[dish of learning notes dog learning C] evaluation expression

Expression evaluation

   The order in which expressions are evaluated is partly determined by the operators priority and associativity decision . The operands of some expressions may need to be converted to other types in the evaluation process .

  Implicit type conversion

   Improve the overall shape
   significance ： The integer operation of the expression is in CPU Executed in the corresponding computing device of ,CPU Inner integer arithmetic unit （ALU） The byte length of the operands of It's usually int Byte length of , It's also CPU The length of a general-purpose register . therefore , Even two char Type addition , stay CPU When executing, it should be converted to CPU The standard length of the inner operands . Universal CPU It is difficult to realize two directly 8 Bits and bytes are directly added （ Although there may be such byte addition instructions in machine instructions ）. So all kinds of expressions The length is less than int The integer value of the length , Must be converted to int perhaps unsigned int, Then it can be sent in CPU To perform the operation .
   How to improve ： according to Sign bit of variable data type To improve .

int main() {
    
	char a = 3;
	//00000011
	char b = 127;
	//01111111
	char c = a + b;
	// Improve the overall shape 
	//a - 00000000 00000000 00000000 00000011
	//b - 00000000 00000000 00000000 01111111
	//c - 00000000 00000000 00000000 10000010
	//char Type only 8 A bit ,c To truncate 
	//c - 10000010
	printf("%d", c);
	//%d What is printed is an integer , therefore c To do integer lifting 
	// Integer promotion is promoted according to the sign bit of the data type of the variable 
	//10000010 The sign bit of is 1, So make up 1
	//11111111 11111111 11111111 10000010 -  Complement code 
	//11111111 11111111 11111111 10000001 -  Inverse code 
	//10000000 00000000 00000000 01111110 - -126 -  Original code 

	return 0;
}

  Arithmetic conversion

   If the operands of an operator belong to different types , Then unless one of the operands is converted to the type of the other operand , Otherwise, the operation cannot be carried out .

If the type of an operand is low in the list , Then first convert to the type of another operand and perform the operation .
long double
double
float
unsigned long int
long int
unsigned int
int

   Arithmetic conversions are To types with higher accuracy convert .
   Arithmetic conversion must be reasonable , Otherwise there will be problems , If accuracy is lost .

int main() {
    
	int a = 4;
	float f = 4.5f;
	a + f;
	//a Will send to float transformation 
	return 0;
}

  Operator properties

   There are three factors that affect the evaluation of complex expressions ：
   1. The priority of the operator
   2. The associativity of operators
   3. Whether to control the order of evaluation

int main() {
    
	int a = 1;
	int b = 2;
	int c = a + b * 7;// Priority determines the order of calculation 
	int d = a + b + 7;// Priority does not work , Associativity determines order 
	return 0;
}

   The priority and associativity of operators are to make the code run more logically and accurately , Not for people to show off their skills , Write a bunch of operators to show how well you master priority and associativity ;
   If an expression cannot determine the unique calculation path through the attributes of the operator , Then this expression is problematic .
   attach ：C List of priorities and associativity of language operators