Making decisions
Real programs constantly choose between actions: log the user in or show an error, charge full price or apply a discount. The if statement is how C expresses that choice. The condition is evaluated and if it is true the block runs; otherwise the else block (if present) runs instead.
int score = 72;
if (score >= 90) printf("A\n");
else if (score >= 60) printf("Pass\n");
else printf("Fail\n");else if and switch
When you have more than two branches, else if chains keep the code linear and easy to read. For matching a single value against many constants, some languages offer a switch or match statement that is cleaner than a long chain.
Truthy and falsy
C treats certain values as automatically "true" or "false" inside a condition. Zero, empty strings and the null value are usually falsy, while everything else is truthy. Learning these rules avoids subtle bugs.