Re: Right shift >> weird result... Why?

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 25 May 2006 23:43:02 -0400

Message-ID:

<127cua7dfh6d65@corp.supernews.com>

Victor Bazarov wrote:

Kai-Uwe Bux wrote:

Mick_fae_Glesga wrote:

Ah, OK, thanks... I was hoping it would be zero....

Well, do you know of any other way of asking 'What is the highest set
bit in this number?' does such a command exist? or do I have to
look at each bit individually?

What about:

template < typename Unsigned >
Unsigned highest_bit ( Unsigned u ) {
Unsigned result = 0;
while ( u != 0 ) {
   ++result;
   u >>= 1;
}
return ( result );
}

#include <iostream>

int main ( void ) {
unsigned long u;
while ( std::cin >> u ) {
   std::cout << u << " --> " << highest_bit(u) << '\n';
}
}

Best

Kai-Uwe Bux

Or

    unsigned highest_bit(unsigned a) {
        return log(a) / log(2);
    }

Hm, I just don't trust floating point operations:

template < typename Unsigned >
Unsigned highest_bit_kub ( Unsigned u ) {
  Unsigned result = 0;
  while ( u != 0 ) {
    ++result;
    u >>= 1;
  }
  return ( result );
}

#include <cmath>

template < typename Unsigned >
Unsigned highest_bit_vb ( Unsigned u ) {
  return std::log(u) / std::log(2);
}

#include <iostream>
#include <iomanip>

int main ( void ) {
  unsigned long u;
  for ( u = 0; u < 25; ++u ) {
    std::cout << std::setw(3) << u
              << " highest_bit_kub: " << highest_bit_kub(u)
              << ", highest_bit_vb: " << highest_bit_vb(u) << '\n';
  }
}

Output on my machine:

  0 highest_bit_kub: 0, highest_bit_vb: 0
  1 highest_bit_kub: 1, highest_bit_vb: 0
  2 highest_bit_kub: 2, highest_bit_vb: 1
  3 highest_bit_kub: 2, highest_bit_vb: 1
  4 highest_bit_kub: 3, highest_bit_vb: 2
  5 highest_bit_kub: 3, highest_bit_vb: 2
  6 highest_bit_kub: 3, highest_bit_vb: 2
  7 highest_bit_kub: 3, highest_bit_vb: 2
  8 highest_bit_kub: 4, highest_bit_vb: 2
  9 highest_bit_kub: 4, highest_bit_vb: 3
10 highest_bit_kub: 4, highest_bit_vb: 3
11 highest_bit_kub: 4, highest_bit_vb: 3
12 highest_bit_kub: 4, highest_bit_vb: 3
13 highest_bit_kub: 4, highest_bit_vb: 3
14 highest_bit_kub: 4, highest_bit_vb: 3
15 highest_bit_kub: 4, highest_bit_vb: 3
16 highest_bit_kub: 5, highest_bit_vb: 4
17 highest_bit_kub: 5, highest_bit_vb: 4
18 highest_bit_kub: 5, highest_bit_vb: 4
19 highest_bit_kub: 5, highest_bit_vb: 4
20 highest_bit_kub: 5, highest_bit_vb: 4
21 highest_bit_kub: 5, highest_bit_vb: 4
22 highest_bit_kub: 5, highest_bit_vb: 4
23 highest_bit_kub: 5, highest_bit_vb: 4
24 highest_bit_kub: 5, highest_bit_vb: 4

Note in particular the lines 7,8,9 as opposed to 3,4 and 15,16.

Best

Kai-Uwe Bux