bitwise operators

[77scrapper77]

hi,

I need to write a function that will check whether x is nonzero. Return 0
if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
<< >> and I am not allowed to use an loops.

My program so far looks like:

int isNonZero(int x) {


x = (1 << x) & x;
x = ~x;
x = x & 1;
return x;


}

All cases work except for the below.


Test isNonZero(-2147483648[0x80000000]) failed.
Gives 0[0x0]. Should be 1[0x1]

If tried everything I could think of and I am all out of ideas. Any one
have any idea how I could fix my code to deal with this negative number?


I also need to write one that will check whether x is nonzero using only ~
& ^ | + << >>

My code so far is:
int isLess(int x, int y) {

int a =0;

a = x ^ y;
a = y >> a;
a = a + 1;
a = !a;
return !a;
}

Which only works for about half the cases (on small non-negative numbers)


Any input would be appreciated.
Thanks,

[infobahn]

Re: bitwise operators

77scrapper77 wrote:[color=blue]
>
> I need to write a function that will check whether x is nonzero. Return 0
> if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
> << >> and I am not allowed to use an loops.
>
> My program so far looks like:
>
> int isNonZero(int x) {
>
> x = (1 << x) & x;[/color]

Undefined behaviour for most values of x. The Standard has this to
say about bit-shifting:

"If the value of the right operand is negative or is greater than
or equal to the width in bits of the promoted left operand, the
behavior is undefined."

Also, = is an operator, and it's not in your list of allowed operators.

Personally, I don't think it's possible in correct C with just the
operators that you are allowed to use (although I am ready to be
proved wrong).

[Michael Mair]

Re: bitwise operators

77scrapper77 wrote:[color=blue]
> hi,
>
> I need to write a function that will check whether x is nonzero. Return 0
> if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
> << >> and I am not allowed to use an loops.
>
> My program so far looks like:
>
> int isNonZero(int x) {[/color]
This function name invades the implementation' s namespace
as it starts with "is". Make that IsNonZero().[color=blue]
>
>
> x = (1 << x) & x;[/color]

Undefined behaviour, see infobahn's message.
[color=blue]
> x = ~x;
> x = x & 1;
> return x;
>
>
> }
>
> All cases work except for the below.[/color]

Nope: x=0 ... (~((1<<0) & 0)) & 1 == 1.
So your most important case does not work.

I do not see at all how you can do anything sensible for signed
integers as they could have negative zeros for 1s complement and
sign-magnitude representations . Seems to be a completely hopeless
task. Even for 2s complement only, I do not have the least clue
how to do it.
[color=blue]
> Test isNonZero(-2147483648[0x80000000]) failed.
> Gives 0[0x0]. Should be 1[0x1]
>
> If tried everything I could think of and I am all out of ideas. Any one
> have any idea how I could fix my code to deal with this negative number?[/color]

No. Not at all.

[color=blue]
> I also need to write one that will check whether x is nonzero using only ~[/color]

ITYM whether x<y
[color=blue]
> & ^ | + << >>
>
> My code so far is:
> int isLess(int x, int y) {[/color]

Again: The function name.
[color=blue]
> int a =0;
>
> a = x ^ y;
> a = y >> a;[/color]

You seem to be completely clueless what the shift operators are doing.
[color=blue]
> a = a + 1;
> a = !a;
> return !a;[/color]

! is an operator, too, and not on your list.
If it was not, your IsNonZero() function's body would consist of
{return !!x;}
[color=blue]
> }
>
> Which only works for about half the cases (on small non-negative numbers)
>
> Any input would be appreciated.[/color]

Are you allowed to use if, else, ...?
Either you are not telling us the whole truth or you should look
for someone different to give you C problems.


Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Ben Pfaff]

Re: bitwise operators

"77scrapper 77" <gregarious_gir l77@nospam.hotm ail.com> writes:
[color=blue]
> I need to write a function that will check whether x is nonzero. Return 0
> if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
> << >> and I am not allowed to use an loops.[/color]

int is_nonzero (int x)
{
switch (x) {
case 0:
return 0;
default:
return 1;
}
}
--
"Some programming practices beg for errors;
this one is like calling an 800 number
and having errors delivered to your door."
--Steve McConnell

[Old Wolf]

Re: bitwise operators

77scrapper77 wrote:[color=blue]
> hi,
>
> I need to write a function that will check whether x is nonzero.[/color]
Return 0[color=blue]
> if x is = to 0 and 1 otherwise. The open operators I can use are ~ &[/color]
^ | +[color=blue]
> << >> and I am not allowed to use an loops.
>[/color]

int isNonZero(int x)
{
if (x)
return 1;
return 0;
}

[77scrapper77]

Re: bitwise operators

I can't use any loops or if statements. I can use = though.

