odd/even bitwise and

[Serve Laurijssen]

Some people prefer to use
"if (x & 1)"
to see if a number is odd or even. Is this completely portable according to
the standard?

[Ahmed S. Badran]

Re: odd/even bitwise and


"Serve Laurijssen" <cs@nospam.comp .com> wrote in message
news:eJucc.5394 $RU5.83970@zonn et-reader-1...[color=blue]
> "if (x & 1)"
> to see if a number is odd or even. Is this completely portable according[/color]
to[color=blue]
> the standard?[/color]

I don't know what does this have to do with the standard, but the thing is
an odd number will ALWAYS have bit 0 set to '1' and an even number will
always have bit 0 set to '0'. This is a matter of binary representation.

Ahmed

[Ahmed S. Badran]

Re: odd/even bitwise and


"Serve Laurijssen" <cs@nospam.comp .com> wrote in message
news:eJucc.5394 $RU5.83970@zonn et-reader-1...[color=blue]
> "if (x & 1)"
> to see if a number is odd or even. Is this completely portable according[/color]
to[color=blue]
> the standard?[/color]

I don't know what does this have to do with the standard, but the thing is
an odd number will ALWAYS have bit 0 set to '1' and an even number will
always have bit 0 set to '0'. This is a matter of binary representation.

Ahmed

[CBFalconer]

Re: odd/even bitwise and

Serve Laurijssen wrote:[color=blue]
>
> Some people prefer to use
> "if (x & 1)"
> to see if a number is odd or even. Is this completely portable
> according to the standard?[/color]

Only if x is some form of unsigned integer.

--
Chuck F (cbfalconer@yah oo.com) (cbfalconer@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[CBFalconer]

Re: odd/even bitwise and

Serve Laurijssen wrote:[color=blue]
>
> Some people prefer to use
> "if (x & 1)"
> to see if a number is odd or even. Is this completely portable
> according to the standard?[/color]

Only if x is some form of unsigned integer.

--
Chuck F (cbfalconer@yah oo.com) (cbfalconer@wor ldnet.att.net)
Available for consulting/temporary embedded and systems.
<http://cbfalconer.home .att.net> USE worldnet address!

[pete]

Re: odd/even bitwise and

Serve Laurijssen wrote:[color=blue]
>
> Some people prefer to use
> "if (x & 1)"
> to see if a number is odd or even.
> Is this completely portable according to the standard?[/color]

If x is an unsigned integer type
or a signed type with a non negative value, then it's portable.

--
pete

[pete]

Re: odd/even bitwise and

Serve Laurijssen wrote:[color=blue]
>
> Some people prefer to use
> "if (x & 1)"
> to see if a number is odd or even.
> Is this completely portable according to the standard?[/color]

If x is an unsigned integer type
or a signed type with a non negative value, then it's portable.

--
pete

[pete]

Re: odd/even bitwise and

Ahmed S. Badran wrote:[color=blue]
>
> "Serve Laurijssen" <cs@nospam.comp .com> wrote in message
> news:eJucc.5394 $RU5.83970@zonn et-reader-1...[color=green]
> > "if (x & 1)"
> > to see if a number is odd or even.
> > Is this completely portable according to the standard?[/color]
>
> I don't know what does this have to do with the standard,[/color]

The answer to the question, is in the standard.
[color=blue]
> but the thing is an odd number will ALWAYS have bit 0 set to
> '1' and an even number will always have bit 0 set to '0'.[/color]

That's not true for one's complement
representations of negative integers.
[color=blue]
> This is a matter of binary representation.[/color]

The standard specifies more than one
way to represent negative integers.

--
pete

[pete]

Re: odd/even bitwise and

Ahmed S. Badran wrote:[color=blue]
>
> "Serve Laurijssen" <cs@nospam.comp .com> wrote in message
> news:eJucc.5394 $RU5.83970@zonn et-reader-1...[color=green]
> > "if (x & 1)"
> > to see if a number is odd or even.
> > Is this completely portable according to the standard?[/color]
>
> I don't know what does this have to do with the standard,[/color]

The answer to the question, is in the standard.
[color=blue]
> but the thing is an odd number will ALWAYS have bit 0 set to
> '1' and an even number will always have bit 0 set to '0'.[/color]

That's not true for one's complement
representations of negative integers.
[color=blue]
> This is a matter of binary representation.[/color]

The standard specifies more than one
way to represent negative integers.

