在 C 编程语言中：将 3（数字 3）分配给变量的顺序是什么？ 至于哪个变量会收到 3 第一，第二和第三？

Question

Question Continued: In C programming Language: For the Question below: What order would 3 (Number 3) be assigned to the variables? As in which variable would receive 3 first, second and third? And which variable would have 3 in the end?

Question: A = B = C = 3

Further explanation of What I'm asking/My attempts to understand this concept:

According to the image I've attached stating the Associativity of Operators, the Assignment operators should be from left to right no?

So should 3 be assigned to A, then B, and then C?

According to a practice question solution it is the opposite, 3 being assigned to C, then B, then A, so am very confused why it's right to left? When the Associativity of Operators say it's left to right!

Answer 1

The expression A = B = C = 3 is parsed in C as A = (B = (C = 3)) . The assignment operator associates right-to-left.

However, the actual assignment is specified as a side effect of the expression, and the order in which these side effects occur is not specified by the C standard.

The image in the question is wrong to show the order of assignment operators as left to right, and the source of the image should be regarded with suspicion. The association of assignment operators arises out of the grammar rules in the C standard, where 6.5.16 shows one rule as:

assignment-expression : unary-expression assignment-operator assignment-expression

The fact that the right operand is an assignment expression means that in X = Y , Y can be another assignment expression, such as Z = 4 , but X cannot be. So A = B = C = 3 must be parsed as C = 3 being an assignment expression inside of B = … , and B = C = 3 must be an assignment expression inside of A = … . Contrast this with a rule for one of the additive operators in C 6.5.6:

additive-expression : additive-expression - multiplicative-expression

In that rule, the additive-expression is on the left, so A - B - C necessarily groups as (A - B) - C .