Vector Algorithms

Here we’ll look at some of the standard library algorithms that don’t modify the container. We’ll use std::vector for all these examples, but other containers in C++ work as well.

Counting repetitions

We can use std::ranges::count to count how many instances of an element exist in a vector:

Listing 85 count_example.cpp

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

int main() {

    std::vector<int> v{1, 1, 4, 6, 10, 1, 3, -2, -8, 0};

    auto num = std::ranges::count(v, 1);
    std::cout << "the number of times 1 appears in v is " << num << std::endl;

}

Note

If we did this manually, using a loop, we would have to write:

int count{};
for (auto e : v) {
    if (e == 1) {
        count++;
    }
}

so the ranges library version is much more compact.

A variation, std::ranges::count_if takes a function that is used to determine if we count an element (e.g., only counting odd elements).

Checking if any element matches

We can use std::ranges::any_of to see if any element in the vector satisfies a condition.

This requires that we pass in a function the works on a single element and returns a bool.

Listing 86 any_element.cpp

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


bool is_even(int e) {
    return e % 2 == 0;
}


int main() {

    std::vector<int> a{1, 3, 5, 7, 9, 11, 13};
    std::vector<int> b{1, 3, 5, 6, 7, 9, 11, 13};

    auto test = std::ranges::any_of(a, is_even);
    std::cout << "any of a are even? " << test << std::endl;

    auto test2 = std::ranges::any_of(b, is_even);
    std::cout << "any of b are even? " << test2 << std::endl;

}

There is also an all_of and none_of variant.

Finding an element

Here’s an example of using find on a vector (using std::ranges::find):

Listing 87 find_example.cpp

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

int main() {

    const std::vector<int> container{100, 200, 300, 400, 500, 600};

    // search through the entire vector to find the first instance of the
    // element "400"

    auto pos = std::ranges::find(container, 400);

    if (pos == container.end()) {
        std::cout << "element not found" << std::endl;
        return 1;
    }

    std::cout << "element found: " << *pos << std::endl;

    // output the remaining elements after the one we searched for

    for (int value : std::ranges::subrange(std::next(pos), container.end())) {
        std::cout << value << std::endl;
    }
}

This returns an iterator that points to the first match in the vector.

Important

Notice the check:

if (pos == container.end()) {
    std::cout << "element not found" << std::endl;
    return 1;
}

If std::ranges::find does not find a match, then it returns an iterator that points to one past the end of the vector (that’s what .end() is).

If we want to know the index of the element we found, we could use std::ranges::distance and ask for the distance from the beginning of the vector:

Listing 88 distance_example.cpp

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

int main() {

    const std::vector<int> container{100, 200, 300, 400, 500, 600};

    auto pos = std::ranges::find(container, 400);

    if (pos == container.end()) {
        std::cout << "element not found" << std::endl;
        return 1;
    }

    // here we seek the distance from the beginning of the vector
    auto idx = std::ranges::distance(container.cbegin(), pos);

    std::cout << "index = " << idx << std::endl;

    // note: we need to be careful here and ensure that idx is >= 0.
    // and < the size of the vector.  We could be accessing out
    // of bounds if the value we searched for was not in the vector

    std::cout << "value = " << container[idx] << std::endl;
}

Caution

std::ranges::distance will return a signed integer, since the distance count be negative, depending on the starting point we used. But we can only index a vector using an unsigned integer (that is what std::size_t is, so we need to be careful when we do the indexing here. We know we are safe because we use .begin() as the starting point.