"C-language" symbol precedence

I. The problem arises

Today to see Ali's pen test, see a very interesting topic, but it is easy to make mistakes.

　　Title: The following function, the value of the 32bit system foo (2^31-3) is:

int foo (int  x) {return x&-x;}

　Answer: If you want to correct this problem, first of all to understand the C language of the priority level of the symbol, minus (-) the priority is higher than ^, so 2^31-3=2^28, there is a trap is C language that ^ is an XOR operation instead of a power function, so 2^28=30, and then calculate 30 &-30 results. And because the data in computer memory is in the form of twos complement, the number of participating bit operations is in the form of complement. So we need to convert 30 and 30 to complement, then bitwise AND operation, the result is 2. We can also use the program to see the value of X generated during execution as follows:

#include<iostream>using namespacestd;intFoointx) {cout<<"x ="<< x <<Endl; returnX &-x;}voidMain () {intres =0; Res= Foo (2^ to-3); cout<<"res ="<< Res <<Endl;}

Two. Priority of characters

Priority level	Operator	Name or meaning	Use form	Combination direction	Description
1	[]	Array subscript	array name [constant expression]	Left to right
	()	Parentheses	(expression)/function name (formal parameter list)
	.	Member selection (object)	Object. Member name
	-	Member selection (pointer)	Object pointers, member names
2	-	Minus sign operator	-expression	Right to Left	Monocular operator
	Type	Forcing type conversions	(data type) expression
	++	Self-increment operator	+ + variable name/variable name + +		Monocular operator
	--	Self-decrement operator	--Variable name/variable name--		Monocular operator
	*	Value operator	* Pointer variable		Monocular operator
	&	Fetch address operator	& Variable Name		Monocular operator
	!	Logical non-operator	! An expression		Monocular operator
	~	Bitwise inverse operator	~ Expression		Monocular operator
	sizeof	Length operator	sizeof (expression)
3	/	Except	An expression/expression	Left to right	Binocular operator
	*	By	Expression-expression *		Binocular operator
	%	Remainder (modulo)	Integer expression/integer expression		Binocular operator
4	+	Add	An expression + an expression	Left to right	Binocular operator
	-	Reducing	Expression-expression		Binocular operator
5	<<	Move left	Variables << expressions	Left to right	Binocular operator
	>>	Move right	Variables >> expressions		Binocular operator
6	>	Greater than	Expressions > expressions	Left to right	Binocular operator
	>=	Greater than or equal	Expression->= expression		Binocular operator
	<	Less than	Expressions < expressions		Binocular operator
	<=	Less than or equal	Expression-<= expression		Binocular operator
7	==	Equals	Expression-= = Expression	Left to right	Binocular operator
	!=	Not equal to	Expression! = Expression		Binocular operator
8	&	Bitwise-AND	Expressions & Expressions	Left to right	Binocular operator
9	^	Bitwise XOR OR	An expression ^ expression	Left to right	Binocular operator
10	|	Bitwise OR	Expression-expression	Left to right	Binocular operator
11	&&	Logic and	Expressions && Expressions	Left to right	Binocular operator
12	||	Logical OR	An expression | | An expression	Left to right	Binocular operator
13	?:	Conditional operators	Expression 1? Expression 2: Expression 3	Right to Left	Trinocular operator
14	=	Assignment operators	variable = expression	Right to Left
	/=	Assign value after addition	Variable-/= expression
	*=	Multiply post-Assign value	Variable-*= expression
	%=	Assign value after modulo	Variable-%= expression
	+=	Add after Assignment	Variable + = expression
	-=	Reduced-value Assignment	Variable-= expression
	<<=	Assign value after left shift	Variable-<<= expression
	>>=	Assign value after right shift	Variable->>= expression
	&=	Bitwise AND post-assigned values	Variable-&= expression
	^=	Bitwise XOR or post-assignment	Variable-^= expression
	|=	Bitwise OR post-assigned value	Variable-|= expression
15	,	Comma operator	expression, expression,...	Left to right	Left-to-right sequential operation

Description: operators of the same precedence, the order of operations is determined by the binding direction.

Three. Priority formula

parenthesis member first; parentheses operator [] () member operator. -

All monocular second; All monocular operators such as + + + (positive)-(negative) pointer operation *&

Multiplication Yozo, plus minus four; this "remainder" means the residual operation is%

Shift five, relationship six, shift operator:<< >>, relationship:> < >= <=, etc.

Equals (and) unequal row seventh; = = = = =

Bits and Xor and bits or; these are bitwise operations: Bits and (&) XOR (^) bits or (|)

"Three points of the world" eighty or ninety;

logical or heel; logical Operator: | | and &&

12 and 11; Note order: Priority (| |) Bottom-to-priority (&&)

The condition is higher than the assignment, the three-mesh operator precedence is ranked to 13-bit only than the assignment operator and the "," high

Lowest comma operation level! The comma operator has the lowest precedence

Resources

[1] http://blog.chinaunix.net/uid-23577393-id-2733234.html

[2] http://blog.csdn.net/zhlfox2006/article/details/11854799

[3] Http://www.aichengxu.com/article/c%E8%AF%AD%E8%A8%80/31501_11.html

"C-language" symbol precedence