--
pete

[osmium]

Re: odd/even bitwise and

Ahmed S. Badran writes:
[color=blue][color=green]
> > "if (x & 1)"
> > to see if a number is odd or even. Is this completely portable according[/color]
> to[color=green]
> > the standard?[/color]
>
> I don't know what does this have to do with the standard, but the thing is
> an odd number will ALWAYS have bit 0 set to '1' and an even number will
> always have bit 0 set to '0'. This is a matter of binary representation.[/color]

It depends on the hardware. If x is a one's complement representation of 0
the test might fail. It strikes me as code intended to impress someone with
one's erudition, I would not do it.

[osmium]

Re: odd/even bitwise and

Ahmed S. Badran writes:
[color=blue][color=green]
> > "if (x & 1)"
> > to see if a number is odd or even. Is this completely portable according[/color]
> to[color=green]
> > the standard?[/color]
>
> I don't know what does this have to do with the standard, but the thing is
> an odd number will ALWAYS have bit 0 set to '1' and an even number will
> always have bit 0 set to '0'. This is a matter of binary representation.[/color]

It depends on the hardware. If x is a one's complement representation of 0
the test might fail. It strikes me as code intended to impress someone with
one's erudition, I would not do it.

[Ahmed S. Badran]

Re: odd/even bitwise and

pete wrote:[color=blue]
> Ahmed S. Badran wrote:[color=green]
>>
>> "Serve Laurijssen" <cs@nospam.comp .com> wrote in message
>> news:eJucc.5394 $RU5.83970@zonn et-reader-1...[color=darkred]
>>> "if (x & 1)"
>>> to see if a number is odd or even.
>>> Is this completely portable according to the standard?[/color]
>>
>> I don't know what does this have to do with the standard,[/color]
>
> The answer to the question, is in the standard.
>[color=green]
>> but the thing is an odd number will ALWAYS have bit 0 set to
>> '1' and an even number will always have bit 0 set to '0'.[/color]
>
> That's not true for one's complement
> representations of negative integers.
>[color=green]
>> This is a matter of binary representation.[/color]
>
> The standard specifies more than one
> way to represent negative integers.[/color]
Ok, just to re-elaborate and make the answer complete, I never took negative
numbers into consideration with my previous answer, so my previous answer is
valid/correct with all positive integers. Thanks for pointing that out guys.

Ahmed

[Ahmed S. Badran]

Re: odd/even bitwise and

pete wrote:[color=blue]
> Ahmed S. Badran wrote:[color=green]
>>
>> "Serve Laurijssen" <cs@nospam.comp .com> wrote in message
>> news:eJucc.5394 $RU5.83970@zonn et-reader-1...[color=darkred]
>>> "if (x & 1)"
>>> to see if a number is odd or even.
>>> Is this completely portable according to the standard?[/color]
>>
>> I don't know what does this have to do with the standard,[/color]
>
> The answer to the question, is in the standard.
>[color=green]
>> but the thing is an odd number will ALWAYS have bit 0 set to
>> '1' and an even number will always have bit 0 set to '0'.[/color]
>
> That's not true for one's complement
> representations of negative integers.
>[color=green]
>> This is a matter of binary representation.[/color]
>
> The standard specifies more than one
> way to represent negative integers.[/color]
Ok, just to re-elaborate and make the answer complete, I never took negative
numbers into consideration with my previous answer, so my previous answer is
valid/correct with all positive integers. Thanks for pointing that out guys.

Ahmed

[Rogério Brito]

Re: odd/even bitwise and

Serve Laurijssen wrote:[color=blue]
> Some people prefer to use
> "if (x & 1)"
> to see if a number is odd or even. Is this completely portable according to
> the standard?[/color]

No. That is only possible if the representation of the integers use
twos-complement.

To see if a number is odd or even, use the modulus operator (e.g., x%2).

--
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Rogério Brito - rbrito@ime.usp. br - http://www.ime.usp.br/~rbrito
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

[Rogério Brito]

Re: odd/even bitwise and

Serve Laurijssen wrote:[color=blue]
> Some people prefer to use
> "if (x & 1)"
> to see if a number is odd or even. Is this completely portable according to
> the standard?[/color]

No. That is only possible if the representation of the integers use
twos-complement.

To see if a number is odd or even, use the modulus operator (e.g., x%2).

--
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Rogério Brito - rbrito@ime.usp. br - http://www.ime.usp.br/~rbrito
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=