What are bitwise operators?

75 down vote favorite 54

I'm someone who writes code just for fun and haven't really delved into it in either an academic or professional setting, so stuff like these bitwise operators really escapes me. I was reading an article about JavaScript, which apparently supports bitwise operations. I keep seeing this operation mentioned in places, and I've tried reading about to figure out what exactly it is, but I just don't seem to get it at all. So what are they? Clear examples would be great! :D Just a few more questions - what are some practical applications of bitwise operations? When might you use them? boolean  boolean-logic  bitwise-operators share improve this question edited Oct 25 '14 at 12:35 Peter Mortensen 9,253 10 64 98 asked  Nov 9 '08 at 23:59 click 384 5 5

I'm someone who mainly writes code just for fun and I've written a functional (but limited) microkernel. In x86 assembly language. For fun. :D –  Artelius  Nov 10 '08 at 0:13 2   For further questions, you might want to add a new SO question and reference this one. You'll probably get a better set of answers that way. –  Greg Hewgill  Nov 10 '08 at 0:39 add a comment

9 Answers

active oldest votes

up vote 113 down vote accepted

Since nobody has broached the subject of why these are useful: I use bitwise operations a lot when working with flags. For example, if you want to pass a series of flags to an operation (say, File.Open, with Read mode and Write mode both enabled), you could pass them as a single value. This is accomplished by assigning each possible flag it's own bit in a bitset (byte, short, int, or long). For example: Read: 00000001 Write: 00000010 So if you want to pass read AND write, you would pass (READ | WRITE) which then combines the two into  00000011 Which then can be decrypted on the other end like: if ((flag & Read) != 0) { //... which checks 00000011 & 00000001 which returns 00000001 which is not 0, so the flag does specify READ. You can use XOR to toggle various bits. I've used this when using a flag to specify directional inputs (Up, Down, Left, Right). For example, if a sprite is moving horizontally, and I want it to turn around: Up: 00000001 Down: 00000010 Left: 00000100 Right: 00001000 Current: 00000100 I simply XOR the current value with (LEFT | RIGHT) which will turn LEFT off and RIGHT on, in this case. Bit Shifting is useful in several cases. x << y is the same as x * 2y if you need to quickly multiply by a power of two, but watch out for shifting a 1-bit into the top bit - this makes the number negative unless it's unsigned. It's also useful when dealing with different sizes of data. For example, reading an integer from four bytes: int val = (A << 24) | (B << 16) | (C << 8) | D; Assuming that A is the most-significant byte and D the least. It would end up as: A = 01000000 B = 00000101 C = 00101011 D = 11100011 val = 01000000 00000101 00101011 11100011 Colors are often stored this way (with the most significant byte either ignored or used as Alpha): A = 255 = 11111111 R = 21 = 00010101 G = 255 = 11111111 B = 0 = 00000000 Color = 11111111 00010101 11111111 00000000 To find the values again, just shift the bits to the right until it's at the bottom, then mask off the remaining higher-order bits: Int Alpha = Color >> 24 Int Red = Color >> 16 & 0xFF Int Green = Color >> 8 & 0xFF Int Blue = Color & 0xFF 0xFF is the same as 11111111. So essentially, for Red, you would be doing this: Color >> 16 = (filled in 00000000 00000000)11111111 00010101 (removed 11111111 00000000) 00000000 00000000 11111111 00010101 & 00000000 00000000 00000000 11111111 = 00000000 00000000 00000000 00010101 (The original value) share improve this answer edited Jun 24 '13 at 16:55 answered  Nov 10 '08 at 0:39 Ed Marty 29.6k 13 80 132

11   +1 this is the only good answer given –  TrojanName  Jul 13 '11 at 12:38 1   great description! Thanks! –  monkut  Aug 29 '13 at 6:09 add a comment

up vote 18 down vote

Bitwise operators are operators that work on a bit at a time. AND is 1 only if both of its inputs are 1. OR is 1 if one or more of its inputs are 1. XOR is 1 only if exactly one of its inputs are 1. NOT is 1 only if its input are 0. These can be best described as truth tables. Inputs possibilities are on the top and left, the resultant bit is one of the four (two in the case of NOT since it only has one input) values shown at the intersection of the two inputs. AND|0 1 OR|0 1 ---+---- ---+---- 0|0 0 0|0 1 1|0 1 1|1 1 XOR|0 1 NOT|0 1 ---+---- ---+--- 0|0 1 |1 0 1|1 0 One example is if you only want the lower 4 bits of an integer, you AND it with 15 (binary 1111) so: 203: 1100 1011 AND 15: 0000 1111 ------------------ IS 11: 0000 1011 share improve this answer edited May 26 '13 at 7:15 answered  Nov 10 '08 at 0:04 paxdiablo 444k 107 869 1329

