CODEWITHSUNDEEP
c#.netalgorithmmath.net-core
configurator
configurator

Reputation: 41670

How to check if a number is a power of 2

Today I needed a simple algorithm for checking if a number is a power of 2.

The algorithm needs to be:

  1. Simple
  2. Correct for any ulong value.

I came up with this simple algorithm:

private bool IsPowerOfTwo(ulong number)
{
    if (number == 0)
        return false;

    for (ulong power = 1; power > 0; power = power << 1)
    {
        // This for loop used shifting for powers of 2, meaning
        // that the value will become 0 after the last shift
        // (from binary 1000...0000 to 0000...0000) then, the 'for'
        // loop will break out.

        if (power == number)
            return true;
        if (power > number)
            return false;
    }
    return false;
}

But then I thought: How about checking if log2 x is an exactly a round number? When I checked for 2^63+1, Math.Log() returned exactly 63 because of rounding. So I checked if 2 to the power 63 is equal to the original number and it is, because the calculation is done in doubles and not in exact numbers.

private bool IsPowerOfTwo_2(ulong number)
{
    double log = Math.Log(number, 2);
    double pow = Math.Pow(2, Math.Round(log));
    return pow == number;
}

This returned true for the given wrong value: 9223372036854775809.

Is there a better algorithm?

Upvotes: 738

Views: 410913

Answers (30)

Hans Brende
Hans Brende

Reputation: 8647

Many answers solve this cleverly for integers, but here's how, using only basic floating-point operations, to solve it for double!

const double HALF_EPS = 1.1102230246251565e-16; // 2^-53

bool isPowerOf2(double n) {
    return (
        n > 1e-292 ? n + n*HALF_EPS - n == 0 :
        n > 0 && n + n/HALF_EPS == n/HALF_EPS
    );
}

For C# of course, you can simply use Double.isPow2 instead, but that doesn't translate to other languages. This answer will work for double-precision floating point numbers in any language, under the assumption that the rounding mode is the default IEEE 754 roundTiesToEven and subnormals aren't flushed to zero (if they are then you can simply increase 1e-292 to 1e-291).

See my answer here for a complete proof of correctness (and why you should not use log2 to determine whether a number is a power of 2).

Upvotes: 0

RARE Kpop Manifesto
RARE Kpop Manifesto

Reputation: 2915

Here's a clean approach if you want to avoid using any and all bitwise ops (this is generic pseudo-code instead of being exactly C#) :

func is_power_of_2( uint64 n ) {

    return (n > 4) ? 2**63 % n == 0 \
                   :     n % 3 != 0
}

A single modulo(%) op isn't all that expensive computationally.

Upvotes: 0

velocity
velocity

Reputation: 2106

I've been reading the Java documentation for Random.nextInt(int bound) and saw this nice piece of code which checks whether the parameter is a power of 2, which says (part of the code) :

if ((bound & -bound) == bound) // ie, bound is a power of 2

let's test it

for (int i=0; i<=8; i++) {
  System.out.println(i+" = " + Integer.toBinaryString(i));
}

>>
0 = 0
1 = 1
2 = 10
3 = 11
4 = 100
5 = 101
6 = 110
7 = 111
8 = 1000
// the left most 0 bits where cut out of the output

for (int i=-1; i>=-8; i--) {
  System.out.println(i+" = " + Integer.toBinaryString(i));
}

>>
-1 = 11111111111111111111111111111111
-2 = 11111111111111111111111111111110
-3 = 11111111111111111111111111111101
-4 = 11111111111111111111111111111100
-5 = 11111111111111111111111111111011
-6 = 11111111111111111111111111111010
-7 = 11111111111111111111111111111001
-8 = 11111111111111111111111111111000

did you notice something ? positive and negative Power 2 numbers have opposite bits in binary representation (negative power 2 numbers are 1's complement of positive power 2 numbers.

if we apply a logical AND over X and -X, where X is a Power 2 number, we get the positive absolute value of that number X as a result :)

for (int i=0; i<=8; i++) {
  System.out.println(i + " & " + (-i)+" = " + (i & (-i)));
}

>>
0 & 0 = 0    <-
1 & -1 = 1   <-
2 & -2 = 2   <-
3 & -3 = 1
4 & -4 = 4   <-
5 & -5 = 1
6 & -6 = 2
7 & -7 = 1
8 & -8 = 8   <-

Upvotes: 6

Rushil Patel
Rushil Patel

Reputation: 27

unsigned int n; // we want to see if n is a power of 2
bool f;         // the result goes here 

f = (n & (n - 1)) == 0;

Note that 0 is considered a power of 2 here. To remedy this, use:

f = n && !(n & (n - 1));

Upvotes: 0

displayName
displayName

Reputation: 14399

The following addendum to the accepted answer may be useful for some people:

A power of two, when expressed in binary, will always look like 1 followed by n zeroes where n is greater than or equal to 0. Ex:

Decimal  Binary
1        1     (1 followed by 0 zero)
2        10    (1 followed by 1 zero)
4        100   (1 followed by 2 zeroes)
8        1000  (1 followed by 3 zeroes)
.        .
.        .
.        .

and so on.

