C: Bit operations

1. Negative numbers-two types of complements (twos complement)

Generally use the two class complement to indicate a negative number, the leftmost one is a sign bit.

When converting a decimal negative to a binary symbol number, you first add 1 to the number, then the absolute value, then convert it to binary, and finally the binary is mended.

[1]-5

[2]-4

[3]4

[4]0000 0100

[5]1111 1011

Converts a negative number from binary to decimal, first to all its bits, and then converts the result to decimal, changing its symbol, and then minus 1.

[1]1111 1011

[2]0000 0100

[3]4

[4]-4

[5]-5

When using a two-class complement to represent a number, the maximum positive number that can be stored with n bits is 2n-1-1, with an n-bit-stored minimum negative -2n-1.

Assuming that the integer occupies 32 bits (4 bytes), the maximum positive number that can be stored is 231-1, the smallest negative number is 231, and an unsigned integer, or unsigned int, has a range of 0 to 232-1.

2. Bitwise-and-bitwise OR

Bitwise and often used to mask certain bits in a number,

Word &= 0x1//except the rightmost 4 digits, the remaining bits are cleared

Bitwise or often used to set certain bits to 1

Word |= 0x1//Sets the rightmost 4-bit to 1

3. Xor or operation

A different or operation can exchange two values without using a temporary variable.

/*exchange two integer values*/
void swap(int　* a, int　* b)
{
　　*a ^= *b;
　　*b ^= *a;
　　*a ^= *b;
}

If a number and 1 of the difference or operation, can achieve the reverse effect, 0^1=1,1^1=0.

This assumes that the machine has an integer length of 32 bits and a reverse of 31 digits of the integer 0 to get the largest integer.

int main()
{
　　int a=0;
　　printf("%i,%i",a,a ^ 0x7fffffff);
　　return 0;
}