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* [Bug c++/29597] New: log expression returns different results when casting
@ 2006-10-25 21:58 brad_atcheson at yahoo dot ca
2006-10-25 22:05 ` [Bug c++/29597] " pinskia at gcc dot gnu dot org
2006-10-26 6:39 ` bangerth at dealii dot org
0 siblings, 2 replies; 3+ messages in thread
From: brad_atcheson at yahoo dot ca @ 2006-10-25 21:58 UTC (permalink / raw)
To: gcc-bugs
The following code should return the base2 logarithm of 64 (ie 6)
double d = log(64.0) / log(2.0);
int i = (int)d;
cout << i;
And indeed it does output "6". However, if you attempt to cast it on the same
line, then it returns 5.
int i2 = (int)( log(64.0) / log(2.0) );
int i3 = static_cast<int>( log(64.0) / log(2.0) );
cout << i2 << i3;
This outputs "55" instead of the expected "66". Visual Studio 2003 outputs
"66".
gcc 4.1.0, Suse Linux 10.1.
--
Summary: log expression returns different results when casting
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: brad_atcheson at yahoo dot ca
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29597
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/29597] log expression returns different results when casting
2006-10-25 21:58 [Bug c++/29597] New: log expression returns different results when casting brad_atcheson at yahoo dot ca
@ 2006-10-25 22:05 ` pinskia at gcc dot gnu dot org
2006-10-26 6:39 ` bangerth at dealii dot org
1 sibling, 0 replies; 3+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-10-25 22:05 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2006-10-25 22:05 -------
This comes from precission of the floating point.
*** This bug has been marked as a duplicate of 323 ***
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |DUPLICATE
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29597
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug c++/29597] log expression returns different results when casting
2006-10-25 21:58 [Bug c++/29597] New: log expression returns different results when casting brad_atcheson at yahoo dot ca
2006-10-25 22:05 ` [Bug c++/29597] " pinskia at gcc dot gnu dot org
@ 2006-10-26 6:39 ` bangerth at dealii dot org
1 sibling, 0 replies; 3+ messages in thread
From: bangerth at dealii dot org @ 2006-10-26 6:39 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from bangerth at dealii dot org 2006-10-26 06:39 -------
To be more concrete: (int)(expression) rounds down. So if your quotient
of logarithms happens to be computed to 5.999999999999999, then you
will get a result of 5 after casting to int. You should round results,
not just cast them.
W.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29597
^ permalink raw reply [flat|nested] 3+ messages in thread
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2006-10-25 21:58 [Bug c++/29597] New: log expression returns different results when casting brad_atcheson at yahoo dot ca
2006-10-25 22:05 ` [Bug c++/29597] " pinskia at gcc dot gnu dot org
2006-10-26 6:39 ` bangerth at dealii dot org
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