Given an unsigned int, what's the fastest way to get the “indices” of given bits?

Question

So, for example, for. 0110 has bits 1 and 2, 1000 has bit 3 set to 1111, has bits 0,1,2,3 of a set

Answer 1

If there really is only 4 bits, then the fastest method will certainly include a lookup table. There are a total of 16 different possibilities.

Answer 2

The best internet link for all of these beat hacks is bit-twisting hacks

Answer 3

. 32- ( , unsigned int).

/Allan

Answer 4

for( int i = 0; variable ; ++i, variable >>= 1 ) {
  if( variable & 1 )
    // store bit index - i
}

Answer 5

.NET, , , .

( BitTools).

[Flags]
public enum Int32Bits
{
    // Lookup table but nicer
    None = 0,
    Bit1 = 1,        Bit2  = 1 << 1,  Bit3  = 1 << 2,  Bit4  = 1 << 3,  Bit5  = 1 << 4,  Bit6  = 1 << 5,  Bit7  = 1 << 6,  Bit8  = 1 << 7,
    Bit9  = 1 << 8,  Bit10 = 1 << 9,  Bit11 = 1 << 10, Bit12 = 1 << 11, Bit13 = 1 << 12, Bit14 = 1 << 13, Bit15 = 1 << 14, Bit16 = 1 << 15,
    Bit17 = 1 << 16, Bit18 = 1 << 17, Bit19 = 1 << 18, Bit20 = 1 << 19, Bit21 = 1 << 20, Bit22 = 1 << 21, Bit23 = 1 << 22, Bit24 = 1 << 23,
    Bit25 = 1 << 24, Bit26 = 1 << 25, Bit27 = 1 << 26, Bit28 = 1 << 27, Bit29 = 1 << 28, Bit30 = 1 << 29, Bit31 = 1 << 30, Bit32 = 1 << 31,
}

public static class BitTools
{
    public static Boolean IsSet(Int32 value, Int32Bits bitToCheck)
    {
        return ((Int32Bits)value & bitToCheck) == bitToCheck;
    }

    public static Boolean IsSet(UInt32 value, Int32Bits bitToCheck)
    {
        return ((Int32Bits)value & bitToCheck) == bitToCheck;
    }

    public static Boolean IsBitSet(this Int32 value, Int32Bits bitToCheck)
    {
        return ((Int32Bits)value & bitToCheck) == bitToCheck;
    }
    public static Boolean IsBitSet(this UInt32 value, Int32Bits bitToCheck)
    {
        return ((Int32Bits)value & bitToCheck) == bitToCheck;
    }
}

:

static void Main(string[] args)
{
    UInt32 testValue =  5557; //1010110110101;

    if (BitTools.IsSet(testValue, Int32Bits.Bit1))
    {
        Console.WriteLine("The first bit is set!");
    }
    if (testValue.IsBitSet(Int32Bits.Bit5))
    {
        Console.WriteLine("The fifth bit is set!");
    }
    if (!testValue.IsBitSet(Int32Bits.Bit2))
    {
        Console.WriteLine("The second bit is NOT set!");
    }
}

(U) Int Int * Bits IsSet IsBitSet.

EDIT: , ints, .

Answer 6

, .

" ", .NET BitArray true/false.

BitArray

Answer 7

@Allan Wind...

. -, , .. - .

firstbit = (x & 0x00000001) 
secondbit = (x & 0x00000002) 
thirdbit = (x & 0x00000004)   //<-- I'm not saying to store these values, just giving an example. 
...

x86 32- , , 32- .

.

x86 x64, , .

Answer 8

int, ( ). , .

. , MSB 32- int. nibble upper_idxs, nibble lower_idxs. 24 lower_idxs 28 upper_idxs. , , 16 20 , 8 "".

, : -)

Answer 9

:

• Extract each set bit set_bit= x & -x; x&= x - 1;

• Subtract 1 and count the number of bits.

Answer 10

I think this will help

import java.util.*;
public class bitSet {

    public static void main(String[]args) {
        Scanner scnr = new Scanner(System.in);
        int x = scnr.nextInt();
        int i = 0;
        while (i<32) {
            if ( ((x>>i)&1) == 1) {
                System.out.println(i);
            }
            i++;
        }
    }
}