When we subtract 1 from these kind of numbers, they become 0 followed by n ones and again n is same as above. Ex:

Decimal    Binary
1 - 1 = 0  0    (0 followed by 0 one)
2 - 1 = 1  01   (0 followed by 1 one)
4 - 1 = 3  011  (0 followed by 2 ones)
8 - 1 = 7  0111 (0 followed by 3 ones)
.          .
.          .
.          .

and so on.

Coming to the crux

What happens when we do a bitwise AND of a number x, which is a power of 2, and x - 1?

The one of x gets aligned with the zero of x - 1 and all the zeroes of x get aligned with ones of x - 1, causing the bitwise AND to result in 0. And that is how we have the single line answer mentioned above being right.

Further adding to the beauty of accepted answer above -

So, we have a property at our disposal now:

When we subtract 1 from any number, then in the binary representation the rightmost 1 will become 0 and all the zeroes to the right of that rightmost 1 will now become 1.

One awesome use of this property is in finding out - How many 1s are present in the binary representation of a given number? The short and sweet code to do that for a given integer x is:

byte count = 0;
for ( ; x != 0; x &= (x - 1)) count++;
Console.Write("Total ones in the binary representation of x = {0}", count);

Another aspect of numbers that can be proved from the concept explained above is "Can every positive number be represented as the sum of powers of 2?".

Yes, every positive number can be represented as the sum of powers of 2. For any number, take its binary representation. Ex: Take number 117.

The binary representation of 117 is 1110101

Because  1110101 = 1000000 + 100000 + 10000 + 0000 + 100 + 00 + 1
we have  117     = 64      + 32     + 16    + 0    + 4   + 0  + 1

Upvotes: 33

eraoul
eraoul

Reputation: 1151

In C, I tested the i && !(i & (i - 1) trick and compared it with __builtin_popcount(i), using gcc on Linux, with the -mpopcnt flag to be sure to use the CPU's POPCNT instruction. My test program counted the # of integers in the interval [0, 2^31) that were a power of two.

At first I thought that i && !(i & (i - 1) was 10% faster, even though I verified that POPCNT was used in the disassembly where I used__builtin_popcount.

However, I realized that I had included an if statement, and branch prediction was probably doing better on the bit twiddling version. I removed the if and POPCNT ended up faster, as expected.

Results:

Intel(R) Core(TM) i7-4771 CPU max 3.90GHz

Timing (i & !(i & (i - 1))) trick
30

real    0m13.804s
user    0m13.799s
sys     0m0.000s

Timing POPCNT
30

real    0m11.916s
user    0m11.916s
sys     0m0.000s

AMD Ryzen Threadripper 2950X 16-Core Processor max 3.50GHz

Timing (i && !(i & (i - 1))) trick
30

real    0m13.675s
user    0m13.673s
sys 0m0.000s

Timing POPCNT
30

real    0m13.156s
user    0m13.153s
sys 0m0.000s

Note that here the Intel CPU seems slightly slower than AMD with the bit twiddling, but has a much faster POPCNT; the AMD POPCNT doesn't provide as much of a boost.

popcnt_test.c:

#include "stdio.h"

// Count # of integers that are powers of 2 up to (not including) 2^31;
int main() {
  int n;
  for (int z = 0; z < 20; z++){
      n = 0;
      for (unsigned long i = 0; i < 1<<30; i++) {
       #ifdef USE_POPCNT
        n += (__builtin_popcount(i)==1); // Was: if (__builtin_popcount(i) == 1) n++;
       #else
        n += (i && !(i & (i - 1)));  // Was: if (i && !(i & (i - 1))) n++;
       #endif
      }
  }

  printf("%d\n", n);
  return 0;
}

Run tests:

gcc popcnt_test.c -O3 -o test.exe
gcc popcnt_test.c -O3 -DUSE_POPCNT -mpopcnt -o test-popcnt.exe

