Questions tagged [bitwise-operators]
low-level, primitive operations that directly manipulate bit patterns and binary numerals
51
questions
0
votes
0answers
47 views
Semantics of simulating a 64bit integer with two 32bit integers (hi, lo)
I am working in a system which only supports 32bit integers, in order to have a 64bit (unsigned) integer I decided to simply use two 32bit integers with one being the upper 32 bits (hi), and the other ...
2
votes
2answers
394 views
What's the purpose of multiplying by 1024x1024?
I'm working with an old C module that was originally ran on Power PC architecture and compiled with gnu 3.0.6
I'm porting it to run in a VS2012 project on Intel hardware.
The module creates a 3D ...
-4
votes
5answers
193 views
Do decimal equivalents to binary number values hold significance in software programming?
It seems as though in software engineering, we care more about these "on and off switch" usages of binary numbers more than the actual values of them numbers... For example, say I have the number:
...
1
vote
2answers
98 views
C++ - BitVector logic
I have implemented a simple bit vector class. However, I have some problems with understanding, how to push data to it.
In a standard vector, push_back inserts new element et the end. A similar ...
3
votes
3answers
132 views
What effects does memory space have on bitwise shifts?
It is true that the bitwise left shift operation (shl) doubles the value of the integer being shifted. However, when constrained to a finite space, such as 8 bits for example, left shift will begin to ...
1
vote
1answer
147 views
How to understand and design functions with enumeration arguments that work in tandem with bitwise operators in C++ and Qt?
In the C++, there are 6 bitwise operators:
Symbol Operator
& bitwise AND
| bitwise inclusive OR
^ bitwise XOR (eXclusive OR)
<< left shift
>> right shift
~...
1
vote
3answers
597 views
How can I query, increment & decrement arbitrary-length integers encoded into a bit-array?
I'm in the process of implementing a counting Bloom filter. This data structure is defined as a bit-array and a "width" parameter, W.
The bit array stores unsigned integers, whose size is determined ...
0
votes
1answer
48 views
Best practices for reading dynamic byte streams: is line-by-line comparison with a mask the best way?
I receive sensor data as a binary stream of bytes. This stream is not always the same length, and does not include the same data set each time. If the sensor did not send a field, it is simply absent, ...
80
votes
7answers
14k views
Why are bit masks called “masks” and what purpose do they serve?
Why are "bit masks" called like this?
I know that they are mainly used for bitwise operations and the usage of bit masks is more efficient than the usage of separate variables.
However my question ...
0
votes
1answer
166 views
Is it possible to store and query combinations of boolean-like information in an integer?
With one boolean, it is simply 0 and 1, and it's straight-forward to query.
With two booleans, it becomes necessary to specify what each number means, ex:
0 F-F
1 F-T
2 T-F
3 T-T
Here it's easy to ...
0
votes
4answers
356 views
Shortening a boolean AND with third operand
I'm trying to calculate the sum of 2 bits using basic binary arithmetic and currently, I'm doing this:
function Add(bool a, bool b, bool carry)
{
return
{
Result: a ^ b ^ carry,
...
32
votes
1answer
3k views
Why was the caret used for XOR instead of exponentiation?
Not that it's really a problem for anyone that has faced this syntactic issue before, but I see a wild amount of confusion stemming from the use of the caret (^) as the XOR operation in lieu of the ...
9
votes
2answers
1k views
Is it possible to define all bitwise operators using a 'bitwise nand' similar to how all boolean logic can be built using just 'boolean nand'?
Nand is known as a 'universal' logic gate, because it allows you define all other boolean logic gates:
not(x) = nand(x,x)
and(x, y) = not(nand(x, y))
or(x, y) = nand(not(x), not(y))
nor(x, y) = not(...
1
vote
2answers
189 views
Is there a conceptual 2-bit-input logical operation that does: 0 0 = 0, 0 1 = 1, 1 0 = 0, 1 1 = 0?
Reason: In an LR35902 (the processor in the Game Boy) assembly program of mine, I'm trying to figure out what bits have changed to 1's when comparing A with B, which would need a bitwise operation ...
4
votes
5answers
4k views
How does bitwise information storage in (32-bit) int variables work?
In this book I'm reading I'm going over bitwise operators. Its says the following in the book.
Bitwise operations can potentially store a lot of information in a small amount of memory. Many ...
0
votes
1answer
1k views
Getting an array index (0,1,2,..8) from bit masking value (1,2,4,8..256) without using log2(n). Maybe a design issue
I'm working on a component where I put in data and I get different data as a result. The input is always the same (3 Objects).
