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The number is odd?

P: n/a
Hi to all,
How can verify if a number is odd in C++?

Thank You and best Regards.
Gaetano

Nov 2 '06 #1
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28 Replies


P: n/a
nick048 <ni*************@moonsoft.itwrote:
Hi to all,
How can verify if a number is odd in C++?
If the number is a scalar, then you can do this:

bool odd = !!(number & 1);

david
Nov 2 '06 #2

P: n/a

nick048 wrote:
Hi to all,
How can verify if a number is odd in C++?

Thank You and best Regards.
Gaetano
You can do as David suggested - or prefer the less obscure (and at
least as fast)
number % 2 != 0

/Peter

Nov 2 '06 #3

P: n/a
Geo

david wrote:
nick048 <ni*************@moonsoft.itwrote:
Hi to all,
How can verify if a number is odd in C++?

If the number is a scalar, then you can do this:

bool odd = !!(number & 1);

david
Why !! ?

Nov 2 '06 #4

P: n/a
nick048 wrote:
Hi to all,
How can verify if a number is odd in C++?

Thank You and best Regards.
Gaetano
If an odd number is one that is ot divisible by two then you can just
divide that number by 2, take the remainder and see if it is 1 or 0;

Ben
Nov 2 '06 #5

P: n/a

Geo wrote:
david wrote:
nick048 <ni*************@moonsoft.itwrote:
Hi to all,
How can verify if a number is odd in C++?
If the number is a scalar, then you can do this:

bool odd = !!(number & 1);

david
Why !! ?
Typo/thinko I expect. Either that or there is some subtle reason not to
just do this:

bool odd( number & 1 );
K

Nov 2 '06 #6

P: n/a
Kirit S?lensminde <ki****************@gmail.comwrote:
Geo wrote:
>david wrote:
nick048 <ni*************@moonsoft.itwrote:
Hi to all,
How can verify if a number is odd in C++?

If the number is a scalar, then you can do this:

bool odd = !!(number & 1);

david
Why !! ?
(number & 1) is synonym to (number % 2), the remainder of the division by 2.

I was thinking the logic & operator would be at least as fast as % (modulo).

The !! is there to convert a scalar to a bool, but it may be unneeded,
especially in this case where the result is only 0 or 1.
Typo/thinko I expect. Either that or there is some subtle reason not to
just do this:
bool odd( number & 1 );
This is ok, but your reasons are unclear to me:
Is this preferred/cleaner/mandatory to initialize a bool ?
int x = 3; int x(3); aren't they identical ?
bool is not a basic type ?
Nov 2 '06 #7

P: n/a
david <de**@free.frwrites:
nick048 <ni*************@moonsoft.itwrote:
>Hi to all,
How can verify if a number is odd in C++?

If the number is a scalar, then you can do this:

bool odd = !!(number & 1);
AFAIK not portable. Will fail on systems with ones' complement when
number is negative (or -0). The valid way is:

bool odd = number % 2;
/* note that it can produce, -1, 0 or 1 */

If you want a value 0 or 1 use either:

int odd = !!(number % 2);

or:

int odd = number % 2 != 0;

On systems with twos' complement compiler will probably optimize those
codes to: number & 1.

--
Best regards, _ _
.o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michal "mina86" Nazarewicz (o o)
ooo +--<mina86*tlen.pl>---<jid:mina86*chrome.pl>--ooO--(_)--Ooo--
Nov 2 '06 #8

P: n/a
Michal Nazarewicz <mi****@tlen.plwrote:
> bool odd = !!(number & 1);
AFAIK not portable. Will fail on systems with ones' complement when
number is negative (or -0).
Right. I was not aware that this could exist.

david
Nov 2 '06 #9

P: n/a
Kirit Sælensminde:
>Why !! ?

!!a

is short-hand for:

(bool)a

Typo/thinko I expect. Either that or there is some subtle reason not to
just do this:

bool odd( number & 1 );

Because it's dressed up to look like you're passing an argument to a
constructor. Intrinisc types are distinct from user-defined types... don't
lump them into the same category and come out with bastardisations such as:

int i(1);

, it just looks stupid, plus it makes one have to consider the "if it looks
like a declaration" rule.