echo "Timing (i && !(i & (i - 1))) trick"
time ./test.exe

echo
echo "Timing POPCNT"
time ./test-opt.exe

Upvotes: 0

Cato
Cato

Reputation: 3701

I'm assuming 1 is a power of two, which it is, it's 2 to the power of zero

 bool IsPowerOfTwo(ulong testValue)
 {
  ulong bitTest = 1;
  while (bitTest != 0)
  {
    if (bitTest == testValue) return true;
    bitTest = bitTest << 1;
  }

  return false;
}

Upvotes: 0

WaterGenie
WaterGenie

Reputation: 139

There were a number of answers and posted links explaining why the n & (n-1) == 0 works for powers of 2, but I couldn't find any explanation of why it doesn't work for non-powers of 2, so I'm adding this just for completeness.

For n = 1 (2^0 = 1), 1 & 0 = 0, so we are fine.

For odd n > 1, there are at least 2 bits of 1 (left-most and right-most bits). Now n and n-1 will only differ by the right-most bit, so their &-sum will at least have a 1 on the left-most bit, so n & (n-1) != 0:

n:          1xxxx1  for odd n > 1
n-1:        1xxxx0
            ------
n & (n-1):  1xxxx0 != 0

Now for even n that is not a power of 2, we also have at least 2 bits of 1 (left-most and non-right-most). Here, n and n-1 will differ up to the right-most 1 bit, so their &-sum will also have at least a 1 on the left-most bit:

        right-most 1 bit of n
                 v
n:          1xxxx100..00 for even n
n-1:        1xxxx011..11
            ------------
n & (n-1):  1xxxx000..00 != 0

Upvotes: 2

Dr.jacky
Dr.jacky

Reputation: 3547

Kotlin:

fun isPowerOfTwo(n: Int): Boolean {
    return (n > 0) && (n.and(n-1) == 0)
}

or

fun isPowerOfTwo(n: Int): Boolean {
    if (n == 0) return false
    return (n and (n - 1).inv()) == n
}

inv inverts the bits in this value.

Note:
log2 solution doesn't work for large numbers, like 536870912 ->

import kotlin.math.truncate
import kotlin.math.log2

fun isPowerOfTwo(n: Int): Boolean {
    return (n > 0) && (log2(n.toDouble())) == truncate(log2(n.toDouble()))
}

Upvotes: 1

Uche Igbokwe
Uche Igbokwe

Reputation: 49

There is a one liner in .NET 6

// IsPow2 evaluates whether the specified Int32 value is a power of two.
Console.WriteLine(BitOperations.IsPow2(128));            // True

Upvotes: 2

Vivek Nuna
Vivek Nuna

Reputation: 1

It's very easy in .Net 6 now.

using System.Numerics;

bool isPow2 = BitOperations.IsPow2(64); // sets true

Here is the documentation.

Upvotes: 9

Aleksandar Biševac
Aleksandar Biševac

Reputation: 29

This one returns if the number is the power of two up to 64 value ( you can change it inside for loop condition ("6" is for 2^6 is 64);

const isPowerOfTwo = (number) => {
  let result = false;
  for (let i = 1; i <= 6; i++) {
    if (number === Math.pow(2, i)) {
      result = true;
    }
  }
  return result;
};

console.log(isPowerOfTwo(16));
console.log(isPowerOfTwo(10));

Upvotes: 0

ABHISHEK PARMAR
ABHISHEK PARMAR

Reputation: 47

in this approach , you can check if there is only 1 set bit in the integer and the integer is > 0 (c++).

bool is_pow_of_2(int n){
    int count = 0;
    for(int i = 0; i < 32; i++){
        count += (n>>i & 1);
    }
    return count == 1 && n > 0;
}

Upvotes: 1

Marcus Moo
Marcus Moo

Reputation: 196

This is another method to do it as well

package javacore;

import java.util.Scanner;

public class Main_exercise5 {
    public static void main(String[] args) {
        // Local Declaration
        boolean ispoweroftwo = false;
        int n;
        Scanner input = new Scanner (System.in);
        System.out.println("Enter a number");
        n = input.nextInt();
        ispoweroftwo = checkNumber(n);
        System.out.println(ispoweroftwo);
    }
    
    public static boolean checkNumber(int n) {
        // Function declaration
        boolean ispoweroftwo= false;
        // if not divisible by 2, means isnotpoweroftwo
        if(n%2!=0){
            ispoweroftwo=false;
            return ispoweroftwo;
        }
        else {
            for(int power=1; power>0; power=power<<1) {
                if (power==n) {
                    return true;
                }
                else if (power>n) {
                    return false;
                }
            }
        }
        return ispoweroftwo;
    }
}

