if, else, and else if

The ladder

if (score >= 90) {
    grade = 'A';
} else if (score >= 80) {
    grade = 'B';
} else {
    grade = 'F';
}

Conditions test top to bottom; the FIRST true branch runs and the rest are skipped. The final else catches everything remaining — put the most specific test first.

Brace discipline

Braces are optional for a single statement — and a standing trap:

if (x > 0)
    printf("positive\n");
    printf("always prints!\n");   // NOT part of the if

Rule for this course: always brace.

Ternary: a tiny if as an expression

const char* sign = (x >= 0) ? "non-negative" : "negative";

Use it for choosing a value; keep real logic in if.