[C Advanced] 16, bit operators

Summary

1) Bit operator in C language:

Operator	significance	rule
&	Bitwise and	All 1s get 1, and 0s get 0
I	Bitwise or	1 is 1 and all 0s are 0
^	Bitwise exclusive or	Same as 0, different as 1
~	Negate	1 becomes 0, 0 becomes 1
<<	Shift left	High bits are discarded, low bits are filled with 0
·>>	Shift right	High bit complement sign bit, low bit discarded

2) Points to note when moving left and right:

• left operand must be of integer type

  • Char and short are implicitly converted and then operated
• right operand must be: [0,31], otherwise the result is undefined (different compilers process different results)
• Left shift operator << shifts the binary bits of the equivalent to multiplying by the n power of 2, is more efficient
• right shift operator>> shifts the binary bit of the equivalent to dividing by the n power of 2, is more efficient
• Avoid mixed operations such as bitwise operators, logical operators, and mathematical operators. If this is necessary, use parentheses to indicate the order of calculation. ( single calculation shift ratio, according to logic three assignments)

3) Points to note in bit operation:

• There is no short-circuit rule for bit operation, will participate in the operation
• result of the bit operation is an integer, not 0 or 1
• The priority of bit operation is higher than logic operation ( single arithmetic shift ratio is assigned by logic three)

4) Exchange the values of two integer variables:

• Demo1: part and method: do not use intermediate variables, but there is an overflow problem

  #define SWAP(a, b) \
  {                  \
    a = a + b;       \
    b = a - b;       \
    a = a - b;       \
  }

• Demo2: bit operation method: XOR ( two identical values XOR result is 0, any number and 0 XOR is still itself)

  #define SWAP(a, b)    \
  {                     \
    a = a ^ b;          \
    b = a ^ b;          \
    a = a ^ b;          \
  }

Anatomy of bitwise operators

The C language was originally designed to develop the UNIX operating system. The operating system runs on a hardware platform, which inevitably involves bit operations, and requires operations on the bits (0 and 1) of the hardware. The bit operation in C language directly maps to the bit operation in the hardware system.
The bit operator directly operates on the bit bit, and its is the most efficient.

1. Bitwise operators in C language

Operator	significance	rule
&	Bitwise and	All 1s get 1, and 0s get 0
I	Bitwise or	1 is 1 and all 0s are 0
^	Bitwise exclusive or	Same as 0, different as 1
~	Negate	1 becomes 0, 0 becomes 1
<<	Shift left	High bits are discarded, low bits are filled with 0
·>>	Shift right	High bit complement sign bit, low bit discarded

2. Points to note about shifting left and right

• left operand must be of integer type

  • char and short are implicitly converted to int
• right operand of 16167e38ecbc47 must be: [0,31], otherwise the result is undefined (different compilers handle different results)
• Left shift operator << shifts the binary bits of the equivalent to multiplying by the n power of 2, is more efficient

• right shift operator>> shifts the binary bit of the equivalent to dividing by the n power of 2, which is more efficient

  • Demo1

    d << 2;               // error, 左操作数必须是整型
    int x = 10 >> 1;      // ok，"右移等效除以2的n次方"，输出5
    int y = -1 << 1;      // ok，"左移等效乘以2的n次方"，输出-2
  
    int z = 3 << -1;      // -1不在[0,31]范围中，结果未定义，不同编译器不同处理
            // 编译器亦提示了错误：Possible overflow in shift operation
    
    // gcc 1，            左移-1相当于右移1，那就除以2，得到了1
    // bcc -2147483648，  int的最小值
    // vc 0，             本来就未定义，给你0把

• Avoid mixed operations such as bitwise operators, logical operators, and mathematical operators. If this is necessary, use parentheses to indicate the order of calculation. ( single calculation shift ratio, according to logic three assignments)

  • Demo2

  0x1 << 2 + 3 输出多少？
  等价于 0x1 << (2 + 3)，输出为32
  
  // 如果这行代码作者本意是先左移，再加法，实际执行结果就不是期望的
  // 所以在写代码时要尽量用括号()来表示清楚计算次序

3. Points of Attention in Bit Operation

• There is no short-circuit rule for bit operations, will participate in the operation
• The result of bit operation is an integer, not 0 or 1

• The priority of bit operation is higher than logic operation ( single arithmetic shift ratio is assigned by logic three)

  int i = 0;
  int j = 0;
  int k = 0;
  
  if(++i | ++j & ++k)        // 所有的运算数都会执行到，因此i j k判断后都为1
    printf("cc");

4. Simple example of bit operation application

Exchange the values of two integer variables

• Demo1: part and method: do not use intermediate variables, but there is an overflow problem

  #define SWAP(a, b) \
  {                  \
    a = a + b;       \
    b = a - b;       \
    a = a - b;       \
  }

• Demo2: -bit arithmetic mode: use XOR ( two identical values XOR result is 0, any number and 0 XOR is still itself)

  #define SWAP(a, b)    \
  {                     \
    a = a ^ b;          \
    b = a ^ b;          \
    a = a ^ b;          \
  }

This article is summarized from "C Language Advanced Course" by Tang Zuolin from "Ditai Software Academy".
If there are any errors or omissions, please correct me.