# Subtracting unsigned ints

Discussion in 'C Programming' started by bg_ie@yahoo.com, Jun 20, 2006.

1. ### Guest

Hi,

I have a function which compares two unsigned ints and returns their
difference in order to establish which is the greater of the two.

int Compare(unsigned int time_left, unsigned int time_right)
{
return (time_left - time_right);
}

usage -

if( Compare(time_left, time_right) > 0)
// the time has come.

The problem is that when time_left is 5 and time_right is 10, I get -5,
but if time_left is 5 and time_right is 0xc0000005 (a number greater
than 5) I get a positive value when I wish to get a negitive one.

I understand why I get this positive value - because the negaitve bit
is set in time_right, but how can I change my code so that I get the
desired results.

Barry.

, Jun 20, 2006

2. ### Ian CollinsGuest

wrote:
> Hi,
>
> I have a function which compares two unsigned ints and returns their
> difference in order to establish which is the greater of the two.
>
> int Compare(unsigned int time_left, unsigned int time_right)
> {
> return (time_left - time_right);
> }
>
> usage -
>
> if( Compare(time_left, time_right) > 0)
> // the time has come.
>
> The problem is that when time_left is 5 and time_right is 10, I get -5,
> but if time_left is 5 and time_right is 0xc0000005 (a number greater
> than 5) I get a positive value when I wish to get a negitive one.
>
> I understand why I get this positive value - because the negaitve bit
> is set in time_right, but how can I change my code so that I get the
> desired results.
>

If the exact difference doesn't matter;

return (time_left > time_right);

or

return ((time_left > time_right) ? 1 : 0);

--
Ian Collins.

Ian Collins, Jun 20, 2006

3. ### Peter NilssonGuest

wrote:
> Hi,
>
> I have a function which compares two unsigned ints and returns their
> difference in order to establish which is the greater of the two.

You can't subtract two unsigned ints and yield a negative number.

> int Compare(unsigned int time_left, unsigned int time_right)
> {
> return (time_left - time_right);
> }
>
> usage -
>
> if( Compare(time_left, time_right) > 0)
> // the time has come.
>
> The problem is that when time_left is 5 and time_right is 10, I get -5,
> but if time_left is 5 and time_right is 0xc0000005 (a number greater
> than 5) I get a positive value when I wish to get a negitive one.

Try...

if (time_left < time_right) return -1;
if (time_left > time_right) return +1;
return 0;

Or...

return (time_left < time_right) ? -1 : (time_left > time_right);

Or...

return (time_left > time_right) - (time_left < time_right);

>
> I understand why I get this positive value - because the negaitve bit
> is set in time_right, ...

No, time_right is an unsigned int. There is _no_ negative (sign) bit.

The problem is that arithmetic on unsigned ints is performed
modulo one more than UINT_MAX.

--
Peter

Peter Nilsson, Jun 20, 2006
4. ### NeluGuest

writes:

> Hi,
>
> I have a function which compares two unsigned ints and returns their
> difference in order to establish which is the greater of the two.
>
> int Compare(unsigned int time_left, unsigned int time_right)
> {
> return (time_left - time_right);
> }
>
> usage -
>
> if( Compare(time_left, time_right) > 0)
> // the time has come.
>
> The problem is that when time_left is 5 and time_right is 10, I get -5,
> but if time_left is 5 and time_right is 0xc0000005 (a number greater
> than 5) I get a positive value when I wish to get a negitive one.
>
> I understand why I get this positive value - because the negaitve bit
> is set in time_right, but how can I change my code so that I get the
> desired results.
>

It doesn't matter what you set. It's supposed to be an unsigned int so
it gets promoted to something higher than int (assuming that int has 4
bytes) to be able to store the positive value. They probably get
promoted to long so you have a difference between two long
arguments. But, you are returning int even if the unsigned value
doesn't fit in an int.

--
Ioan - Ciprian Tandau
tandau _at_ freeshell _dot_ org (hope it's not too late)
(... and that it still works...)

