Program to Count Number of Set Bits in an (Big) Array _ GeeksforGeeks

5/12/12 Program to count number of set bits in an (big) array | GeeksforGeeks

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Program to count number of set bits in an (big) array

March 12, 2011

Given an integer array of length N (an arbitrarily large number). How to count number of setbits in the array?

The simple approach would be, create an efficient method to count set bits in a word (mostprominent size, usually equal to bit length of processor), and add bits from individual elementsof array.

Various methods of counting set bits of an integer exists, see this for example. These methodsrun at best O(logN) where N is number of bits. Note that on a processor N is fixed, count canbe done in O(1) time on 32 bit machine irrespective of total set bits. Overall, the bits in arraycan be computed in O(n) time, where ‘n’ is array size.



However, a table look up will be more efficient method when array size is large. Storing table

look up that can handle 232 integers will be impractical.

The following code illustrates simple program to count set bits in a randomly generated 64 Kinteger array. The idea is to generate a look up for first 256 numbers (one byte), and breakevery element of array at byte boundary. A meta program using C/C++ preprocessorgenerates the look up table for counting set bits in a byte.

The mathematical derivation behind meta program is evident from the following table (Addthe column and row indices to get the number, then look into the table to get set bits in thatnumber. For example, to get set bits in 10, it can be extracted from row named as 8 andcolumn named as 2),

0, 1, 2, 3 0 - 0, 1, 1, 2 -------- GROUP_A(0) 4 - 1, 2, 2, 3 -------- GROUP_A(1) 8 - 1, 2, 2, 3 -------- GROUP_A(1)12 - 2, 3, 3, 4 -------- GROUP_A(2)16 - 1, 2, 2, 3 -------- GROUP_A(1)20 - 2, 3, 3, 4 -------- GROUP_A(2)24 - 2, 3, 3, 4 -------- GROUP_A(2)28 - 3, 4, 4, 5 -------- GROUP_A(3) ... so on

From the table, there is a patten emerging in multiples of 4, both in the table as well as in thegroup parameter. The sequence can be generalized as shown in the code.

Complexity:

All the operations takes O(1) except iterating over the array. The time complexity is O(n)where ‘n’ is size of array. Space complexity depends on the meta program that generates lookup.

Code:

#include <stdio.h>#include <stdlib.h>#include <time.h> /* Size of array 64 K */#define SIZE (1 << 16) /* Meta program that generates set bit count array of first 256 integers */ /* GROUP_A - When combined with META_LOOK_UP generates count for 4x4 elements */



#define GROUP_A(x) x, x + 1, x + 1, x + 2 /* GROUP_B - When combined with META_LOOK_UP generates count for 4x4x4 elements */ #define GROUP_B(x) GROUP_A(x), GROUP_A(x+1), GROUP_A(x+1), GROUP_A(x+2) /* GROUP_C - When combined with META_LOOK_UP generates count for 4x4x4x4 elements */ #define GROUP_C(x) GROUP_B(x), GROUP_B(x+1), GROUP_B(x+1), GROUP_B(x+2) /* Provide appropriate letter to generate the table */ #define META_LOOK_UP(PARAMETER) \ GROUP_##PARAMETER(0), \ GROUP_##PARAMETER(1), \ GROUP_##PARAMETER(1), \ GROUP_##PARAMETER(2) \ int countSetBits(int array[], size_t array_size){ int count = 0; /* META_LOOK_UP(C) - generates a table of 256 integers whose sequence will be number of bits in i-th position where 0 <= i < 256 */ /* A static table will be much faster to access */ static unsigned char const look_up[] = { META_LOOK_UP(C) }; /* No shifting funda (for better readability) */ unsigned char *pData = NULL; for(size_t index = 0; index < array_size; index++) { /* It is fine, bypass the type system */ pData = (unsigned char *)&array[index]; /* Count set bits in individual bytes */ count += look_up[pData[0]]; count += look_up[pData[1]]; count += look_up[pData[2]]; count += look_up[pData[3]]; } return count;



Contributed by Venki. Please write comments if you find anything incorrect, or you want toshare more information about the topic discussed above.

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2 comments so far

1. pranav says:October 13, 2011 at 7:57 PM

please someone explain clearly GROUP_A, GROUP_B, etc. concept and meaning ofthe definition of :

} /* Driver program, generates table of random 64 K numbers */int main(){ int index; int random[SIZE]; /* Seed to the random-number generator */ srand((unsigned)time(0)); /* Generate random numbers. */ for( index = 0; index < SIZE; index++ ) { random[index] = rand(); } printf("Total number of bits = %d\n", countSetBits(random, SIZE)); return 0;}

Send 2people



#define META_LOOK_UP(PARAMETER) \GROUP_##PARAMETER(0), \GROUP_##PARAMETER(1), \GROUP_##PARAMETER(1), \GROUP_##PARAMETER(2) \

ReplyVenki says:October 14, 2011 at 12:41 AM

@pranav, the idea is to create table of 256 entries. The table contains number ofset bits in the sequential index of table. The macro META_LOOK_UP(C)generates the table. After preprocessing stage the definition of look_up will besomething like

static unsigned char const look_up[] = { 0, 1, 1, 2, 1, 2, 2, 3, ... 6, 7, 7, 8 };

The code in main, simply splits the array into individual bytes and looks up the bitcount value in the table.

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Program to Count Number of Set Bits in an (Big) Array _ GeeksforGeeks