IntermediatePractice in the editor

C++ Algorithms

The header algorithm contains dozens of ready-made algorithms, and numeric adds a few for numbers. They work on any container through iterators, usually begin() and end(). Using them instead of hand-written loops makes code shorter, clearer and less buggy.

Sorting, searching and counting

ExampleTry it in the editor

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

int main() {
    std::vector<int> v = {42, 7, 19, 88, 3, 19};
    std::sort(v.begin(), v.end());
    for (int x : v) std::cout << x << " ";
    std::cout << "\n";

    auto it = std::find(v.begin(), v.end(), 88);
    if (it != v.end()) std::cout << "88 found at position " << (it - v.begin()) << "\n";

    std::cout << "19 appears " << std::count(v.begin(), v.end(), 19) << " times\n";
    std::cout << "sum " << std::accumulate(v.begin(), v.end(), 0) << "\n";
    std::cout << "max " << *std::max_element(v.begin(), v.end()) << "\n";
    return 0;
}

Output

3 7 19 19 42 88 
88 found at position 5
19 appears 2 times
sum 178
max 88

With lambdas

Many algorithms take a condition or a rule as a lambda: count_if, find_if, sort with a custom order, transform and remove_if.

ExampleTry it in the editor

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

int main() {
    std::vector<int> marks = {45, 72, 88, 39, 91};
    int passed = std::count_if(marks.begin(), marks.end(), [](int m) { return m >= 50; });
    std::cout << passed << " passed\n";

    auto first = std::find_if(marks.begin(), marks.end(), [](int m) { return m > 80; });
    std::cout << "first above 80: " << *first << "\n";

    std::string word = "hello";
    std::transform(word.begin(), word.end(), word.begin(), [](unsigned char c) { return std::toupper(c); });
    std::cout << word << "\n";

    std::vector<int> v = {1, -2, 3, -4, 5};
    v.erase(std::remove_if(v.begin(), v.end(), [](int x) { return x < 0; }), v.end());
    for (int x : v) std::cout << x << " ";
    std::cout << "\n";
    return 0;
}

Output

3 passed
first above 80: 88
HELLO
1 3 5

remove_if moves the kept elements to the front and returns where the rest starts; erase then cuts them off. This "erase-remove" pair is a classic; C++20's std::erase_if(v, ...) does both.

Other useful algorithms

Algorithm	Does
`std::reverse`	reverses the order
`std::binary_search`	fast search in a sorted range
`std::unique`	removes neighbouring duplicates (sort first)
`std::fill`, `std::iota`	fills with one value, or with 1, 2, 3, …
`std::all_of`, `std::any_of`, `std::none_of`	test a condition on all elements
`std::min`, `std::max`, `std::minmax_element`	smallest and largest

Ranges (C++20)

The ranges versions take the whole container instead of a pair of iterators, and views can filter and transform lazily:

ExampleTry it in the editor

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

int main() {
    std::vector<int> v = {5, 2, 8, 1, 9, 4};
    std::ranges::sort(v);
    for (int x : v | std::views::filter([](int x) { return x % 2 == 0; })
                   | std::views::transform([](int x) { return x * x; })) {
        std::cout << x << " ";
    }
    std::cout << "\n";
    return 0;
}

Output

4 16 64

Why use algorithms

A named algorithm says what the code does: std::count_if is clearer than a loop with a counter, and it is tested and fast. Most algorithms take a range as two iterators, begin and end, so the same function works with vectors, arrays, lists and strings. Remember that std::remove does not shrink a container; combine it with erase, or use C++20's std::erase_if.

Practice exercises

Sort a vector of words alphabetically, then by length with a lambda.
Use std::any_of to check whether a vector contains a negative number.
Remove duplicates from a vector with sort, unique and erase.