Operator priority, associativity, and whether to control the evaluation order [detailed explanation]

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Introduction to this issue
This paper mainly introduces ： Three properties of operators ： priority 、 associativity 、 Whether to control the order of evaluation ; And those that cannot be constrained by these three attributes , So as to determine the problem expression of the unique calculation path .

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The properties of the operator

To evaluate a complex expression , You must be familiar with the properties of each operator in the expression . Operators have three properties ：

1. The priority of the operator
2. The associativity of operators
3. Whether to control the order of evaluation

Be careful ： Which of the two adjacent operands should be executed first , Depending on the priority of the operator , However, if two operators have the same priority, it depends on the associativity of the operators .

priority	The operator	describe	Operation class	Usage examples	Result Type	associativity	Whether to control the order of evaluation
1	（）	Aggregation group	N/A	（ expression ）	Same as expression	N/A	no
	（）	Function call		rexp(rexp,…,rexp)	rexp	L/R	no
	[ ]	Subscript reference		rexp[rexp]	lexp	L/R	no
	.	Access structure members		lexp. Member name	lexp	L/R	no
	->	Access structure pointer member		rexp-> Member name	lexp	L/R	no
2	!	Logic is not	Monocular operations	! rexp	rexp	R/L	no
	~	According to the not		~ rexp	rexp	R/L	no
	++	Self increasing 1		++ lexp、lexp ++	rexp	R/L	no
	- -	Self reduction 1		- - lexp、lexp - -	rexp	R/L	no
	+	Monocular , Means to find a positive value		+ rexp	rexp	R/L	no
	-	Monocular , Means to find a negative value		- rexp	rexp	R/L	no
	*	Indirect access to		* rexp	lexp	R/L	no
	&	Address fetch		& lexp	rexp	R/L	no
	（ Type name ）	Cast		( Type name )rexp	rexp	R/L	no
	sizeof	Find the number of bytes occupied		sizeof rexp、sizeof( type )	rexp	R/L	no
3	*、/、%	ride 、 except 、 Integer remainder	Arithmetic operation	rexp * rexp、rexp / rexp、rexp % rexp	rexp	L/R	no
4	+、-	Add 、 reduce		rexp + rexp、rexp - rexp	rexp	L/R	no
5	<<、>>	Shift left 、 Shift right	An operation	rexp << rexp、rexp >> rexp	rexp	L/R	no
6	<、<=、>、>=	Less than 、 Less than or equal to 、 Greater than 、 Greater than or equal to	Relationship between operation	rexp < rexp、rexp <= rexp、rexp > rexp、rexp >= rexp	rexp	L/R	no
7	==、!=	be equal to 、 It's not equal to		rexp == rexp、rexp != rexp	rexp	L/R	no
8	&	Bitwise AND	An operation	rexp & rexp	rexp	L/R	no
9	^	Bitwise XOR		rexp ^ rexp	rexp	L/R	no
10	|	Press bit or		rexp | rexp	rexp	L/R	no
11	&&	Logic and	Logical operations	rexp && rexp	rexp	L/R	yes
12	||	Logic or		rexp || rexp	rexp	L/R	yes
13	？：	Conditional operation	Three mesh operation	rexp ？ rexp：rexp	rexp	R/L	yes
15	=、+=、-=、*=、/=、%=、&=、^=、|=、<<=、>>=	The assignment operation	Binocular operation	lexp = rexp	rexp	R/L	no
16	,	Comma expression	Sequential operation	rexp,rexp	rexp	L/R	yes

Be careful ： In the table above rexp Is an R-value expression , Represents its value ;lexp It's an lvalue expression , Represents its memory space .N/A It means empty ,R/L It means from right to left ,L/R It means from left to right .

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Problem expression

1. a * b + c * d + e * f

   At first glance, there seems to be no problem , But in fact, the expression cannot be unique through priority and associativity Calculate the path , Because priority only determines which of the two adjacent operators executes first . Just like the expression a * b + c * d + e * f, We are not sure which multiplication to take first , May ask first a * b, Or maybe c * d, Or maybe e * d. If you find any multiplication, you will change the result of the other two multiplication , Then we cannot determine the unique value of the expression .

2. c + --c

   The expression has a definite and unique calculation path , But I'm not sure c The value is obtained in –c Before or after execution , So the results are unpredictable , There is ambiguity .

3. Problem code

#include<stdio.h>
int fun()
{
    
	static int count = 1;
	return ++count;
}
int main()
{
    
	int answer;
	answer = fun() - fun() * fun();
	printf("%d\n",answer);
	return 0;
}

   The same is true of this code , We can only determine the evaluation order after the function is called , However, it is impossible to determine the sequence of function calls by the priority of operators . The difference between who calls first and who calls later will inevitably lead to the failure of the calculated value , It is impossible to get exact results .