Re: mixing signed and unsigned

From:

Kai-Uwe Bux <jkherciueh@gmx.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 18 Feb 2007 03:28:26 -0500

Message-ID:

<er92nc$k94$1@murdoch.acc.Virginia.EDU>

John Harrison wrote:

Kai-Uwe Bux wrote:

John Harrison wrote:

I have a problem. I want to compare an integral value, n, against three
half open ranges as follows

[-A, 0) // range 1
[0, B) // range 2
[B, C} // range 3

Each range corresponds to a different outcome and if the integral value
isn't within any of the ranges, that's a fourth outcome. So far so easy,
the problem is that n is a signed quantity and A, B, C are unsigned
quantities. Apart from this obscure corner of my code this makes perfect
sense, so I don't want to change the signedness of anything.

How to I write these tests so that my code is reasonably understandable,
rather than a horrible mess of casts and compiler warnings?

One more point, of the unsigned quantity, only B is guaranteed small
enough that it could be safely cast to a signed integer.

What about:

#include <iostream>

int main ( void ) {
  unsigned long A = 30;
  unsigned long B = 20;
  unsigned long C = 100;
  long x = 0;
  while ( std::cin >> x ) {
    if ( x >= 0 ) {
      if ( x < B ) {
        std::cout << "range 2";
      } else if ( x < C ) {
        std::cout << "range 3";
      } else {
        std::cout << "above all ranges";
      }
    } else {
      if ( ( -x ) <= A ) {
        std::cout << "range 1";
      } else {
        std::cout << "below all ranges";
      }
    }
    std::cout << '\n';
  }
}

This only compares positive values. As long as the unsigned type is large
enough to represent the absolute values of the signed type, you will be
fine.

I thought of something like that, but two problems.

-x would overflow if x == std::numeric_limits<ptrdiff_t>::min()

Oops, you'r right. I forgot a cast.

"above all" and "below all" are the same outcome and I'd prefer not to
write the same code twice

The two cases are, of course, not the same. You just want them to be the
same. In that case, just use a goto :-)

So, the corrected code is:

#include <iostream>
#include <limits>

int main ( void ) {
  unsigned long A = 30;
  unsigned long B = 20;
  unsigned long C = 100;
  int x = 0;
  while ( std::cin >> x ) {
    if ( x >= 0 ) {
      if ( x < B ) {
        std::cout << "range 2";
      } else if ( x < C ) {
        std::cout << "range 3";
      } else {
      out_of_range:
        std::cout << "out_of_range";
      }
    } else {
      // we cast first so that unary - does not cause an overflow.
      if ( -static_cast<unsigned long>(x) <= A ) {
        std::cout << "range 1";
      } else {
        // we do not distinguish underflow and overflow:
        goto out_of_range;
      }
    }
    std::cout << '\n';
  }
}

Of course, there are ways to get around the goto, e.g., you could put the
common code into a function.

Best

Kai-Uwe Bux