Nelu, Jun 20, 2006
5. ### Eric SosmanGuest

Nelu wrote:

> writes:
>
>
>>Hi,
>>
>>I have a function which compares two unsigned ints and returns their
>>difference in order to establish which is the greater of the two.
>>
>>int Compare(unsigned int time_left, unsigned int time_right)
>>{
>> return (time_left - time_right);
>>}
>>
>>usage -
>>
>>if( Compare(time_left, time_right) > 0)
>> // the time has come.
>>
>>The problem is that when time_left is 5 and time_right is 10, I get -5,
>>but if time_left is 5 and time_right is 0xc0000005 (a number greater
>>than 5) I get a positive value when I wish to get a negitive one.
>>
>>I understand why I get this positive value - because the negaitve bit
>>is set in time_right, but how can I change my code so that I get the
>>desired results.
>>

>
>
> It doesn't matter what you set. It's supposed to be an unsigned int so
> it gets promoted to something higher than int (assuming that int has 4
> bytes) to be able to store the positive value. They probably get
> promoted to long so you have a difference between two long
> arguments. But, you are returning int even if the unsigned value
> doesn't fit in an int.

You were doing fine until the second sentence, which is
wrong and misleading: Wrong because unsigned int is not "promotable"
at all, and misleading because the four-byte int is a red herring
that has nothing to do with anything. The third sentence, since
its premise is a falsehood, is simply meaningless.

Your final sentence, though, correctly identifies the crux
of the problem. Others have offered solutions.

--
Eric Sosman
lid

Eric Sosman, Jun 20, 2006
6. ### peteGuest

Nelu wrote:
>
> writes:
>
> > Hi,
> >
> > I have a function which compares two unsigned ints and returns their
> > difference in order to establish which is the greater of the two.
> >
> > int Compare(unsigned int time_left, unsigned int time_right)
> > {
> > return (time_left - time_right);
> > }
> >
> > usage -
> >
> > if( Compare(time_left, time_right) > 0)
> > // the time has come.
> >
> > The problem is that when time_left is 5 and time_right is 10,
> > I get -5,
> > but if time_left is 5 and time_right is 0xc0000005 (a number greater
> > than 5) I get a positive value when I wish to get a negitive one.
> >
> > I understand why I get this positive value
> > - because the negaitve bit
> > is set in time_right, but how can I change my code so that I get the
> > desired results.
> >

>
> It doesn't matter what you set. It's supposed to be an unsigned int so
> it gets promoted to something higher than int (assuming that int has 4
> bytes) to be able to store the positive value. They probably get
> promoted to long so you have a difference between two long
> arguments. But, you are returning int even if the unsigned value
> doesn't fit in an int.

I don't understand what you mean.
There's nothing getting promoted in the shown code.
The type of (time_left - time_right) is unsigned int.
The only conversion in the shown code
is that the return value of the function,
is (time_left - time_right) converted to type int,
which is implementation defined
if (time_left - time_right) is greater than INT_MAX.

--
pete

pete, Jun 20, 2006
7. ### Jack KleinGuest

On 19 Jun 2006 18:06:25 -0700, wrote in comp.lang.c:

> Hi,
>
> I have a function which compares two unsigned ints and returns their
> difference in order to establish which is the greater of the two.
>
> int Compare(unsigned int time_left, unsigned int time_right)
> {
> return (time_left - time_right);
> }
>
> usage -
>
> if( Compare(time_left, time_right) > 0)
> // the time has come.
>
> The problem is that when time_left is 5 and time_right is 10, I get -5,
> but if time_left is 5 and time_right is 0xc0000005 (a number greater
> than 5) I get a positive value when I wish to get a negitive one.
>
> I understand why I get this positive value - because the negaitve bit
> is set in time_right, but how can I change my code so that I get the
> desired results.
>

What happens if the difference is has greater magnitude that a signed
int can hold? If one of the values is UINT_MAX and the other is 0,
the difference between them is either positive or negative UINT_MAX,
and that won't fit in a signed int on any platform you are ever likely

Still assuming it fits (untested):

int Compare(unsigned int time_left, unsigned int time_right)
{
int result;

if (time_left >= time_right)
{
result = time_left - time_right;
}
else
{
result = -(int)(time_right - time_left);
}
return result;
}

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://c-faq.com/
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

Jack Klein, Jun 20, 2006
8. ### NeluGuest

Eric Sosman <> writes:

> Nelu wrote:
>
> > writes:
> >
> >>Hi,
> >>
> >>I have a function which compares two unsigned ints and returns their
> >>difference in order to establish which is the greater of the two.
> >>
> >>int Compare(unsigned int time_left, unsigned int time_right)
> >>{
> >> return (time_left - time_right);
> >>}
> >>
> >>usage -
> >>
> >>if( Compare(time_left, time_right) > 0)
> >> // the time has come.
> >>
> >>The problem is that when time_left is 5 and time_right is 10, I get -5,
> >>but if time_left is 5 and time_right is 0xc0000005 (a number greater
> >>than 5) I get a positive value when I wish to get a negitive one.
> >>
> >>I understand why I get this positive value - because the negaitve bit
> >>is set in time_right, but how can I change my code so that I get the
> >>desired results.
> >>

> > It doesn't matter what you set. It's supposed to be an unsigned int
> > so
> > it gets promoted to something higher than int (assuming that int has 4
> > bytes) to be able to store the positive value. They probably get
> > promoted to long so you have a difference between two long
> > arguments. But, you are returning int even if the unsigned value
> > doesn't fit in an int.

