18.3 — Introduction to standard library algorithms

New programmers typically spend a lot of time writing custom loops to perform relatively simple tasks, such as sorting or counting or searching arrays. These loops can be problematic, both in terms of how easy it is to make an error, and in terms of overall maintainability, as loops can be hard to understand.

Because searching, counting, and sorting are such common operations to do, the C++ standard library comes with a bunch of functions to do these things in just a few lines of code. Additionally, these standard library functions come pre-tested, are efficient, work on a variety of different container types, and many support parallelization (the ability to devote multiple CPU threads to the same task in order to complete it faster).

The functionality provided in the algorithms library generally fall into one of three categories:

• Inspectors -- Used to view (but not modify) data in a container. Examples include searching and counting.
• Mutators -- Used to modify data in a container. Examples include sorting and shuffling.
• Facilitators -- Used to generate a result based on values of the data members. Examples include objects that multiply values, or objects that determine what order pairs of elements should be sorted in.

These algorithms live in the algorithms library. In this lesson, we’ll explore some of the more common algorithms -- but there are many more, and we encourage you to read through the linked reference to see everything that’s available!

Note: All of these make use of iterators, so if you’re not familiar with basic iterators, please review lesson 18.2 -- Introduction to iterators.

Using std::find to find an element by value

std::find searches for the first occurrence of a value in a container. std::find takes 3 parameters: an iterator to the starting element in the sequence, an iterator to the ending element in the sequence, and a value to search for. It returns an iterator pointing to the element (if it is found) or the end of the container (if the element is not found).

For example:

#include <algorithm>
#include <array>
#include <iostream>

int main()
{
    std::array arr{ 13, 90, 99, 5, 40, 80 };

    std::cout << "Enter a value to search for and replace with: ";
    int search{};
    int replace{};
    std::cin >> search >> replace;

    // Input validation omitted

    // std::find returns an iterator pointing to the found element (or the end of the container)
    // we'll store it in a variable, using type inference to deduce the type of
    // the iterator (since we don't care)
    auto found{ std::find(arr.begin(), arr.end(), search) };

    // Algorithms that don't find what they were looking for return the end iterator.
    // We can access it by using the end() member function.
    if (found == arr.end())
    {
        std::cout << "Could not find " << search << '\n';
    }
    else
    {
        // Override the found element.
        *found = replace;
    }

    for (int i : arr)
    {
        std::cout << i << ' ';
    }

    std::cout << '\n';

    return 0;
}

Sample run when the element is found

Enter a value to search for and replace with: 5 234
13 90 99 234 40 80

Sample run when the element isn’t found

Enter a value to search for and replace with: 0 234
Could not find 0
13 90 99 5 40 80

Using std::find_if to find an element that matches some condition

Sometimes we want to see if there is a value in a container that matches some condition (e.g. a string that contains a specific substring) rather than an exact value. In such cases, std::find_if is perfect.

The std::find_if function works similarly to std::find, but instead of passing in a specific value to search for, we pass in a callable object, such as a function pointer (or a lambda, which we’ll cover later). For each element being iterated over, std::find_if will call this function (passing the element as an argument to the function), and the function can return true if a match is found, or false otherwise.

Here’s an example where we use std::find_if to check if any elements contain the substring “nut”:

#include <algorithm>
#include <array>
#include <iostream>
#include <string_view>

// Our function will return true if the element matches
bool containsNut(std::string_view str)
{
    // std::string_view::find returns std::string_view::npos if it doesn't find
    // the substring. Otherwise it returns the index where the substring occurs
    // in str.
    return str.find("nut") != std::string_view::npos;
}

int main()
{
    std::array<std::string_view, 4> arr{ "apple", "banana", "walnut", "lemon" };

    // Scan our array to see if any elements contain the "nut" substring
    auto found{ std::find_if(arr.begin(), arr.end(), containsNut) };

    if (found == arr.end())
    {
        std::cout << "No nuts\n";
    }
    else
    {
        std::cout << "Found " << *found << '\n';
    }

    return 0;
}

Output

Found walnut

If you were to write the above example by hand, you’d need at least three loops (one to loop through the array, and two to match the substring). The standard library functions allow us to do the same thing in just a few lines of code!

