Iterators and the Algorithm Family

The idea

Every container hands out iterators — generalised positions — via begin() and end(). begin() points at the first element; end() points one past the last (a half-open range [begin, end); end is never dereferenced). Algorithms speak this language, which is why they work identically on vector, array, string...

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

std::vector<int> v{5, 3, 9, 1};

std::sort(v.begin(), v.end());                    // {1, 3, 5, 9}
auto it = std::find(v.begin(), v.end(), 5);       // iterator to 5, or v.end()
bool has = (it != v.end());
int n_odd  = std::count_if(v.begin(), v.end(), [](int x){ return x % 2 != 0; });
int sum    = std::accumulate(v.begin(), v.end(), 0);
std::reverse(v.begin(), v.end());
auto smallest = *std::min_element(v.begin(), v.end());

Why prefer algorithms over hand-rolled loops

1. Intent. std::sort announces its purpose; a loop must be read to be understood.
2. Correctness. std::sort is battle-tested; your quicksort is a weekend project.
3. Range-for can't do everything. Sorting, searching, folding — range-for has no answer; algorithms do.

When a loop you write is just for (auto& x : c) if (p(x)) ++n;, the algorithm version (std::count_if) says the same thing in one line that cannot be misread.

Ranges: the C++20 evolution (a glance)

C++20 adds std::ranges::sort(v) — same algorithms, container passed directly, fewer .begin()s. Know the name; this course teaches the iterator form because it underlies both and remains the lingua franca of documentation and real code.