--

Frederick Gotham
Nov 2 '06 #10

P: n/a
Geo

Frederick Gotham wrote:
Kirit Sælensminde:
Why !! ?


!!a

is short-hand for:

(bool)a
Is it really?
Where does it say that ?
What's the point any way since implicit conversion to bool is going to
happen for the assignement in the same way that it would happen for the
'!' !!!.
--

Frederick Gotham
Nov 2 '06 #11

P: n/a
Geo:
> !!a

is short-hand for:

(bool)a
Is it really?
Where does it say that ?

! converts its value to bool, then inverts it. The second ! will invert it
again. It has the overall effect of:

(bool)a

What's the point any way since implicit conversion to bool is going to
happen for the assignement in the same way that it would happen for the
'!' !!!.

You're right. I've only ever seen it used to suppress compiler warnings.
The following gives a warning on many compilers:

bool b = 5 - 3;

--

Frederick Gotham
Nov 2 '06 #12

P: n/a
Frederick Gotham:
! converts its value to bool, then inverts it. The second ! will invert it
again. It has the overall effect of:

(bool)a

If talking about intrinsic types, of course.

--

Frederick Gotham
Nov 2 '06 #13

P: n/a
Geo

Frederick Gotham wrote:
Geo:
!!a

is short-hand for:

(bool)a
Is it really?
Where does it say that ?


! converts its value to bool, then inverts it. The second ! will invert it
again. It has the overall effect of:

(bool)a

What's the point any way since implicit conversion to bool is going to
happen for the assignement in the same way that it would happen for the
'!' !!!.
I was questioning how it worked, just why it was better/clearer than
(bool) or static_cast<bool>() ?
>
You're right. I've only ever seen it used to suppress compiler warnings.
The following gives a warning on many compilers:

bool b = 5 - 3;
Why would it warn about that but not !(5-3) ?
>
--

Frederick Gotham
Nov 2 '06 #14

P: n/a
Geo wrote:
Frederick Gotham wrote:
>Geo:
>>> !!a

is short-hand for:

(bool)a

Is it really?
Where does it say that ?


! converts its value to bool, then inverts it. The second ! will
invert it again. It has the overall effect of:

(bool)a

>>What's the point any way since implicit conversion to bool is going
to happen for the assignement in the same way that it would happen
for the '!' !!!.

I was questioning how it worked, just why it was better/clearer than
(bool) or static_cast<bool>() ?
Because it saves you typing. Because it looks cool. Because when you
refer to it in a conversation with a colleague you can say "bang-bang-ay"
instead of "ay-cast-2-bool" or some such. Bang! Bang! Isn't it cool?

(Oh, that's why it is "better". Clearer? Of course not. But who cares
when it's so cool?)

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
Nov 2 '06 #15

P: n/a
Geo

Victor Bazarov wrote:
Geo wrote:
Frederick Gotham wrote:
Geo:

!!a

is short-hand for:

(bool)a

Is it really?
Where does it say that ?
! converts its value to bool, then inverts it. The second ! will
invert it again. It has the overall effect of:

(bool)a
What's the point any way since implicit conversion to bool is going
to happen for the assignement in the same way that it would happen
for the '!' !!!.
I was questioning how it worked, just why it was better/clearer than
(bool) or static_cast<bool>() ?

Because it saves you typing. Because it looks cool. Because when you
refer to it in a conversation with a colleague you can say "bang-bang-ay"
instead of "ay-cast-2-bool" or some such. Bang! Bang! Isn't it cool?

(Oh, that's why it is "better". Clearer? Of course not. But who cares
when it's so cool?)
I guess, and I suppose I might use it, just for the coolness, but I've
never met anyone who calls '!' bang.... or '#' pound... but let's not
go there :)
>
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
Nov 2 '06 #16

P: n/a
Geo <gg@remm.orgwrote:
never met anyone who calls [...] '#' pound... but let's not
go there :)
Actually, we already did :)
http://groups.google.com/group/comp....092e9d325aa4f/

and even before I started reading this group:
http://groups.google.com/group/comp....7c712f09983b7/

--
Marcus Kwok
Replace 'invalid' with 'net' to reply
Nov 2 '06 #17

P: n/a
Geo wrote:
>(Oh, that's why it is "better". Clearer? Of course not. But who cares
when it's so cool?)