Using std::count and std::count_if to count how many occurrences there are

std::count and std::count_if search for all occurrences of an element or an element fulfilling a condition.

In the following example, we’ll count how many elements contain the substring “nut”:

#include <algorithm>
#include <array>
#include <iostream>
#include <string_view>

bool containsNut(std::string_view str)
{
	return str.find("nut") != std::string_view::npos;
}

int main()
{
	std::array<std::string_view, 5> arr{ "apple", "banana", "walnut", "lemon", "peanut" };

	auto nuts{ std::count_if(arr.begin(), arr.end(), containsNut) };

	std::cout << "Counted " << nuts << " nut(s)\n";

	return 0;
}

Output

Counted 2 nut(s)

Using std::sort to custom sort

We previously used std::sort to sort an array in ascending order, but std::sort can do more than that. There’s a version of std::sort that takes a function as its third parameter that allows us to sort however we like. The function takes two parameters to compare, and returns true if the first argument should be ordered before the second. By default, std::sort sorts the elements in ascending order.

Let’s use std::sort to sort an array in reverse order using a custom comparison function named greater:

#include <algorithm>
#include <array>
#include <iostream>

bool greater(int a, int b)
{
    // Order @a before @b if @a is greater than @b.
    return (a > b);
}

int main()
{
    std::array arr{ 13, 90, 99, 5, 40, 80 };

    // Pass greater to std::sort
    std::sort(arr.begin(), arr.end(), greater);

    for (int i : arr)
    {
        std::cout << i << ' ';
    }

    std::cout << '\n';

    return 0;
}

Output

99 90 80 40 13 5

Once again, instead of writing our own custom loop functions, we can sort our array however we like in just a few lines of code!

Our greater function needs 2 arguments, but we’re not passing it any, so where do they come from? When we use a function without parentheses (), it’s only a function pointer, not a call. You might remember this from when we tried to print a function without parentheses and std::cout printed “1”. std::sort uses this pointer and calls the actual greater function with any 2 elements of the array. We don’t know which elements greater will be called with, because it’s not defined which sorting algorithm std::sort is using under the hood. We talk more about function pointers in a later chapter.

  std::sort(arr.begin(), arr.end(), greater); // call our custom greater function

  std::sort(arr.begin(), arr.end(), std::greater{}); // use the standard library greater comparison
  // Before C++17, we had to specify the element type when we create std::greater
  std::sort(arr.begin(), arr.end(), std::greater<int>{}); // use the standard library greater comparison

#include <iostream>
#include <iterator>
#include <utility>

void sort(int* begin, int* end)
{
    for (auto startElement{ begin }; startElement != end-1; ++startElement)
    {
        auto smallestElement{ startElement };

        // std::next returns a pointer to the next element, just like (startElement + 1) would.
        for (auto currentElement{ std::next(startElement) }; currentElement != end; ++currentElement)
        {
            if (*currentElement < *smallestElement)
            {
                smallestElement = currentElement;
            }
        }

        std::swap(*startElement, *smallestElement);
    }
}

int main()
{
    int array[]{ 2, 1, 9, 4, 5 };

    sort(std::begin(array), std::end(array));

    for (auto i : array)
    {
        std::cout << i << ' ';
    }

    std::cout << '\n';

    return 0;
}

void sort(int* begin, int* end, std::function<bool(int, int)> compare)

if (*currentElement < *smallestElement)

if (compare(*currentElement, *smallestElement))

#include <functional> // std::function
#include <iostream>
#include <iterator>
#include <utility>