[Martin Johansen]

Re: bitwise operators

int isNonZero(int x) {
return(x || 0);
}

[77scrapper77]

Re: bitwise operators

I can't use || either, just |.

[Chris Croughton]

Re: bitwise operators

On Sun, 20 Feb 2005 19:55:08 +0100, Michael Mair
<Michael.Mair@i nvalid.invalid> wrote:
[color=blue]
> 77scrapper77 wrote:[color=green]
>> hi,
>>
>> I need to write a function that will check whether x is nonzero. Return 0
>> if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
>> << >> and I am not allowed to use an loops.
>>
>> My program so far looks like:
>>
>> int isNonZero(int x) {[/color]
> This function name invades the implementation' s namespace
> as it starts with "is". Make that IsNonZero().[/color]

Not true. From section 7.26 "Future Library Directions":

7.26.2 Character handling <ctype.h>

1 Function names that begin with either is or to, and a lowercase
letter may be added to the declarations in the <ctype.h> header.

It is allowed to start an identifier with is and an uppercase letter,
which is what he did. The same is true as far as I can see with most of
the other namespace restrictions (function names starting with str, mem
or wcs ans a lowercase letter are banned, ones with an uppercase letter
are permitted).
[color=blue]
> Are you allowed to use if, else, ...?
> Either you are not telling us the whole truth or you should look
> for someone different to give you C problems.[/color]

It looks like homework to me...

Chris C

[Ben Pfaff]

Re: bitwise operators

Chris Croughton <chris@keristor .net> writes:
[color=blue]
> <Michael.Mair@i nvalid.invalid> wrote:[color=green]
>> 77scrapper77 wrote:[color=darkred]
>>> int isNonZero(int x) {[/color]
>> This function name invades the implementation' s namespace
>> as it starts with "is". Make that IsNonZero().[/color]
>
> Not true. From section 7.26 "Future Library Directions":
>
> 7.26.2 Character handling <ctype.h>
>
> 1 Function names that begin with either is or to, and a lowercase
> letter may be added to the declarations in the <ctype.h> header.[/color]

However, C89 implementations are allowed to have a
case-insensitive linker.
--
int main(void){char p[]="ABCDEFGHIJKLM NOPQRSTUVWXYZab cdefghijklmnopq rstuvwxyz.\
\n",*q="kl BIcNBFr.NKEzjwC IxNJC";int i=sizeof p/2;char *strchr();int putchar(\
);while(*q){i+= strchr(p,*q++)-p;if(i>=(int)si zeof p)i-=sizeof p-1;putchar(p[i]\
);}return 0;}

[Clark S. Cox III]

Re: bitwise operators

On 2005-02-20 12:45:49 -0500, "77scrapper 77"
<gregarious_gir l77@nospam.hotm ail.com> said:
[color=blue]
> hi,
>
> I need to write a function that will check whether x is nonzero. Return 0
> if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
> << >> and I am not allowed to use an loops.[/color]

In C99:

int isNonZero(int x)
{
_Bool b = x;
return b;
}

--
Clark S. Cox, III
clarkcox3@gmail .com

[Spacen Jasset]

Re: bitwise operators

Michael Mair wrote:[color=blue]
> ! is an operator, too, and not on your list.
> If it was not, your IsNonZero() function's body would consist of
> {return !!x;}
>[color=green]
>> }[/color][/color]

This has it. That's the first thing I thought of. Any excuse to stack up
operators.

If you don't like that then, try return !!!!x; instead


It's best not to use tricks though, readability is paramount.

also, this will suffice, return x ? 1 : 0 ;

[77scrapper77]

Re: bitwise operators

I can't use the ! operator.

[roman ziak]

Re: bitwise operators

77scrapper77 wrote:[color=blue]
> hi,
>
> I need to write a function that will check whether x is nonzero. Return 0
> if x is = to 0 and 1 otherwise. The open operators I can use are ~ & ^ | +
> << >> and I am not allowed to use an loops.
>
> My program so far looks like:
>
> int isNonZero(int x) {
>
>
> x = (1 << x) & x;
> x = ~x;
> x = x & 1;
> return x;
>
>
> }[/color]

I am not sure if standard defines the case when you are shifting left by
more than the bit size of integer.

But how about:

N-1
---
[ \ (x >> i) ] & 1
/
---
i=0

where N is the bit size of word, so for 4 bit machine the expression is:

( x + (x>>1) + (x>>2) + (x>>4) ) & 1

[roman ziak]

Re: bitwise operators

> N-1[color=blue]
> ---
> [ \ (x >> i) ] & 1
> /
> ---
> i=0
>
> where N is the bit size of word, so for 4 bit machine the expression is:
>
> ( x + (x>>1) + (x>>2) + (x>>4) ) & 1[/color]

correction:

( x + (x>>1) + (x>>2) + (x>>3) ) & 1