From these 3 Objects up to 9 other Objects can be calculated. One ...
1
vote
2answers
407 views
LSB-propagating left shift, conceptual equivalent of sign-propagating right shift [closed]
How would I write something that fills with the right-most bit (<<< is used to denote this non-existent operator):
1 <<< 7: "11111111" and 0 <<< 7: "00000000"
9 <<&...
0
votes
1answer
495 views
How to calculate big O notation according to number width?
I'm trying to understand big O with the bitwise operations. I have 2 functions those are solving the same question from different perspective.
num1BitsSecondSolution starts to shift the number right ...
2
votes
3answers
200 views
Is there any low level way to get shifted or unshifted bits which results from bitwise operations?
I was playing with bitwise operations and a question about counting true bits of any positive integer value, so I solved the problem with bit shifting, so I just thought if there would be some way to ...
0
votes
3answers
720 views
Bits - Least-Significant/Lowest is 0th or 1st; zero or one indexed
Question
Is there a rough consensus if the bitmask 0x01 is properly said to have the "zeroth" bit set, or the "first" bit set?
If there isn't rough consensus that there's a generally right answer, ...
3
votes
1answer
528 views
Overflow Exception Checking Problem
Background
I have to call a method that needs to return a signed integer (see code block below) after converting from an unsigned integer. The reason for this is that I have to do bit-wise math that ...
0
votes
1answer
850 views
Is there a way to do bitwise operation with more than two state
I know how make bit operation. I'm wondering if you need more than 2 states is there a way to do it using bit instead of array and % operators. Because those are really slow.
So in case the array ...
17
votes
2answers
17k views
Is there any advantage to c-style bit manipulation over std::bitset?
I work almost exclusively in C++11/14, and usually cringe when I see code like this:
std::int64_t mArray;
mArray |= someMask << 1;
This is just an example; I'm talking about bit-wise ...
3
votes
1answer
901 views
How do you set and check a single bit in a programming language that does not support bitwise operations?
I'm using a programming language (more a scripting language) that does not support any bitwise operators like AND, OR, XOR, NOT (and shift as well).
Common arithmetic and logical operations like + - *...
0
votes
2answers
2k views
boolean operations in C using bitfields
I am trying to implement boolean data type in C. Basically, I am working with sets.
The following code can be used to access each bit but I am unsure whether I can represent sets using this method.
...
28
votes
11answers
9k views
When I test out the difference in time between shifting and multiplying in C, there is no difference. Why?
I have been taught that shifting in binary is much more efficient than multiplying by 2^k. So I wanted to experiment, and I used the following code to test this out:
#include <time.h>
#include &...
11
votes
3answers
33k views
What does “(int) value & 0x1, (int) value & 0x2, (int) value & 0x4, (int) value & 0x8” mean?"
The "value" ranges from 0 to 15 (its possible values). When will those 4 "if" conditions be met? If my (int)value = 2 does this mean 0010?
if ((int)value & 0x1)
{
...
7
votes
1answer
2k views
In Java, why use bit hacks when non-bitwise technique it is more readable?
Is there any legitimate use for bit manipulation hacks in higher-level languages such as Java?
I can see them being useful in speed-sensitive low-level and computation-intensive programs, e.g. ...
2
votes
2answers
198 views
gcc -S seems a bit misshapen with shifting and ANDing bits
Example:
int c = 4;
int p = 5;
if (p & (1 << c))
printf("ok\n");
else
printf("nop\n");
gcc -S:
movl -4(%rbp), %eax /* eax holds the variable c */
movl -8(%rbp), %edx /* ...
12
votes
3answers
69k views
How are negative signed values stored?
I was watching this video on the maximum and minimum values of signed integers.
Take an example of a positive signed value - 0000 0001
The first bit denotes that the number is positive and the last ...
0
votes
1answer
147 views
Why Num&sizeMinusOne faster than num&(size-1)
I've been told that when I have a hash table of size m and m=2^k, I can use the & operator as num & (size-1) instead of num % size, to fit the hashCode to my table size.
I've also been told ...
9
votes
3answers
22k views
Speeds of << >> multiplication and division
You can use << to multiply and >> to divide numbers in python when I time them I find using the binary shift way of doing it is 10x faster than dividing or multiplying the regular way.
...
3
votes
4answers
4k views
Why do higher level languages have neither xor nor nand short-circuit operators?
While many higher level languages have bitwise (exclusive or) and bitwise (exclusive and), for instance C, C++, Java, etc. I'm curious why the ( vastly more useful ) logical short-circuit operators ...