// sort accepts a comparison function
void sort(int* begin, int* end, std::function<bool(int, int)> compare)
{
    for (auto startElement{ begin }; startElement != end-1; ++startElement)
    {
        auto smallestElement{ startElement };

        for (auto currentElement{ std::next(startElement) }; currentElement != end; ++currentElement)
        {
            // the comparison function is used to check if the current element should be ordered
            // before the currently "smallest" element.
            if (compare(*currentElement, *smallestElement))
            {
                smallestElement = currentElement;
            }
        }

        std::swap(*startElement, *smallestElement);
    }
}

int main()
{
    int array[]{ 2, 1, 9, 4, 5 };

    // use std::greater to sort in descending order
    // (We have to use the global namespace selector to prevent a collision
    // between our sort function and std::sort.)
    ::sort(std::begin(array), std::end(array), std::greater{});

    for (auto i : array)
    {
        std::cout << i << ' ';
    }

    std::cout << '\n';

    return 0;
}

Using std::for_each to do something to all elements of a container

std::for_each takes a list as input and applies a custom function to every element. This is useful when we want to perform the same operation to every element in a list.

Here’s an example where we use std::for_each to double all the numbers in an array:

#include <algorithm>
#include <array>
#include <iostream>

void doubleNumber(int& i)
{
    i *= 2;
}

int main()
{
    std::array arr{ 1, 2, 3, 4 };

    std::for_each(arr.begin(), arr.end(), doubleNumber);

    for (int i : arr)
    {
        std::cout << i << ' ';
    }

    std::cout << '\n';

    return 0;
}

Output

2 4 6 8

This often seems like the most unnecessary algorithm to new developers, because equivalent code with a range-based for-loop is shorter and easier. But there are benefits to std::for_each. Let’s compare std::for_each to a range-based for-loop.

std::ranges::for_each(arr, doubleNumber); // Since C++20, we don't have to use begin() and end().
// std::for_each(arr.begin(), arr.end(), doubleNumber); // Before C++20

for (auto& i : arr)
{
    doubleNumber(i);
}

With std::for_each, our intentions are clear. Call doubleNumber with each element of arr. In the range-based for-loop, we have to add a new variable, i. This leads to several mistakes that a programmer could do when they’re tired or not paying attention. For one, there could be an implicit conversion if we don’t use auto. We could forget the ampersand, and doubleNumber wouldn’t affect the array. We could accidentally pass a variable other than i to doubleNumber. These mistakes cannot happen with std::for_each.

Additionally, std::for_each can skip elements at the beginning or end of a container, for example to skip the first element of arr, std::next can be used to advance begin to the next element.

std::for_each(std::next(arr.begin()), arr.end(), doubleNumber);
// Now arr is [1, 4, 6, 8]. The first element wasn't doubled.

This isn’t possible with a range-based for-loop.

Like many algorithms, std::for_each can be parallelized to achieve faster processing, making it better suited for large projects and big data than a range-based for-loop.

Performance and order of execution

Many of the algorithms in the algorithms library make some kind of guarantee about how they will execute. Typically these are either performance guarantees, or guarantees about the order in which they will execute. For example, std::for_each guarantees that each element will only be accessed once, and that the elements will be accessed in forwards sequential order.

While most algorithms provide some kind of performance guarantee, fewer have order of execution guarantees. For such algorithms, we need to be careful not to make assumptions about the order in which elements will be accessed or processed.

For example, if we were using a standard library algorithm to multiply the first value by 1, the second value by 2, the third by 3, etc… we’d want to avoid using any algorithms that didn’t guarantee a forwards sequential execution order!

The following algorithms guarantee sequential execution: std::for_each, std::copy, std::copy_backward, std::move, and std::move_backward. Many other algorithms (particular those that use a forward iterator) are implicitly sequential due to the forward iterator requirement.

Ranges in C++20

Having to explicitly pass arr.begin() and arr.end() to every algorithm is a bit annoying. But fear not -- C++20 adds ranges, which allow us to simply pass arr. This will make our code even shorter and more readable.

Conclusion

The algorithms library has a ton of useful functionality that can make your code simpler and more robust. We only cover a small subset in this lesson, but because most of these functions work very similarly, once you know how a few work, you can make use of most of them.