Counting unset bits in C++ Bitset

24 Sep 2023

When working with binary data in C++, the std::bitset class provides a convenient way to manipulate and access individual bits. However, there may be scenarios where you need to count the number of unset (0) bits in a std::bitset. In this blog post, we’ll explore a simple approach to accomplish this task efficiently.

The Problem

Given a std::bitset, the goal is to count the number of unset (0) bits in it. This can be useful in various situations, such as determining the number of available resources, calculating parity, or checking for data corruption.

The Solution

We can leverage the built-in count method of the std::bitset class to count the number of set (1) bits. To count the unset bits, we subtract the count of set bits from the total number of bits in the bitset.

std::bitset<8> myBitset("11001010");
int unsetBits = myBitset.size() - myBitset.count();

Here, we create a std::bitset called myBitset with an initial binary value of “11001010”. The size method returns the total number of bits in the bitset, and the count method returns the number of set bits. By subtracting the count from the size, we get the count of unset bits.

Example Usage

Let’s consider a practical example where we want to count the unset bits in a bitset representing the permission flags of a file. We can define a helper function like this:

#include <iostream>
#include <bitset>

int countUnsetBits(const std::bitset<8>& bitset) {
    return bitset.size() - bitset.count();
}

int main() {
    std::bitset<8> filePermissions("01010101");
    int unsetBits = countUnsetBits(filePermissions);

    std::cout << "Number of unset bits: " << unsetBits << std::endl;
    
    return 0;
}

In this example, we define a bitset called filePermissions representing the permission flags of a file. We then call the countUnsetBits helper function to count the unset bits. The result is printed to the console.

Conclusion

Counting the number of unset bits in a std::bitset is a straightforward task in C++. By subtracting the count of set bits from the total number of bits, we can efficiently obtain the desired result. This can be useful in various applications where bit-level operations are required.

#C++ #Bitset