I guess, and I suppose I might use it, just for the coolness, but I've
never met anyone who calls '!' bang.... or '#' pound... but let's not
go there :)
I prefer the Intercal nomenclature for chars. For example " is called rabbit
ears.

For those that does not known Intercal, is one of the first languages that
completely avoided the need for a GO TO instruction. It uses COME FROM
instead.

It's a nice language, but many people write coolest code using Standard C++.

--
Salu2
Nov 2 '06 #18

P: n/a
Geo wrote:
>
I guess, and I suppose I might use it, just for the coolness, but I've
never met anyone who calls '!' bang.... or '#' pound... but let's not
go there :)
Are you serious? What do you call # if not "pound"? I suppose you
could call it "sharp", but among people I know (not C# programmers)
everyone calls it "pound". Is this a geographical thing? (I'm in the U.S.)
Nov 3 '06 #19

P: n/a
david wrote:
>
(number & 1) is synonym to (number % 2), the remainder of the division by 2.
If the number is negative, then (number & 1) might give
the wrong result.
I was thinking the logic & operator would be at least as fast as % (modulo).
The compiler will select the fastest operation, you don't
need to worry. Even the worst compilers will get this right.
The !! is there to convert a scalar to a bool, but it may be unneeded,
especially in this case where the result is only 0 or 1.
Other integral types (and pointer types, for that matter), can
be implicitly converted to bool. You do not need to explicitly
perform the conversion.
bool is not a basic type ?
Actually it is.

Nov 3 '06 #20

P: n/a
Mark P wrote:
Geo wrote:

I guess, and I suppose I might use it, just for the coolness, but I've
never met anyone who calls '!' bang.... or '#' pound... but let's not
go there :)

Are you serious? What do you call # if not "pound"? I suppose you
could call it "sharp", but among people I know (not C# programmers)
everyone calls it "pound". Is this a geographical thing? (I'm in the U.S.)
Everyone I know calls it "hash". This is particularly
appropriate for describing the language "C hash" too.

Nov 3 '06 #21

P: n/a
Mark P wrote:
Geo wrote:
>>
I guess, and I suppose I might use it, just for the coolness, but I've
never met anyone who calls '!' bang.... or '#' pound... but let's not
go there :)

Are you serious? What do you call # if not "pound"? I suppose you
could call it "sharp", but among people I know (not C# programmers)
everyone calls it "pound". Is this a geographical thing? (I'm in the
U.S.)
You obviously haven't used a UK keyboard, where shift3 is the £ (GBP)
symbol. # is hash.

--
Ian Collins.
Nov 3 '06 #22

P: n/a
Old Wolf wrote:
Mark P wrote:
>Geo wrote:
>>I guess, and I suppose I might use it, just for the coolness, but I've
never met anyone who calls '!' bang.... or '#' pound... but let's not
go there :)
Are you serious? What do you call # if not "pound"? I suppose you
could call it "sharp", but among people I know (not C# programmers)
everyone calls it "pound". Is this a geographical thing? (I'm in the U.S.)

Everyone I know calls it "hash". This is particularly
appropriate for describing the language "C hash" too.
But what about the language C tic-tac-toe-board?

Where in the world are you located?
Nov 3 '06 #23

P: n/a
Mark P wrote:
Old Wolf wrote:
>Mark P wrote:
>>Geo wrote:
I guess, and I suppose I might use it, just for the coolness, but
I've never met anyone who calls '!' bang.... or '#' pound... but
let's not go there :)

Are you serious? What do you call # if not "pound"? I suppose you
could call it "sharp", but among people I know (not C# programmers)
everyone calls it "pound". Is this a geographical thing? (I'm in
the U.S.)

Everyone I know calls it "hash". This is particularly
appropriate for describing the language "C hash" too.

