Algorithms

Algorithms

The <algorithm> header holds the "what to do" part of the STL. The algorithms work on [begin, end) ranges, which are exactly the iterators from the previous lesson.

Sort

#include <algorithm>
#include <vector>

std::vector<int> v = {5, 2, 7, 1};
std::sort(v.begin(), v.end());
// v is now {1, 2, 5, 7}

sort rearranges the whole range into ascending order. Give it the two-iterator range.

Find

auto it = std::find(v.begin(), v.end(), 7);
if (it != v.end()) {
    std::cout << "found\n";
}

find returns an iterator to the first match, or end() when nothing matches.

Count

int c = std::count(v.begin(), v.end(), 2);

count sums how many times a value appears inside the range.

min_element and max_element

auto smallest = std::min_element(v.begin(), v.end());
auto largest  = std::max_element(v.begin(), v.end());
std::cout << *smallest << " " << *largest;   // 1 7

Both return iterators, so dereference them to read the actual values.

Why iterators bind it together

Every algorithm takes the same shape: algorithm(begin, end, ...). That is why the library can sort a vector, find a value in a list, or count inside an array - one concept, all containers.

TL;DR

• Algorithms live in <algorithm> and work on iterator ranges.
• sort(begin, end) sorts; find(begin, end, value) returns an iterator or end().
• count(begin, end, x) tallies appearances of x.
• min_element and max_element return iterators to the smallest or biggest element.
• One range pattern works across every container.