5
votes
2answers
2k views
Why is ~0 a Sequence of 1s [closed]
Why is ~0 a sequence of 1 bits and not just a single bit? Where do the extra 1 bits come from? Does this mean ~1 a sequence of 0 bits or just a single 0 bit?
My understanding was that the ~ operator ...
1
vote
1answer
742 views
Next power of 2 for a number (in search for better “bit-twiddling” way)
I just wonder if there exists better (i.e. faster?) way to get the next
power of 2 for a given number than the following one (maybe some
better sort of "bit-twiddling" hack is possible?) ...
static ...
5
votes
1answer
2k views
Clearing the lowest set bit of a number
I can see in this tutorial on bit manipulation, under the heading "Extracting every last bit", that -
Suppose we wish to find the lowest set bit of x (which is known to be
non-zero). If we ...
0
votes
3answers
735 views
Using Power of 2 numbers to represent types
Let's say that we have some values, represented by power of 2:
TYPE_1 = 1
TYPE_2 = 2
TYPE_3 = 4
TYPE_4 = 8
...
I need to store some of these types in one value.
Example:
To represent TYPE_1 with ...
4
votes
2answers
1k views
Concept of bit fields
Whenever I read a code like this:
struct node
{
int x : 2;
int p : 4;
}n;
with bit fields involved, I get really confused, as to how they are represented in memory, what ...
63
votes
2answers
6k views
Why do bitwise operators have lower priority than comparisons?
Could someone explain the rationale, why in a bunch of most popular languages (see note below) comparison operators (==, !=, <, >, <=, >=) have higher priority than bitwise operators (&, |, ^...
61
votes
7answers
71k views
Using scoped enums for bit flags in C++
An enum X : int (C#) or enum class X : int (C++11) is a type that has a hidden inner field of int that can hold any value. In addition, a number of predefined constants of X are defined on the enum. ...
2
votes
2answers
1k views
What kind of specific projects can I do to master bitwise operations in C++? Also is there a canonical book? [closed]
I don't use C++ or bitwise operations at my current job but I'm thinking of applying to companies where it is a requirement to be fluent with them (on their tests anyway).
So my question is: Can ...
10
votes
3answers
42k views
Is there something special about the number 65535?
2¹⁶-1 & 2⁵ = 2⁵ (or? obviously ?)
A developer asked me today what is bitwise 65535 & 32 i.e. 2¹⁶-1 & 2⁵ = ?
I thought at first spontaneously 32 but it seemed to easy whereupon I thought ...
8
votes
3answers
29k views
Ternary operator (condition ? foo : bar) and the XOR (^) operator
I have read in a recent code review that both ternary operator (condition ? foo : bar) and the XOR operator ^ are rarely used in Java. Is it true?
If yes, is this because they are less readable? or ...
0
votes
2answers
17k views
Flipping the desired bit of an integer number [duplicate]
Suppose you were given an integer number in decimal notation. This when represented in binary will be a series of 0's and 1's. This sequence varies in length with the magnitude of the number. Now ...
6
votes
3answers
5k views
How does bit flipping / complementing work?
I am currently learning about bitwise operation, so bear with me. I understand AND, OR, and shifting. What I don't understand is bit flipping.
So, 5 is 0101. When someone says to me "flip those", it ...
34
votes
13answers
4k views
How good does a well-rounded programmer need to be with bit-wise operations? [closed]
I have been browsing some OpenJDK code recently and have found some intriguing pieces of code there that has to do with bit-wise operations. I even asked a question about it on StackOverflow.
...
12
votes
5answers
6k views
What is the benefit of studying bitwise operators? [duplicate]
What is the benefit of studying bitwise operators (Bitwise Not, Bitwise AND, Bitwise OR, Bitwise XOR, Left Shift, Signed Right Shift, Unsigned Right Shift etc.)?
Will we really use these operators in ...
4
votes
3answers
377 views
How about multiple bits performance in common databases?
If there are 20 bit columns (non-null boolean), is it better to explicitly hold them in an integer type (int32)? Or, will the underlying database merge them in a single integer?
I'm using PostgreSQL, ...
14
votes
4answers
11k views
What's your favorite bit-wise technique? [closed]
A few days ago, StackExchange member Anto inquired about valid uses for bit-wise operators. I stated that shifting was faster than multiplying and dividing integers by powers of two. StackExchange ...
19
votes
15answers
4k views
What are bit operators good for? [closed]
Programming languages often come with various bit operators (e.g. bitwise left- and right shift, bitwise AND, OR, XOR...). These don't get used though very much, or at least such has my experience ...
