bit-manipulation

问题 For example: int age; cin >> age; cout << "You are " << age << " years old!" << endl; Why do we use the "<<" and ">>" operators here? What are they doing? I somewhat understand bit-shifting, but I don't get how that works here. 回答1: They are called the stream insertion operator ( << ) and the stream extraction operator ( >> ). These are the same operators as the left and right bit shift operators (even though they have different names). The bit shift operators are overloaded, so that when the

问题 I am going through 'The C language by K&R'. Right now I am doing the bitwise section. I am having a hard time in understanding the following code. int mask = ~0 >> n; I was playing on using this to mask n left side of another binary like this. 0000 1111 1010 0101 // random number My problem is that when I print var mask it still negative -1. Assuming n is 4. I thought shifting ~0 which is -1 will be 15 (0000 1111). thanks for the answers 回答1: Performing a right shift on a negative value

问题 To allow for different formatting options on a method that displays a news story, I've created an enumeration that can be passed in to specify how it gets displayed. [Flags] private enum NewsStyle { Thumbnail = 0, Date = 1, Text = 2, Link = 4, All = 8 } string FormatNews( DataRow news, NewsStyle style ) { StringBuilder HTML = new StringBuilder(); // Should the link be shown if ( ((newsStyle & NewsStyle.All) == NewsStyle.All || (newsStyle & NewsStyle.Link) == NewsStyle.Link)) { HTML

问题 My current project requires extensive use of bit fields. I found a simple, functional recipe for bit a field class but it was lacking a few features I needed, so I decided to extend it. I've just got to implementing __str__ and __repr__ and I want to make sure I'm following convention. __str__ is supposed to be informal and concice, so I've made it return the bit field's decimal value (i.e. str(bit field 11) would be "3" . __repr__ is supposed to be a official representation of the object, so

问题 I have an arbitrary 8-bit binary number e.g., 11101101 I have to swap all the pair of bits like: Before swapping: 11-10-11-01 After swapping: 11-01-11-10 I was asked this in an interview ! 回答1: In pseudo-code: x = ((x & 0b10101010) >> 1) | ((x & 0b01010101) << 1) It works by handling the low bits and high bits of each bit-pair separately and then combining the result: The expression x & 0b10101010 extracts the high bit from each pair, and then >> 1 shifts it to the low bit position. Similarly

问题 I have an arbitrary 8-bit binary number e.g., 11101101 I have to swap all the pair of bits like: Before swapping: 11-10-11-01 After swapping: 11-01-11-10 I was asked this in an interview ! 回答1: In pseudo-code: x = ((x & 0b10101010) >> 1) | ((x & 0b01010101) << 1) It works by handling the low bits and high bits of each bit-pair separately and then combining the result: The expression x & 0b10101010 extracts the high bit from each pair, and then >> 1 shifts it to the low bit position. Similarly

问题 I have an arbitrary 8-bit binary number e.g., 11101101 I have to swap all the pair of bits like: Before swapping: 11-10-11-01 After swapping: 11-01-11-10 I was asked this in an interview ! 回答1: In pseudo-code: x = ((x & 0b10101010) >> 1) | ((x & 0b01010101) << 1) It works by handling the low bits and high bits of each bit-pair separately and then combining the result: The expression x & 0b10101010 extracts the high bit from each pair, and then >> 1 shifts it to the low bit position. Similarly

问题 looking at http://ref.x86asm.net/coder32.html I found two opcodes that match for the statement xor eax,eax 1) opcode 31 XOR r/m16/32 r16/32 2) opcode 33 XOR r16/32 r/m16/32 both refers to 32bit register for operand1 and operand2. So, is there any differences in this specific case of the XORing two 32bit registers ? 回答1: x86 has 2 redundant ways to encode a 2-register instance of any of the basic ALU instructions that have r/m source and r/m destination forms. This redundancy is a consequence

问题 I am trying to translate some js code into python and having trouble converting bitwise operators. I already introduced ctypes.c_int in python but the results still do not match. For the >>> in js I used as suggested here. A minimal example of my (not working) code: Javascript: let R = [7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 7, 12, 17, 22, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20, 5, 9, 14, 20, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 4, 11, 16, 23, 6, 10, 15, 21, 6, 10, 15, 21, 6,

问题 Followup to this I figured out what went wrong in the linked post. The Sigma0 and Sigma1 boxes weren't doing what they should: modulo 2 addition of the rotated versions of either A or E. Basically, it should work like this: Notice that if the number of bits in a position is even the result is 0, if odd it's 1. To do this I made a truth table and karnaugh maps: LINK I simplified the resulting expression to !A!BC + A(B XNOR C) And I tried to apply it to Sigma0 and Sigma1. But it's not working.