Step 2 of 6
Loop over a vector
Range-based for works on vectors. Use const auto& to avoid copying each element:
for (const auto& x : v) { ... }
for (auto& x : v) { x *= 2; } // & to modify in place
Pass vectors to functions by const& (read-only) or & (to modify). Passing by value copies the whole thing.
Your turn: write std::vector<int> running_total(const std::vector<int>& v) that returns the prefix sums. {1, 2, 3, 4} becomes {1, 3, 6, 10}.