expressive.

main.cpp


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

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    // Lambda predicate to check if a number is even
    auto is_even = [](int n) {
        return n % 2 == 0;
    };

    // Use std::count_if with the predicate
    int even_count = std::count_if(numbers.begin(), numbers.end(), is_even);

    std::cout << "Number of even numbers: " << even_count << std::endl;

    return 0;
}

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Section 2. Chapter 1

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main.cpp


              1234567891011121314151617181920
            
#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    // Lambda predicate to check if a number is even
    auto is_even = [](int n) {
        return n % 2 == 0;
    };

    // Use std::count_if with the predicate
    int even_count = std::count_if(numbers.begin(), numbers.end(), is_even);

    std::cout << "Number of even numbers: " << even_count << std::endl;

    return 0;
}

Everything was clear?

Thanks for your feedback!

Section 2. Chapter 1