Upvotes: 0

Anil Gupta
Anil Gupta

Reputation: 145

I see many answers are suggesting to return n && !(n & (n - 1)) but to my experience if the input values are negative it returns false values. I will share another simple approach here since we know a power of two number have only one set bit so simply we will count number of set bit this will take O(log N) time.

while (n > 0) {
    int count = 0;
    n = n & (n - 1);
    count++;
}
return count == 1;

Check this article to count no. of set bits

Upvotes: 0

jbat100
jbat100

Reputation: 16827

Mark gravell suggested this if you have .NET Core 3, System.Runtime.Intrinsics.X86.Popcnt.PopCount

public bool IsPowerOfTwo(uint i)
{
    return Popcnt.PopCount(i) == 1
}

Single instruction, faster than (x != 0) && ((x & (x - 1)) == 0) but less portable.

Upvotes: 2

Greg Hewgill
Greg Hewgill

Reputation: 994649

There's a simple trick for this problem:

bool IsPowerOfTwo(ulong x)
{
    return (x & (x - 1)) == 0;
}

Note, this function will report true for 0, which is not a power of 2. If you want to exclude that, here's how:

bool IsPowerOfTwo(ulong x)
{
    return (x != 0) && ((x & (x - 1)) == 0);
}

Explanation

First and foremost the bitwise binary & operator from MSDN definition:

Binary & operators are predefined for the integral types and bool. For integral types, & computes the logical bitwise AND of its operands. For bool operands, & computes the logical AND of its operands; that is, the result is true if and only if both its operands are true.

Now let's take a look at how this all plays out:

The function returns boolean (true / false) and accepts one incoming parameter of type unsigned long (x, in this case). Let us for the sake of simplicity assume that someone has passed the value 4 and called the function like so:

bool b = IsPowerOfTwo(4)

Now we replace each occurrence of x with 4:

return (4 != 0) && ((4 & (4-1)) == 0);

Well we already know that 4 != 0 evals to true, so far so good. But what about:

((4 & (4-1)) == 0)

This translates to this of course:

((4 & 3) == 0)

But what exactly is 4&3?

The binary representation of 4 is 100 and the binary representation of 3 is 011 (remember the & takes the binary representation of these numbers). So we have:

100 = 4
011 = 3

Imagine these values being stacked up much like elementary addition. The & operator says that if both values are equal to 1 then the result is 1, otherwise it is 0. So 1 & 1 = 1, 1 & 0 = 0, 0 & 0 = 0, and 0 & 1 = 0. So we do the math:

100
011
----
000

The result is simply 0. So we go back and look at what our return statement now translates to:

return (4 != 0) && ((4 & 3) == 0);

Which translates now to:

return true && (0 == 0);

return true && true;

We all know that true && true is simply true, and this shows that for our example, 4 is a power of 2.

Upvotes: 1536

rhodan
rhodan

Reputation: 319

Improving the answer of @user134548, without bits arithmetic:

public static bool IsPowerOfTwo(ulong n)
{
    if (n % 2 != 0) return false;  // is odd (can't be power of 2)

    double exp = Math.Log(n, 2);
    if (exp != Math.Floor(exp)) return false;  // if exp is not integer, n can't be power
    return Math.Pow(2, exp) == n;
}

This works fine for:

IsPowerOfTwo(9223372036854775809)

Upvotes: 1

Freeze Francis
Freeze Francis

Reputation: 518

for any power of 2, the following also holds.

n&(-n)==n

NOTE: fails for n=0 , so need to check for it
Reason why this works is:
-n is the 2s complement of n. -n will have every bit to the left of rightmost set bit of n flipped compared to n. For powers of 2 there is only one set bit.

Upvotes: 4

Michael Burr
Michael Burr

Reputation: 340486

Some sites that document and explain this and other bit twiddling hacks are:

And the grandaddy of them, the book "Hacker's Delight" by Henry Warren, Jr.:

As Sean Anderson's page explains, the expression ((x & (x - 1)) == 0) incorrectly indicates that 0 is a power of 2. He suggests to use:

(!(x & (x - 1)) && x)

to correct that problem.

