C++ Algorithms and Iterators

The header <algorithm> has ready functions which work on ranges described by iterators. An iterator is like a pointer to an item of a container. begin() is the first and end() is one after the last.

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

int main() {
    std::vector<int> v = {5, 2, 9, 1, 7, 2};

    std::sort(v.begin(), v.end());
    for (int x : v) std::cout << x << ' ';
    std::cout << '\n';

    std::cout << std::count(v.begin(), v.end(), 2) << '\n';
    std::cout << *std::min_element(v.begin(), v.end()) << ' ' << *std::max_element(v.begin(), v.end()) << '\n';
    std::cout << std::accumulate(v.begin(), v.end(), 0) << '\n';
    std::cout << std::binary_search(v.begin(), v.end(), 7) << '\n';       // needs a sorted range

    auto it = std::find(v.begin(), v.end(), 9);
    if (it != v.end()) std::cout << "found at index " << it - v.begin() << '\n';
}

transform, for_each and custom rules

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

int main() {
    std::vector<int> v = {1, 2, 3, 4};
    std::vector<int> squares(v.size());
    std::transform(v.begin(), v.end(), squares.begin(), [](int n) { return n * n; });
    for (int x : squares) std::cout << x << ' ';
    std::cout << '\n';

    std::vector<std::string> names = {"Meera", "Al", "Riya", "Karan"};
    std::sort(names.begin(), names.end(), [](const std::string& a, const std::string& b) {
        return a.size() < b.size();
    });
    for (const auto& n : names) std::cout << n << ' ';
    std::cout << '\n';
}

Remove items: the erase-remove idiom

std::remove_if only moves the unwanted items to the end. You must call erase too. In C++20 there is std::erase_if, which does both:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v = {1, 2, 3, 4, 5, 6};
    std::erase_if(v, [](int n) { return n % 2 == 0; });
    for (int x : v) std::cout << x << ' ';
    std::cout << '\n';
}

Iterator loop

#include <iostream>
#include <list>

int main() {
    std::list<char> letters = {'a', 'b', 'c'};
    for (auto it = letters.begin(); it != letters.end(); ++it) {
        std::cout << *it;
    }
    std::cout << '\n';
}

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