Thread: Shouldn't B'1' = 1::bit be true?
Am I missing something here?
agelis=# select B'1'::int;
int4
------
1
(1 row)
agelis=# select 1::bit;
bit
-----
0
(1 row)
agelis=# select B'1'=1;
?column?
----------
f
(1 row)
Last I checked, binary 1 and integer 1 were equal. Is this a bug, or is there
a reason for this that I need educated on? Why does int 1 cast to binary 0?
This is Posgres 7.4.1.
--
Bill Moran
Potential Technologies
http://www.potentialtech.com
Stephan Szabo wrote: > On Wed, 3 Mar 2004, Bill Moran wrote: > > >>Am I missing something here? >> >>agelis=# select B'1'::int; >> int4 >>------ >> 1 >>(1 row) >> >>agelis=# select 1::bit; >> bit >>----- >> 0 >>(1 row) >> >>agelis=# select B'1'=1; >> ?column? >>---------- >> f >>(1 row) >> >>Last I checked, binary 1 and integer 1 were equal. Is this a bug, or is there >>a reason for this that I need educated on? Why does int 1 cast to binary 0? > > Hmm, on my 7.4 machine, I get an error saying that there's no = operator > for bit and integer. If I cast the B'1' into an integer I get a true > result. > > The reason for 1::bit being 0 is that int4 -> bit(n) casts cast to a 32 > bit bit string. IIRC, when bit strings are truncated for size in casts, > the spec says to take from the left. Yeah. I made some changes to the casting config, so that int to bit is cast automatically, instead of requiring an explicit cast. It made the code I was converting easier to manage. I dont' know why my original email took so long to hit the list, but I did find out the same thing you just described. In the end I ended up converting all the bit functions to int, so that I don't have to deal with the casting pecularities, which just seemed like the easiest solution at the time. -- Bill Moran Potential Technologies http://www.potentialtech.com
Bill Moran <wmoran@potentialtech.com> writes:
> Am I missing something here?
Hmm. It seems like int-to-bit casting ought to be aware of the
bit-width one is casting to, and take that number of bits from
the right end of the integer. This would make it be the inverse
of the other direction. Right now it's only an inverse when you
cast to and from bit(32). For shorter bitfield widths, we're
effectively inserting at the right end of the integer, but removing
bits from the left, which is not consistent.
regression=# select B'11000'::bit(5)::int;
int4
------
24
(1 row)
regression=# select 24::int::bit(32);
bit
----------------------------------
00000000000000000000000000011000
(1 row)
regression=# select 24::int::bit(32)::bit(5);
bit
-------
00000
(1 row)
regression=# select 24::int::bit(5);
bit
-------
00000
(1 row)
If we made int-to-bit-N take the rightmost N bits, then the last two
cases would yield different results, but that doesn't seem unreasonable
to me. Or at least it's less unreasonable than bit(5)-to-int not being
the inverse of int-to-bit(5).
Comments?
regards, tom lane
On Wed, 3 Mar 2004, Bill Moran wrote: > Am I missing something here? > > agelis=# select B'1'::int; > int4 > ------ > 1 > (1 row) > > agelis=# select 1::bit; > bit > ----- > 0 > (1 row) > > agelis=# select B'1'=1; > ?column? > ---------- > f > (1 row) > > Last I checked, binary 1 and integer 1 were equal. Is this a bug, or is there > a reason for this that I need educated on? Why does int 1 cast to binary 0? Hmm, on my 7.4 machine, I get an error saying that there's no = operator for bit and integer. If I cast the B'1' into an integer I get a true result. The reason for 1::bit being 0 is that int4 -> bit(n) casts cast to a 32 bit bit string. IIRC, when bit strings are truncated for size in casts, the spec says to take from the left.
"Thomas Swan" <tswan@idigx.com> writes:
> To convert low bits ot high bits you pad 0 bits on the left. To convert
> from high to low you strip bits off the left hand side. This allows
> reasonable behavior.
Unfortunately, the SQL spec is perfectly clear that you pad or strip
zero bits on the *right* of the bit string. We cannot change that.
It might have been better if we had defined int<->bit casts to treat
the first bit of the bit string as the LSB of the integer. But we
didn't, and it's probably too big a change to consider.
regards, tom lane