Upvotes: 107

abelenky
abelenky

Reputation: 64730

bool isPow2 = ((x & ~(x-1))==x)? !!x : 0;

Upvotes: 7

Khaled.K
Khaled.K

Reputation: 5960

Example

0000 0001    Yes
0001 0001    No

Algorithm

  1. Using a bit mask, divide NUM the variable in binary

  2. IF R > 0 AND L > 0: Return FALSE

  3. Otherwise, NUM becomes the one that is non-zero

  4. IF NUM = 1: Return TRUE

  5. Otherwise, go to Step 1

Complexity

Time ~ O(log(d)) where d is number of binary digits

Upvotes: 2

Prakash Jat
Prakash Jat

Reputation: 589

return ((x != 0) && !(x & (x - 1)));

If x is a power of two, its lone 1 bit is in position n. This means x – 1 has a 0 in position n. To see why, recall how a binary subtraction works. When subtracting 1 from x, the borrow propagates all the way to position n; bit n becomes 0 and all lower bits become 1. Now, since x has no 1 bits in common with x – 1, x & (x – 1) is 0, and !(x & (x – 1)) is true.

Upvotes: 5

deft_code
deft_code

Reputation: 59347

Here's a simple C++ solution:

bool IsPowerOfTwo( unsigned int i )
{
    return std::bitset<32>(i).count() == 1;
}

Upvotes: 21

Rezo Megrelidze
Rezo Megrelidze

Reputation: 3060

    bool IsPowerOfTwo(int n)
    {
        if (n > 1)
        {
            while (n%2 == 0)
            {
                n >>= 1;
            }
        }
        return n == 1;
    }

And here's a general algorithm for finding out if a number is a power of another number.

    bool IsPowerOf(int n,int b)
    {
        if (n > 1)
        {
            while (n % b == 0)
            {
                n /= b;
            }
        }
        return n == 1;
    }

Upvotes: 10

sudeepdino008
sudeepdino008

Reputation: 3364

int isPowerOfTwo(unsigned int x)
{
    return ((x != 0) && ((x & (~x + 1)) == x));
}

This is really fast. It takes about 6 minutes and 43 seconds to check all 2^32 integers.

Upvotes: 5

Chethan
Chethan

Reputation: 933

Here is another method I devised, in this case using | instead of & :

bool is_power_of_2(ulong x) {
    if(x ==  (1 << (sizeof(ulong)*8 -1) ) return true;
    return (x > 0) && (x<<1 == (x|(x-1)) +1));
}

Upvotes: 3

bugs king
bugs king

Reputation: 564

bool isPowerOfTwo(int x_)
{
  register int bitpos, bitpos2;
  asm ("bsrl %1,%0": "+r" (bitpos):"rm" (x_));
  asm ("bsfl %1,%0": "+r" (bitpos2):"rm" (x_));
  return bitpos > 0 && bitpos == bitpos2;
}

Upvotes: 4

jerrymouse
jerrymouse

Reputation: 17822

A number is a power of 2 if it contains only 1 set bit. We can use this property and the generic function countSetBits to find if a number is power of 2 or not.

This is a C++ program:

int countSetBits(int n)
{
        int c = 0;
        while(n)
        {
                c += 1;
                n  = n & (n-1);
        }
        return c;
}

bool isPowerOfTwo(int n)
{        
        return (countSetBits(n)==1);
}
int main()
{
    int i, val[] = {0,1,2,3,4,5,15,16,22,32,38,64,70};
    for(i=0; i<sizeof(val)/sizeof(val[0]); i++)
        printf("Num:%d\tSet Bits:%d\t is power of two: %d\n",val[i], countSetBits(val[i]), isPowerOfTwo(val[i]));
    return 0;
}

We dont need to check explicitly for 0 being a Power of 2, as it returns False for 0 as well.

OUTPUT

Num:0   Set Bits:0   is power of two: 0
Num:1   Set Bits:1   is power of two: 1
Num:2   Set Bits:1   is power of two: 1
Num:3   Set Bits:2   is power of two: 0
Num:4   Set Bits:1   is power of two: 1
Num:5   Set Bits:2   is power of two: 0
Num:15  Set Bits:4   is power of two: 0
Num:16  Set Bits:1   is power of two: 1
Num:22  Set Bits:3   is power of two: 0
Num:32  Set Bits:1   is power of two: 1
Num:38  Set Bits:3   is power of two: 0
Num:64  Set Bits:1   is power of two: 1
Num:70  Set Bits:3   is power of two: 0

Upvotes: 3

udhaya
udhaya

Reputation: 55

Find if the given number is a power of 2. 

#include <math.h>

int main(void)
{
    int n,logval,powval;
    printf("Enter a number to find whether it is s power of 2\n");
    scanf("%d",&n);
    logval=log(n)/log(2);
    powval=pow(2,logval);

    if(powval==n)
        printf("The number is a power of 2");
    else
        printf("The number is not a power of 2");

    getch();
    return 0;
}

Upvotes: 3

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