The binary arithmetic operators are +, -, *, /, and the modulus operator %. Integer division truncates any fractional part. The expression
x % y
produces the remainder when x is divided by y, and thus is zero when y divides x exactly. For example, a year is a leap year if it is divisible by 4 but not by 100, except that years divisible by 400 are leap years. Therefore
if ((year% 4 == 0 && year % 100 != 0) || year% 400 == 0) printf( ”%d is a leap year\n”, year); else printf( ”%d is not a leap year\n”, year);
The % operator cannot be applied to float or double. The direction of truncation for / and the sign of the result for % are machine-dependent for negative operands, as is the action taken on overflow or underflow.
The binary + and - operators have the same precedence, which is lower than the precedence of *, /, and %, which is in turn lower than unary + and -.
Arithmetic operators associate left to right.
[The C Programming Language p.41]