Question

I need to fetch last 6 bits of a integer or Uint32. For example if I have a value of 183, I need last six bits which will be 110 111 ie 55.

I have written a small piece of code, but it’s not behaving as expected. Could you guys please point out where I am making a mistake?

int compress8bitTolessBit( int value_to_compress, int no_of_bits_to_compress )
{
    int ret = 0;
    while(no_of_bits_to_compress--)
    {
        std::cout << " the value of bits "<< no_of_bits_to_compress << std::endl;
        ret >>= 1;
        ret |= ( value_to_compress%2 );
        value_to_compress /= 2;
    }
    return ret;
}

int _tmain(int argc, _TCHAR* argv[])
{
    int val = compress8bitTolessBit( 183, 5 );

    std::cout <<" the value is "<< val << std::endl;
      system("pause>nul");
    return 0;
}

Answer 1

You have entered the realm of binary arithmetic. C++ has built-in operators for this kind of thing. The act of “getting certain bits” of an integer is done with an “AND” binary operator.

    0101 0101
AND 0000 1111
    ---------
    0000 0101

In C++ this is:

int n = 0x55 & 0xF;
// n = 0x5

So to get the right-most 6 bits,

int n = original_value & 0x3F;

And to get the right-most N bits,

int n = original_value & ((1 << N) - 1);

Here is more information on

• Binary arithmetic operators in C++
• Binary operators in general