Bit Masking in C++

In this tutorial, we will learn Bit masking in C++ with a program.

What is a bitmask?

We are well aware of the bitwise operators in C++. They are – bitwise and, or, not and exclusive or operators. We use bitmask operations to turn a bit on from off or vice-versa, check if a bit is on or off and for toggling a bit.

Why do we use bit masking?

We use bit masking to store multiple layers of values in the same set of numbers. Let us understand this with an example:

consider the set S={1,2,3,4,5,6,7}.

To represent the set{2,3,5}, we can use the bitmask 0110100.

Using Bitmasks in C++

Bitmasks can be used for the following operations:

To set the ith bit:

Let us consider a value x. We perform x |=x<<i for setting a bit. To put it simply, we left shift x by i bits, and perform the bitwise operation, as it ideally does not change values.

To unset the ith bit:

We use x&=~(x<<i).

To toggle a bit:

For this, we use the operation x^=x<<i.

Bit masking program in C++

#include<iostream>
using namespace std;
void set_mask(int x)
{
   int i;
   cout<<"which position do you want to set?";
   cin>>i;
   cout<<"before "<<x<<endl;
   x|=x<<i;
   cout<<"after "<<x<<endl;

}

void unset_mask(int x)
{
    int i;
    cout<<"enter the position you want to clear: ";
    cin>>i;
    cout<<"before clearing: "<<x<<endl;
    x&=~(x<<i);
    cout<<"after clearing: "<<x<<endl;

}

void toggle_bit(int x)
{
    int i;
    cout<<"Which bit do you want to toggle? ";
    cin>>i;
    cout<<"before toggle: "<<x<<endl;
    x^=x<<i;
    cout<<"After toggle: "<<x<<endl;
}

int main()
{
    int num,ch;
    cout<<"enter a number: ";
    cin>>num;
    while(1)
    {
        cout<<"your choices are: 1.SET MASK\n 2.UNSET/CLEAR MASK\n 3.TOGGLE BIT\n"<<endl;
        cout<<"enter your choice: ";
        cin>>ch;
        switch(ch)
        {
            case 1:set_mask(num);
            break;
            case 2:unset_mask(num);
            break;
            case 3:toggle_bit(num);
            break;
            default:cout<<"invalid choice"<<endl;
            break;

        }

    }

   return 0;


}

Output:
enter a number: 24
your choices are: 1.SET MASK
 2.UNSET/CLEAR MASK
 3.TOGGLE BIT

enter your choice: 1
which position do you want to set?4
before 24
after 408
your choices are: 1.SET MASK
 2.UNSET/CLEAR MASK
 3.TOGGLE BIT

enter your choice: 2
enter the position you want to clear: 4
before clearing: 24
after clearing: 24
your choices are: 1.SET MASK
 2.UNSET/CLEAR MASK
 3.TOGGLE BIT

enter your choice: 3
Which bit do you want to toggle? 2
before toggle: 24
After toggle: 120
your choices are: 1.SET MASK
 2.UNSET/CLEAR MASK
 3.TOGGLE BIT

enter your choice:

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2 responses to “Bit Masking in C++”

Joseph says:
November 23, 2021 at 12:14 pm
This does’nt explain what a bitmask actually is.
Reply
ju says:
July 21, 2022 at 4:08 pm
“Toggle a bit: x^=x<<i"
Did you meant "x ^= 1 << i" instead ?
Reply