unsigned integer?

[Jack]

This is a naive question:

"%u" % -3

I expect it to print 3. But it still print -3.

Also, if I have an int, I can convert it to unsigned int in C:
int i = -3;
int ui = (unsigned int)i;

Is there a way to do this in Python?

[Dan Bishop]

Re: unsigned integer?

On Mar 10, 11:32 am, "Jack" <nos...@invalid .comwrote:

This is a naive question:
>
"%u" % -3
>
I expect it to print 3. But it still print -3.
>
Also, if I have an int, I can convert it to unsigned int in C:
int i = -3;
int ui = (unsigned int)i;
>
Is there a way to do this in Python?

def unsigned(n):
return n & 0xFFFFFFFF

[Duncan Booth]

Re: unsigned integer?

"Jack" <nospam@invalid .comwrote:

This is a naive question:
>
"%u" % -3
>
I expect it to print 3. But it still print -3.

Internally it uses the C runtime to format the number, but if the number
you ask it to print unsigned is negative it uses %d instead of %u. I have
no idea if it is actually possibly to get a different output for %d versus
%u.

>
Also, if I have an int, I can convert it to unsigned int in C:
int i = -3;
int ui = (unsigned int)i;
>
Is there a way to do this in Python?
>

Depeneding on how exactly you want it converted:

i = -3
ui = abs(i)
print ui
ui = (i & 0xffff) # for 16 bit integers
print ui
ui = (i & 0xffffffff) # for 32 bit integers
print ui
ui = (i & 0xfffffffffffff fff) # for 64 bit integers
print ui
ui = (i & 0xfffffffffffff fffffffffffffff ffff) # for 128 bit integers
print ui

which gives the following output:

3
65533
4294967293
184467440737095 51613
340282366920938 463463374607431 768211453

There isn't a unique way to convert a Python integer to an unsigned value
which is why the %u format string cannot do anything other than print the
value. Personally I'd have expected the Python one to either print the
absolute value or throw an exception, but I guess making it an alias for %d
kind of makes sense as well.

[hg]

Re: unsigned integer?

Dan Bishop wrote:

On Mar 10, 11:32 am, "Jack" <nos...@invalid .comwrote:

>This is a naive question:
>>
>"%u" % -3
>>
>I expect it to print 3. But it still print -3.
>>
>Also, if I have an int, I can convert it to unsigned int in C:
> int i = -3;
> int ui = (unsigned int)i;
>>
>Is there a way to do this in Python?

>
def unsigned(n):
return n & 0xFFFFFFFF

or abs(-1) ?

[Dan Bishop]

Re: unsigned integer?

On Mar 10, 11:50 am, Duncan Booth <duncan.bo...@i nvalid.invalid>
wrote:

"Jack" <nos...@invalid .comwrote:

This is a naive question:

>

"%u" % -3

>

I expect it to print 3. But it still print -3.

>
Internally it uses the C runtime to format the number, but if the number
you ask it to print unsigned is negative it uses %d instead of %u. I have
no idea if it is actually possibly to get a different output for %d versus
%u.

%u used to be different from %d, but it changed because of the int/
long unification in Python 2.4.

[Duncan Booth]

Re: unsigned integer?

"Dan Bishop" <danb_83@yahoo. comwrote:

On Mar 10, 11:50 am, Duncan Booth <duncan.bo...@i nvalid.invalid>
wrote:

>"Jack" <nos...@invalid .comwrote:

This is a naive question:

>>

"%u" % -3

>>

I expect it to print 3. But it still print -3.

>>
>Internally it uses the C runtime to format the number, but if the
>number you ask it to print unsigned is negative it uses %d instead of
>%u. I have no idea if it is actually possibly to get a different
>output for %d versus %u.

>
%u used to be different from %d, but it changed because of the int/
long unification in Python 2.4.
>

Yes, I guessed that was it.

The implementation is identical when the value is negative but still
different when the integer is non-negative which is why I questioned
whether it was actually possible to get different output. If not perhaps
both the implementation and the documentation should be simplified.

[Jack]

Re: unsigned integer?

