unsigned integer?

[hg]

def unsigned(n):
return n & 0xFFFFFFFF

or abs(-1) ?

 

[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]

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]

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 & 0xffffffffffffffff) # for 64 bit integers
print ui
ui = (i & 0xffffffffffffffffffffffffffffffff) # for 128 bit integers
print ui

which gives the following output:

3
65533
4294967293
18446744073709551613
340282366920938463463374607431768211453

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.

 

[Dan Bishop]

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]

%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]

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

 

[Paul Rubin]

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]

En Sat, 10 Mar 2007 16:26:08 -0300, Paul Rubin

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.

 

[Paul Rubin]

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]

En Sat, 10 Mar 2007 20:26:13 -0300, Paul Rubin

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.

 

[Paul Rubin]

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

The OP specified that the expected result was 3.

 

[Bjoern Schliessmann]

The OP specified that the expected result was 3.

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

Regards,


Björn

 

[Bjoern Schliessmann]

Dan Bishop wrote:

or abs(-1) ?

Nah! Bitwise operators are cool.

Though ANDing won't make the int unsigned.

Regards,


Björn

 

[Gabriel Genellina]

En Sat, 10 Mar 2007 21:04:00 -0300, Paul Rubin

The OP specified that the expected result was 3.

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