>
> You were doing fine until the second sentence, which is
> wrong and misleading: Wrong because unsigned int is not "promotable"
> at all,
> and misleading because the four-byte int is a red herring
> that has nothing to do with anything. The third sentence, since

The 4 byte thing was a reference to the hex value and the explanation
the OP provided.
I got the promotion thing wrong. I have re-read the standard. I
should've taken another look before posting. Sorry about that.

> its premise is a falsehood, is simply meaningless.

Yes it is.

--
Ioan - Ciprian Tandau
tandau _at_ freeshell _dot_ org (hope it's not too late)
(... and that it still works...)

Nelu, Jun 20, 2006
9. ### CBFalconerGuest

wrote:
>
> I have a function which compares two unsigned ints and returns
> their difference in order to establish which is the greater of
> the two.
>
> int Compare(unsigned int time_left, unsigned int time_right)
> {
> return (time_left - time_right);
> }
>
> usage -
>
> if( Compare(time_left, time_right) > 0)
> // the time has come.
>
> The problem is that when time_left is 5 and time_right is 10, I
> get -5, but if time_left is 5 and time_right is 0xc0000005 (a
> number greater than 5) I get a positive value when I wish to get
> a negitive one.
>
> I understand why I get this positive value - because the negaitve
> bit is set in time_right, but how can I change my code so that I
> get the desired results.

By avoiding overflows and undefined behaviour. Forget 'sign bits'.

int compare(unsigned int left, unsigned int right)
{
return (left < right) - (left > right);
}

--
"I don't know where bin Laden is. I have no idea and really
don't care. It's not that important." - G.W. Bush, 2002-03-13
with September the 11th." - George Walker Bush 2003-09-17

CBFalconer, Jun 20, 2006
10. ### Peter NilssonGuest

Eric Sosman wrote:
> Nelu wrote:
> > writes:
> > > int Compare(unsigned int time_left, unsigned int time_right)
> > > {
> > > return (time_left - time_right);
> > > }

> >
> > ...
> > It doesn't matter what you set. It's supposed to be an unsigned int so
> > it gets promoted to something higher than int (assuming that int has 4
> > bytes) to be able to store the positive value. They probably get
> > promoted to long so you have a difference between two long
> > arguments. But, you are returning int even if the unsigned value
> > doesn't fit in an int.

>
> You were doing fine until the second sentence, which is
> wrong and misleading: Wrong because unsigned int is not "promotable"
> at all, ...

To clarify, although not subject to integral promotion, an unsigned int
operand is potentially subject to arithmetic conversion. However, in
the
case at hand, arithmetic conversion is not applicable since both
operands
are unsigned int.

--
Peter

Peter Nilsson, Jun 20, 2006
11. ### Eric SosmanGuest

CBFalconer wrote:

> wrote:
>
>>I have a function which compares two unsigned ints and returns
>>their difference in order to establish which is the greater of
>>the two.
>>
>>int Compare(unsigned int time_left, unsigned int time_right)
>>{
>> return (time_left - time_right);
>>}
>>
>>usage -
>>
>>if( Compare(time_left, time_right) > 0)
>> // the time has come.
>>
>>The problem is that when time_left is 5 and time_right is 10, I
>>get -5, but if time_left is 5 and time_right is 0xc0000005 (a
>>number greater than 5) I get a positive value when I wish to get
>>a negitive one.
>>
>>I understand why I get this positive value - because the negaitve
>>bit is set in time_right, but how can I change my code so that I
>>get the desired results.

>
>
> By avoiding overflows and undefined behaviour. Forget 'sign bits'.
>
> int compare(unsigned int left, unsigned int right)
> {
> return (left < right) - (left > right);
> }

Aside: I think this is backwards from the O.P.'s intent.
Either exchange the operands of `-' or interchange `<' and `>'.
(Suggestion: Don't make *both* changes. ;-)

--
Eric Sosman
lid

Eric Sosman, Jun 20, 2006