if / else and Boolean Logic

The shape

if (temperature > 30) {
    std::cout << "hot\n";
} else if (temperature > 20) {
    std::cout << "warm\n";
} else {
    std::cout << "cool\n";
}

Conditions run top to bottom; the first true branch wins and the rest are skipped. That ordering is the logic: swapping the first two branches breaks the program even though every condition is "correct".

Combining conditions

bool can_enter = age >= 18 && has_ticket;   // both
bool discount   = is_student || is_senior;  // at least one
bool not_member = !is_member;               // negation

Two classic beginner pitfalls:

1. Range chains. if (0 < x < 10) compiles but does NOT test what you think — it evaluates (0 < x) < 10, i.e. true/false < 10, always true. Write x > 0 && x < 10.
2. Assignment in conditions. if (x = 5) assigns 5, yields 5 (truthy), and runs the branch. Compilers warn (-Wall) — read the warning.

Braces are not optional in this course

C++ allows if (x) std::cout << "yes"; without braces — and that style is how the famous Apple goto fail bug happened: one added line silently left the guard. Always brace your branches, even single-line ones. Modern style guides and this course agree.

Validation mindset

Real programs spend most of their if-budget validating: is this input present? non-empty? in range? The earlier you write checks, the fewer crashes reach your logic. You will practice exactly that in this module's exercises: reject invalid input at the door with clear messages, then compute with confidence.