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3, Bit operation and logic operation

# 1, Bitwise operator analysis

- Bitwise operators in C language

Bit operators directly operate on bit bits, which is the most efficient.

& | Bitwise AND |

| | Bitwise OR |

^ | Bitwise XOR |

~ | Reverse |

<< | Shift left |

>> | Shift right |

- Move left and right attention points
- Left operand must be of integer type
- char and short are implicitly converted to int and then shifted

- The range of right operand must be: [0,31]
- Shift left operator < < shifts the binary bit of the operand to the left
- Rule: discard the high order and make up 0 for the low order

- Shift right operator > > shifts the binary bit of the operand to the right
- Rule: the high bit is used to fill the symbol bit, and the low bit is discarded

- Left operand must be of integer type

The following code:

#include <stdio.h> int main() { printf("%d\n", 3 << 2); printf("%d\n", 3 >> 1); printf("%d\n", -1 >> 1); printf("%d\n", 0x01 << 2 + 3); printf("%d\n", 3 << -1); // oops! return 0; }

The following is the output result:

Note that the priority of four operations is greater than that of bit operations, so the result of 0x01 < < 2 + 3 is 32. Also, the range of the right operand must be: [0,31]. If it is not within this range, the output result of the program is determined by different types of compilers, and the result will be uncertain, just like 3 < < - 1 in this code.

# 2, Tips

- Error proofing criteria:
- Avoid bitwise operators, logical operators and mathematical operators appearing in the same expression at the same time
- When bit operators, logical operators and mathematical operators need to participate in the operation at the same time, try to use parentheses () to express the calculation order

- Tips:
- Shifting n bits to the left is equivalent to multiplying 2 to the nth power, but the efficiency is higher than that of mathematical operators
- Shifting n bits to the right is equivalent to dividing by 2 to the nth power, but it is more efficient than mathematical operators

Here is a code for exchanging the values of two integer variables:

#include <stdio.h> #define SWAP1(a,b) \ { \ int t = a; \ a = b; \ b = t; \ } #define SWAP2(a,b) \ { \ a = a + b; \ b = a - b; \ a = a - b; \ } #define SWAP3(a,b) \ { \ a = a ^ b; \ b = a ^ b; \ a = a ^ b; \ } int main() { int a = 1; int b = 2; //printf("a = %d\n", a); //printf("b = %d\n", b); SWAP1(a,b); printf("a = %d\n", a); printf("b = %d\n\n", b); a = 1; b = 2; SWAP2(a,b); printf("a = %d\n", a); printf("b = %d\n\n", b); a = 1; b = 2; SWAP3(a,b); printf("a = %d\n", a); printf("b = %d\n\n", b); return 0; }

The first method needs to introduce third-party variables. The second method may lead to boundary crossing problems. The third method is more efficient and does not need to introduce third-party variables.

Note the third method: execute a = a ^ b; After, b = a ^ b; It is equivalent to b = a ^ b ^ b; First calculate the following, that is, b = a ^ 0, and the result is b = a; Then execute a = a ^ b; Equivalent to a = a ^ b ^ b, i.e. a = a ^ b ^ a, obviously the result is B.

Tips:

A XOR 0 equals a and a XOR 1 equals non-a.

# 3, Bit operation and logic operation

- Bit operation is different from logical operation:
- There is no short-circuit rule for bit operation, and each operand participates in the operation
- The result of a bit operation is an integer, not 0 or 1
- Bit operation priority is higher than logic operation priority

Here's another judgment condition for confusing change:

#include <stdio.h> int main() { int i = 0; int j = 0; int k = 0; if( ++i | ++j & ++k ) { printf("Run here...\n"); } printf("i = %d, j = %d, k = %d\n\n", i, j, k); i = 0; j = 0; k = 0; if( ++i || ++j && ++k ) { printf("Run here...\n"); } printf("i = %d, j = %d, k = %d\n\n", i, j, k); return 0; }

The following is the output result:

It can be seen that if + + I | + + J & & + + k is wrongly written as + + I | + + J & + K, although they can run, the execution details are different, and bug s may appear in the actual project, which is not easy to check.

# 4, Summary

- Bitwise operators can only be used with integer types
- The range of right operands for shift left and shift right operators must be [0, 31]
- There is no short circuit rule for bit operations, and all operands are evaluated
- The efficiency of bit operation is higher than that of four operations and logic operation
- Operation priority: four operation > bit operation > logic operation