Thanks for all the replies. Because I want to convert an int,
Dan's function actually does it well.

"Jack" <nospam@invalid .comwrote in message
news:I9Kdndp49P JLdm_YnZ2dnUVZ_ t-mnZ2d@comcast.c om...

This is a naive question:
>
"%u" % -3
>
I expect it to print 3. But it still print -3.
>
Also, if I have an int, I can convert it to unsigned int in C:
int i = -3;
int ui = (unsigned int)i;
>
Is there a way to do this in Python?
>

[Paul Rubin]

Re: unsigned integer?

"Jack" <nospam@invalid .comwrites:

Also, if I have an int, I can convert it to unsigned int in C:
int i = -3;
int ui = (unsigned int)i;

I just tried it:

main() {
int i = -3;
unsigned int ui = i;
printf("%d\n", ui);
}

prints -3. What do you want the conversion to do? If you want
the absolute value, use abs().

[Gabriel Genellina]

Re: unsigned integer?

En Sat, 10 Mar 2007 16:26:08 -0300, Paul Rubin
<"http://phr.cx"@NOSPAM. invalidescribió :

"Jack" <nospam@invalid .comwrites:

>Also, if I have an int, I can convert it to unsigned int in C:
> int i = -3;
> int ui = (unsigned int)i;

>
I just tried it:
>
main() {
int i = -3;
unsigned int ui = i;
printf("%d\n", ui);
}
>
prints -3. What do you want the conversion to do? If you want
the absolute value, use abs().

Try again with "%u". Passing i or ui makes no difference, both push the
same value on the stack. C relies on the format string to interpret the
arguments.

--
Gabriel Genellina

[Paul Rubin]

Re: unsigned integer?

"Gabriel Genellina" <gagsl-py2@yahoo.com.a rwrites:

Try again with "%u". Passing i or ui makes no difference, both push
the same value on the stack. C relies on the format string to
interpret the arguments.

If you use %u you get a very large positive value, not +3.

[Gabriel Genellina]

Re: unsigned integer?

En Sat, 10 Mar 2007 20:26:13 -0300, Paul Rubin
<"http://phr.cx"@NOSPAM. invalidescribió :

"Gabriel Genellina" <gagsl-py2@yahoo.com.a rwrites:

>Try again with "%u". Passing i or ui makes no difference, both push
>the same value on the stack. C relies on the format string to
>interpret the arguments.

>
If you use %u you get a very large positive value, not +3.

Exactly, and that's the right value. (unsigned int)(-3) isn't +3.

--
Gabriel Genellina

[Paul Rubin]

Re: unsigned integer?

"Gabriel Genellina" <gagsl-py2@yahoo.com.a rwrites:

If you use %u you get a very large positive value, not +3.

Exactly, and that's the right value. (unsigned int)(-3) isn't +3.

The OP specified that the expected result was 3.

[Bjoern Schliessmann]

Re: unsigned integer?

Paul Rubin wrote:

The OP specified that the expected result was 3.

But that's not what he'd get with his C conversion ;)

Regards,


Björn

--
BOFH excuse #348:

We're on Token Ring, and it looks like the token got loose.

[Bjoern Schliessmann]

Re: unsigned integer?

hg wrote:

Dan Bishop wrote:

>def unsigned(n):
> return n & 0xFFFFFFFF

>
or abs(-1) ?

Nah! Bitwise operators are cool. ;)

Though ANDing won't make the int unsigned.

Regards,


Björn

--
BOFH excuse #23:

improperly oriented keyboard

[Gabriel Genellina]

Re: unsigned integer?

En Sat, 10 Mar 2007 21:04:00 -0300, Paul Rubin
<"http://phr.cx"@NOSPAM. invalidescribió :

"Gabriel Genellina" <gagsl-py2@yahoo.com.a rwrites:

If you use %u you get a very large positive value, not +3.

>Exactly, and that's the right value. (unsigned int)(-3) isn't +3.

>
The OP specified that the expected result was 3.

Ouch! Yes, sorry, I overlooked it. And the C code just made things more
confusing.

--
Gabriel Genellina