But what about the language C tic-tac-toe-board?

Where in the world are you located?
It's a view on the world from a gaol cell. The language is
"See, gaoled!"

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
Nov 3 '06 #24

P: n/a

david wrote:
Kirit S?lensminde <ki****************@gmail.comwrote:
Geo wrote:
david wrote:
nick048 <ni*************@moonsoft.itwrote:
Hi to all,
How can verify if a number is odd in C++?

If the number is a scalar, then you can do this:

bool odd = !!(number & 1);

david
Why !! ?

(number & 1) is synonym to (number % 2), the remainder of the division by 2.

I was thinking the logic & operator would be at least as fast as % (modulo).

The !! is there to convert a scalar to a bool, but it may be unneeded,
especially in this case where the result is only 0 or 1.
Typo/thinko I expect. Either that or there is some subtle reason not to
just do this:
bool odd( number & 1 );

This is ok, but your reasons are unclear to me:
Is this preferred/cleaner/mandatory to initialize a bool ?
int x = 3; int x(3); aren't they identical ?
bool is not a basic type ?
They are identical. It was the lack of the double '!' that I was
referring to.

bool odd = ( number & 1 );
K

Nov 3 '06 #25

P: n/a

Frederick Gotham wrote:
Kirit Sælensminde:
Why !! ?


!!a

is short-hand for:

(bool)a

Typo/thinko I expect. Either that or there is some subtle reason not to
just do this:

bool odd( number & 1 );


Because it's dressed up to look like you're passing an argument to a
constructor. Intrinisc types are distinct from user-defined types... don't
lump them into the same category and come out with bastardisations such as:

int i(1);

, it just looks stupid, plus it makes one have to consider the "if it looks
like a declaration" rule.
I'm not sure that I agree with you on that. The important distinction
between the types we use is whether they follow object or value
semantics, not whether they happen to be in the sub-set of types that
happen to be defined by keywords in the standard.

std::string s( "Hello" );

Presumably you consider the above OK? It invites equal consideration to
the "if it looks like a declaration" rule.
K

Nov 3 '06 #26

P: n/a
On Thu, 2 Nov 2006 10:43:57 +0000 (UTC), david <de**@free.frwrote in
comp.lang.c++:
Kirit S?lensminde <ki****************@gmail.comwrote:
Geo wrote:
david wrote:
nick048 <ni*************@moonsoft.itwrote:
Hi to all,
How can verify if a number is odd in C++?

If the number is a scalar, then you can do this:

bool odd = !!(number & 1);

david
Why !! ?

(number & 1) is synonym to (number % 2), the remainder of the division by 2.
No, it is not. Not for integer types with negative values on 1's
complement systems, which the C++ language specifically supports.

--
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/~a...FAQ-acllc.html
Nov 3 '06 #27

P: n/a
>>"Geo" <gg@remm.orgwrites:
>>>Why !! ?
Frederick Gotham wrote:
>>
!!a

is short-hand for:

(bool)a
"Geo" <gg@remm.orgwrites:
What's the point any way since implicit conversion to bool is going to
happen for the assignement in the same way that it would happen for the
'!' !!!.
In this particular reason it will but consider a code:

#v+
int arr[10] = { /* some initial values */ };
int num = 0;
for (int i = 0; i<10; ++i) num += arr[i] % 2;
#v-

counting how many odd numbers there are in arr. It'll fail since
arr[i]%2 may return -1 so here you have to cast it, use !! or ?: and
!! is the shortest form.

--
Best regards, _ _
.o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o
..o | Computer Science, Michal "mina86" Nazarewicz (o o)
ooo +--<mina86*tlen.pl>---<jid:mina86*chrome.pl>--ooO--(_)--Ooo--
Nov 3 '06 #28

P: n/a
Kirit Sælensminde:
std::string s( "Hello" );

Presumably you consider the above OK?

I use the form for two reasons:

(1) It looks like a function call/constructor call.
(2) It doesn't require a copy-construct as does the other form.

--

Frederick Gotham
Nov 3 '06 #29

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