Bit operations in C++ and their application skills

Bit operations in C are a commonly used operation method among programmers. By using bit operations to process data, some complex computing tasks can be completed more efficiently. This article introduces common bit operation symbols in C and their application techniques, as well as some examples that may be used in actual development.

1. Bitwise operation symbols

C provides six bitwise operation symbols, which can operate on binary bits. Four of them are bitwise operators, and the other Two are shift operators. The bitwise operation symbols are as follows:

& Bitwise AND operation: The result is 1 when both binary bits are 1, otherwise it is 0.

| Bitwise OR operation: The result is 0 when both binary bits are 0, otherwise it is 1.

^ Bitwise XOR operation: the result of two binary bits that are the same is 0, and the result that is different is 1.

~ Bitwise negation operation: negate the binary bit, that is, 0 becomes 1, and 1 becomes 0.

The shift operation symbols are as follows:

Right shift operation: Move the binary number to the right by the specified number of digits, and fill the high bits with 0 or 1 (depending on whether the original number is a positive or negative number).

2. Application skills

Bit operations are widely used in low-level computer operations, and many efficient algorithms can be realized through some clever bit operation techniques. Here are some commonly used techniques:

2.1 Determining parity

For a binary number, when its last digit is 1, the number is odd, and vice versa. Therefore, we can determine the parity of a number through bit operations.

For any integer n, if the result of n & 1 is 0, it means n is an even number, otherwise it is an odd number. For example:

int n = 5;
if( n & 1 ){

cout << "奇数" << endl;

}
else {

cout << "偶数" << endl;

}

2.2 Exchange the values ​​of two numbers

To exchange the values ​​of two variables, the general method is to use a third variable to store the value of one of the variables, and then assign the value of the other variable to it . However, using bit operations does not require the use of additional space for storage, but can be implemented directly through bit XOR operations, that is:

int a = 10, b = 20;
a = a ^ b ;
b = b ^ a;
a = a ^ b;

After execution, the value of variable a is 20, the value of variable b is 10, and the values ​​of the two variables realize each other. Change.

2.3 The number of 1’s in binary numbers

When calculating the number of 1’s in binary numbers, you can use the & operation and right shift to accumulate and solve it one by one. The specific implementation method is as follows:

int countOne(unsigned int n){
int count = 0;
while (n != 0){

   if (n & 1 == 1){
       count++;
   }
   n = n >> 1; //右移一位

}
return count;
}

2.4 Get the i-th bit in binary

We can use the left shift operation to shift the number 1 to the left by i - 1 bits, and then use the bitwise AND operation& , to obtain the i-th bit in the binary number. The specific implementation method is as follows:

bool getBit(unsigned int n, int i){

return (n & (1 << i)) !=0 ;

}

2.5 Set the i-th bit in the binary number to 1

Shift the number 1 to the left by i bits, and then perform a bitwise OR operation on this binary number and the original number to set the i-th bit in the original number to 1.

unsigned int setBit(unsigned int n, int i){

return n | (1 << i);

}

2.6 Set the i-th bit in the binary number to 0

Shift the number 1 to the left by i bits, then invert the binary number and perform a bitwise AND operation with the original number to set the i-th bit in the original number to 0.

unsigned int clearBit(unsigned int n, int i){

return n & ~(1 << i);

}

3. Application example

Bit operation in actual There are also many applications in development, and some related tasks can be completed more efficiently through byte operations. For example, in image processing, the pixels of the image are often binarized, the upper and lower limits are set to 0 and 255, and the target pixel value is changed to 0 or 255. Efficient processing can be achieved using bit operations. The code example is as follows ：

for (int i = 0; i < img.rownum(); i ){

for (int j = 0; j < img.colnum(); j++){        
    if (img.at(i,j) < threshold){
        img.at(i,j) = 0; //设为0
    }
    else {
        img.at(i,j) = 255; //设为255
    }
}

}

By operating on binary digits, it can also be achieved Many efficient algorithms and data structures, such as bitmaps, hash tables, etc.

In short, bit operation is a powerful tool and is widely used in programming. Proficiency in bit operation symbols and techniques can play an active role in implementing efficient algorithms and data structures and improving program operation efficiency.

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