Core Algorithms: find, count, sort, and Friends

The <algorithm> header is a vocabulary of loops other people have already written — tested, named, and complexity-documented. Reading code that uses it is faster than reading hand-rolled loops.

#include <algorithm>
#include <vector>

std::vector<int> v{4, 1, 3, 1, 5};

auto it = std::find(v.begin(), v.end(), 3);      // iterator or end()
int n = std::count(v.begin(), v.end(), 1);       // how many 1s
bool has = std::any_of(v.begin(), v.end(),
                       [](int x) { return x > 4; });
std::sort(v.begin(), v.end());                   // O(n log n), ascending
std::reverse(v.begin(), v.end());
auto mx = *std::max_element(v.begin(), v.end());

Three habits that make algorithms safe:

1. Check before dereferencing. std::find returns end() when nothing matches; comparing against end() is the only safe move.
2. Prefer the _if family with a predicate instead of transforming data to fit a non-_if version.
3. Read the complexity line. sort is O(n log n); count is O(n); nothing here is free.