Iteration Patterns: Range-for, auto, and Iterators

This lesson exists because the STL's core design idea — algorithms that work on ANY container — needs a common currency, and iterators are that currency. Learn the loop styles here and unit 9's entire algorithm library opens up with no new syntax.

Range-based for: Python's for-in, in C++

Since C++11, iterating a container looks like this:

std::vector<int> v = {1, 2, 3};

for (int x : v) {          // x is a COPY of each element
    std::cout << x;
}

for (int& x : v) {         // x is a REFERENCE: modify the real elements
    x *= 2;
}

for (const std::string& s : words) {   // big elements: const& avoids copies
    std::cout << s;
}

The value/reference distinction is lesson 4-2 all over again, applied per element. Copies for cheap reads, & to mutate, const& for big read-only elements.

auto

auto lets the compiler deduce a variable's type from its initializer. for (auto& x : v) adapts if v's element type changes. Use it where the type is obvious from context, write the type out where it teaches the reader.

Iterators, briefly

Range-for is sugar over iterators: pointer-like objects where v.begin() points at the first element and v.end() points one past the last. The pointer walk you wrote in lesson 5-2 is exactly this shape, and every STL algorithm in unit 9 takes a begin/end pair.