Count set bits in an integer using Lookup Table

Last Updated : 03 Dec, 2024

Write an efficient program to count number of 1s in binary representation of an integer.
Examples:

Input : n = 6
Output : 2
Binary representation of 6 is 110 and has 2 set bits

Input : n = 13
Output : 3
Binary representation of 13 is 1101 and has 3 set bits

In the previous post we had seen different method that solved this problem in O(log n) time. In this post we solve in O(1) using lookup table. Here we assume that the size of INT is 32-bits. It’s hard to count all 32 bits in one go using lookup table (” because it’s infeasible to create lookup table of size 2³²-1 “). So we break 32 bits into 8 bits of chunks( How lookup table of size (2⁸-1 ) index : 0-255 ).
LookUp Table
In lookup tale, we store count of set_bit of every
number that are in a range (0-255)
LookupTable[0] = 0 | binary 00000000 CountSetBits 0
LookupTable[1] = 1 | binary 00000001 CountSetBits 1
LookupTable[2] = 1 | binary 00000010 CountSetBits 1
LookupTable[3] = 2 | binary 00000011 CountSetBits 2
LookupTable[4] = 1 | binary 00000100 CountSetBits 1
and so…on upto LookupTable[255].
Let’s take an Example How lookup table work.

Let's number be : 354 
in Binary : 0000000000000000000000101100010

Split it into 8 bits chunks  :
In Binary  :  00000000 | 00000000 | 00000001 | 01100010
In decimal :     0          0          1         98

Now Count Set_bits using LookupTable
LookupTable[0] = 0
LookupTable[1] = 1
LookupTable[98] = 3

so Total bits count : 4

Below is the code implementation of the above approach:

CPP

// C++ Program to count number of set bits // using lookup table in O(1) time  #include <iostream> using namespace std;  // Generate a lookup table for 32 bit integers #define B2(n) n, n + 1, n + 1, n + 2 #define B4(n) B2(n), B2(n + 1), B2(n + 1), B2(n + 2) #define B6(n) B4(n), B4(n + 1), B4(n + 1), B4(n + 2)  // Lookup table that store the reverse of each table unsigned int lookuptable[256]     = { B6(0), B6(1), B6(1), B6(2) };  // function countset Bits Using lookup table // ans return set bits count unsigned int countSetBits(int N) {     // first chunk of 8 bits from right     unsigned int count = lookuptable[N & 0xff] +                           // second chunk from  right                          lookuptable[(N >> 8) & 0xff] +                           // third and fourth chunks                          lookuptable[(N >> 16) & 0xff]                          + lookuptable[(N >> 24) & 0xff];     return count; }  int main() {     unsigned int N = 354;     cout << countSetBits(N) << endl;      return 0; }

Java

// Java count to count number of set bits // using lookup table in O(1) time  // Generate a lookup table for 32 bit integers import java.util.*;  class GFG {     static ArrayList<Integer> lookuptable         = new ArrayList<Integer>();      static void B2(int n)     {         lookuptable.add(n);         lookuptable.add(n + 1);         lookuptable.add(n + 1);         lookuptable.add(n + 2);     }      static void B4(int n)     {         B2(n);         B2(n + 1);         B2(n + 1);         B2(n + 2);     }      static void B6(int n)     {         B4(n);         B4(n + 1);         B4(n + 1);         B4(n + 2);     }      // function countset Bits Using lookup table     // ans return set bits count     static int countSetBits(int N)     {         // adding the bits in chunks of 8 bits         int count = lookuptable.get(N & 0xff)                     + lookuptable.get((N >> 8) & 0xff)                     + lookuptable.get((N >> 16) & 0xff)                     + lookuptable.get((N >> 24) & 0xff);         return count;     }      // Driver Code     public static void main(String[] args)     {         // Lookup table that store the reverse of each table         B6(0);         B6(1);         B6(1);         B6(2);          int N = 354;          // Function Call         System.out.println(countSetBits(N));     } }  // This code is contributed by phasing17

Python

# Python3 count to count number of set bits # using lookup table in O(1) time  # Generate a lookup table for 32 bit integers  lookuptable = []   def B2(n):     lookuptable.extend([n, n + 1, n + 1, n + 2])   def B4(n):     B2(n), B2(n + 1), B2(n + 1), B2(n + 2)   def B6(n):     B4(n), B4(n + 1), B4(n + 1), B4(n + 2)   # Lookup table that store the reverse of each table lookuptable.extend([B6(0), B6(1), B6(1), B6(2)])  # function countset Bits Using lookup table # ans return set bits count   def countSetBits(N):      # adding the bits in chunks of 8 bits     count = lookuptable[N & 0xff] + lookuptable[(N >> 8) & 0xff] + lookuptable[(         N >> 16) & 0xff] + lookuptable[(N >> 24) & 0xff]     return count   # Driver Code N = 354  # Function Call print(countSetBits(N))   # This code is contributed by phasing17

// C# count to count number of set bits // using lookup table in O(1) time  // Generate a lookup table for 32 bit integers  using System; using System.Collections.Generic;  class GFG {     static List<int> lookuptable = new List<int>();      static void B2(int n)     {         lookuptable.Add(n);         lookuptable.Add(n + 1);         lookuptable.Add(n + 1);         lookuptable.Add(n + 2);     }      static void B4(int n)     {         B2(n);         B2(n + 1);         B2(n + 1);         B2(n + 2);     }      static void B6(int n)     {         B4(n);         B4(n + 1);         B4(n + 1);         B4(n + 2);     }      // function countset Bits Using lookup table     // ans return set bits count     static int countSetBits(int N)     {         // adding the bits in chunks of 8 bits         int count = lookuptable[N & 0xff]                     + lookuptable[(N >> 8) & 0xff]                     + lookuptable[(N >> 16) & 0xff]                     + lookuptable[(N >> 24) & 0xff];         return count;     }      // Driver Code     public static void Main(string[] args)     {         // Lookup table that store the reverse of each table         B6(0);         B6(1);         B6(1);         B6(2);          int N = 354;          // Function Call         Console.WriteLine(countSetBits(N));     } }  // This code is contributed by phasing17

JavaScript

// JavaScript count to count number of set bits // using lookup table in O(1) time  // Generate a lookup table for 32 bit integers  let lookuptable = [];   function B2(n) {     lookuptable.push(n);     lookuptable.push(n + 1);     lookuptable.push(n + 1);     lookuptable.push(n + 2); }   function B4(n) {     B2(n), B2(n + 1), B2(n + 1), B2(n + 2) }   function B6(n) {     B4(n), B4(n + 1), B4(n + 1), B4(n + 2) }   // Lookup table that store the reverse of each table lookuptable.push(B6(0)); lookuptable.push(B6(1)); lookuptable.push(B6(1)); lookuptable.push(B6(2));   // function countset Bits Using lookup table // ans return set bits count function countSetBits(N) {     // adding the bits in chunks of 8 bits     let count = lookuptable[N & 0xff] + lookuptable[(N >> 8) & 0xff] + lookuptable[(N >> 16) & 0xff] + lookuptable[(N >> 24) & 0xff];     return count; }   // Driver Code let N = 354;  // Function Call console.log(countSetBits(N));   // This code is contributed by phasing17