Associativity is only needed when the operators in an expression have the same 

precedence. Usually + and - have the same precedence.

Consider the expression 7 - 4 + 2. The result could be either (7 - 4) + 2 = 5 or 7 

- (4 + 2) = 1. The former result corresponds to the case when + and - are left-

associative, the latter to when + and - are right-associative.

Usually the addition, subtraction, multiplication, and division operators are left-associative, 

while the exponentiation, assignment and conditional operators are right-associative. To 

prevent cases where operands would be associated with two operators, or no operator at 

all, operators with the same precedence must have the same associativity.