What USEFUL bitwise operator code tricks should a developer know about?

Question

I must say I have never had cause to use bitwise operators, but I am sure there are some operations that I have performed that would have been more efficiently done with them. How have "shifting" and "OR-ing" helped you solve a problem more efficiently?

Answer 1

Using bitwise operations on strings (characters)

Convert letter to lowercase:

• OR by space => (x | ' ')
• Result is always lowercase even if letter is already lowercase
• eg. ('a' | ' ') => 'a' ; ('A' | ' ') => 'a'

Convert letter to uppercase:

• AND by underline => (x & '_')
• Result is always uppercase even if letter is already uppercase
• eg. ('a' & '_') => 'A' ; ('A' & '_') => 'A'

Invert letter's case:

• XOR by space => (x ^ ' ')
• eg. ('a' ^ ' ') => 'A' ; ('A' ^ ' ') => 'a'

Letter's position in alphabet:

• AND by chr(31)/binary('11111')/(hex('1F') => (x & "\x1F")
• Result is in 1..26 range, letter case is not important
• eg. ('a' & "\x1F") => 1 ; ('B' & "\x1F") => 2

Get letter's position in alphabet (for Uppercase letters only):

• AND by ? => (x & '?') or XOR by @ => (x ^ '@')
• eg. ('C' & '?') => 3 ; ('Z' ^ '@') => 26

Get letter's position in alphabet (for lowercase letters only):

• XOR by backtick/chr(96)/binary('1100000')/hex('60') => (x ^ '`')
• eg. ('d' ^ '`') => 4 ; ('x' ^ '`') => 25

Note: using anything other than the english letters will produce garbage results

Answer 2

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• Bitwise operations on integers(int)

---

Get the maximum integer

int maxInt = ~(1 << 31);
int maxInt = (1 << 31) - 1;
int maxInt = (1 << -1) - 1;

Get the minimum integer

int minInt = 1 << 31;
int minInt = 1 << -1;

Get the maximum long

long maxLong = ((long)1 << 127) - 1;

Multiplied by 2

n << 1; // n*2

Divided by 2

n >> 1; // n/2

Multiplied by the m-th power of 2

n << m; // (3<<5) ==>3 * 2^5 ==> 96

Divided by the m-th power of 2

n >> m; // (20>>2) ==>(20/( 2^2) ==> 5

Check odd number

(n & 1) == 1;

Exchange two values

a ^= b;
b ^= a;
a ^= b;

Get absolute value

(n ^ (n >> 31)) - (n >> 31);

Get the max of two values

b & ((a-b) >> 31) | a & (~(a-b) >> 31);

Get the min of two values

a & ((a-b) >> 31) | b & (~(a-b) >> 31);

Check whether both have the same sign

(x ^ y) >= 0;

Calculate 2^n

2 << (n-1);

Whether is factorial of 2

n > 0 ? (n & (n - 1)) == 0 : false;

Modulo 2^n against m

m & (n - 1);

Get the average

(x + y) >> 1;
((x ^ y) >> 1) + (x & y);

Get the m-th bit of n (from low to high)

(n >> (m-1)) & 1;

Set the m-th bit of n to 0 (from low to high)

n & ~(1 << (m-1));

n + 1

-~n

n - 1

~-n

Get the contrast number

~n + 1;
(n ^ -1) + 1; 

if (x==a) x=b; if (x==b) x=a;

x = a ^ b ^ x;

Answer 3

See the famous Bit Twiddling Hacks
Most of the multiply/divide ones are unnecessary - the compiler will do that automatically and you will just confuse people.

But there are a bunch of, 'check/set/toggle bit N' type hacks that are very useful if you work with hardware or communications protocols.

Answer 4

There's only three that I've ever used with any frequency:

1. Set a bit: a |= 1 << bit;

2. Clear a bit: a &= ~(1 << bit);

3. Test that a bit is set: a & (1 << bit);

Answer 5

Matters Computational: Ideas, Algorithms, Source Code, by Jorg Arndt (PDF). This book contains tons of stuff, I found it via a link at http://www.hackersdelight.org/

Average without overflow

A routine for the computation of the average (x + y)/2 of two arguments x and y is

static inline ulong average(ulong x, ulong y)
// Return floor( (x+y)/2 )
// Use: x+y == ((x&y)<<1) + (x^y)
// that is: sum == carries + sum_without_carries
{
    return (x & y) + ((x ^ y) >> 1);
}

Answer 6

1) Divide/Multiply by a power of 2

foo >>= x; (divide by power of 2)

foo <<= x; (multiply by power of 2)

2) Swap

x ^= y;
y = x ^ y;
x ^= y;

Answer 7

You can compress data, e.g. a collection of integers:

• See which integer values appear more frequently in the collection
• Use short bit-sequences to represent the values which appear more frequently (and longer bit-sequences to represent the values which appear less frequently)
• Concatenate the bits-sequences: so for example, the first 3 bits in the resulting bit stream might represent one integer, then the next 9 bits another integer, etc.

Answer 8

I used bitwise operators to efficiently implement distance calculations for bitstrings. In my application bitstrings were used to represent positions in a discretised space (an octree, if you're interested, encoded with Morton ordering). The distance calculations were needed to know whether points on the grid fell within a particular radius.

Answer 9

Counting set bits, finding lowest/highest set bit, finding nth-from-top/bottom set bit and others can be useful, and it's worth looking at the bit-twiddling hacks site.

That said, this kind of thing isn't day-to-day important. Useful to have a library, but even then the most common uses are indirect (e.g. using a bitset container). Also, ideally, these would be standard library functions - a lot of them are better handled using specialise CPU instructions on some platforms.

Answer 10

While multiplying/dividing by shifting seems nifty, the only thing I needed once in a while was compressing booleans into bits. For that you need bitwise AND/OR, and probably bit shifting/inversion.

Answer 11

I wanted a function to round numbers to the next highest power of two, so I visited the Bit Twiddling website that's been brought up several times and came up with this:

i--;
i |= i >> 1;
i |= i >> 2;
i |= i >> 4;
i |= i >> 8;
i |= i >> 16;
i++;

I use it on a size_t type. It probably won't play well on signed types. If you're worried about portability to platforms with different sized types, sprinkle your code with #if SIZE_MAX >= (number) directives in appropriate places.