Why are there different associativities among operators in C?

Question

Talking about the associativity of operators in C, I was wondering why there are differences associativities among operators that have the same precedence. for example, postfix increment and postfix decrement have left associativity; while prefix increment and prefix decrement have right associativity. Isn't it simple to have just left or right associativity for all the same precedence operators?

Are there any reasons behind that?

Answer 1

Isn't it simple to have just left or right associativity for all the same precedence operators?

Yes and it is the case in C. May be you assumed that prefix and postfix have the same precedence which is wrong. Postfix has a higher precedence than prefix!

Also there is another curious case to consider as to why certain operators have certain associativity. From Wiki,

For example, in C, the assignment a = b is an expression that returns a value (namely, b converted to the type of a) with the side effect of setting a to this value. An assignment can be performed in the middle of an expression. (An expression can be made into a statement by following it with a semicolon; i.e. a = b is an expression but a = b; is a statement). The right-associativity of the = operator allows expressions such as a = b = c to be interpreted as a = (b = c), thereby setting both a and b to the value of c. The alternative (a = b) = c does not make sense because a = b is not an lvalue.

Answer 2

Binary operators are all left-associative except the assignment operator which is right-associative.

Postfix operators are sometimes (for exemple in K&R 2nd) said to be right-associative but this is to express the idea they have higher precedence than unary operators.