add a comment

up vote 15 down vote

It is worth noting that the single-bit truth tables listed as other answers work on only one or two input bits at a time. What happens when you use integers, such as: int x = 5 & 6; The answer lies in the binary expansion of each input: 5 = 0 0 0 0 0 1 0 1 & 6 = 0 0 0 0 0 1 1 0 --------------------- 0 0 0 0 0 1 0 0 Each pair of bits in each column is run through the "AND" function to give the corresponding output bit on the bottom line. So the answer to the above expression is 4. The CPU has done (in this example) 8 separate "AND" operations in parallel, one for each column. I mention this because I still remember having this "AHA!" moment when I learned about this many years ago. share improve this answer edited Nov 10 '08 at 0:16 answered  Nov 10 '08 at 0:11 Greg Hewgill 460k 101 806 1000

Wow, that makes a lot more sense now. It sounded a lot more complicated than it apparently is. Thanks. I'm not sure which to choose as the right answer as there are loads of good ones, and I can't upvote so.. thanks –  click  Nov 10 '08 at 0:21 add a comment

up vote 10 down vote

These are the bitwise operators, all supported in JavaScript: op1 & op2 -- The AND operator compares two bits and generates a result of 1 if both bits are 1; otherwise, it returns 0. op1 | op2 -- The OR operator compares two bits and generates a result of 1 if the bits are complementary; otherwise, it returns 0. op1^ op2 -- The EXCLUSIVE-OR operator compares two bits and returns 1 if either of the bits are 1 and it gives 0 if both bits are 0 or 1. ~op1 -- The COMPLEMENT operator is used to invert all of the bits of the operand. op1 << op2 -- The SHIFT LEFT operator moves the bits to the left, discards the far left bit, and assigns the rightmost bit a value of 0. Each move to the left effectively multiplies op1 by 2. op1 >> op2 -- The SHIFT RIGHT operator moves the bits to the right, discards the far right bit, and assigns the leftmost bit a value of 0. Each move to the right effectively divides op1 in half. The left-most sign bit is preserved. op1 >>> op2 -- The SHIFT RIGHT - ZERO FILL operator moves the bits to the right, discards the far right bit, and assigns the leftmost bit a value of 0. Each move to the right effectively divides op1 in half. The left-most sign bit is discarded. share improve this answer edited Nov 10 '08 at 0:15 answered  Nov 10 '08 at 0:03 Jeff Hillman 4,031 1 18 29

2   Not in all languages. –  Jason Jackson  Nov 10 '08 at 0:07 add a comment

up vote 2 down vote

To break it down a bit more, it has a lot to do with the binary representation of the value in question.  For example (in decimal): x = 8 y = 1 would come out to (in binary): x = 1000 y = 0001 From there, you can do computational operations such as 'and' or 'or'; in this case: x | y = 1000 0001 | ------ 1001 or...9 in decimal Hope this helps.  share improve this answer answered  Nov 10 '08 at 0:14 javamonkey79 8,144 21 77 130

add a comment

up vote 1 down vote

When the term "bitwise" is mentioned, it is sometimes clarifying that is is not a "logical" operator. For example in JavaScript, bitwise operators treat their operands as a sequence of 32 bits (zeros and ones); meanwhile, logical operators are typically used with Boolean (logical) values but can work with non-Boolean types. Take expr1 && expr2 for example. Returns expr1 if it can be converted to false; otherwise, returns expr2. Thus, when used with Boolean values, && returns true if both operands are true; otherwise, returns false. a = "Cat" && "Dog" // t && t returns Dog a = 2 && 4 // t && t returns 4 As others have noted, 2 & 4 is a bitwise AND, so it will return 0. You can copy the following to test.html or something and test: <html> <body> <script> alert("\"Cat\" && \"Dog\" = " + ("Cat" && "Dog") + "\n" + "2 && 4 = " + (2 && 4) + "\n" + "2 & 4 = " + (2 & 4)); </script> share improve this answer edited Nov 10 '08 at 0:33 answered  Nov 10 '08 at 0:26 Eugene Yokota 55.9k 31 131 211

add a comment