📜 WAYPOINT LESSON

Array Mechanics

⭐ beginner⏳ 11 min read📍 Lesson 7 of 281

Declaration, indexing from 0, traversal, and the bounds rule that prevents runtime errors.

Declaration and indexing

int n; in >> n;
vector<long long> a(n);          // indices 0 .. n-1
for (long long& x : a) in >> x;

Indices run 0 to n-1. Accessing a[n] is undefined behavior — the #1 cause of Runtime Error. When a problem says "the i-th element" with 1-based counting, translate: element i lives at a[i-1].

Traversal styles

for (int i = 0; i < n; ++i) ...     // when you need the index
for (long long x : a) ...           // values only
for (int i = n - 1; i >= 0; --i)    // backwards

min/max with a running best

long long best = a[0];              // NOT 0 — values may be negative!
for (long long x : a) best = max(best, x);

Initializing best = 0 fails the moment all values are negative. Start from the first element (or -infinity).

Reverse and rotate

reverse(a.begin(), a.end());                    // in place
rotate(a.begin(), a.begin() + k, a.end());      // left-rotate by k