Bug 29597

Summary: log expression returns different results when casting
Product: gcc Reporter: brad <brad_atcheson>
Component: c++Assignee: Not yet assigned to anyone <unassigned>
Severity: normal CC: 166255, 36876, adam, anoullez, benoit.sibaud, bernert, birger.b, brad_atcheson, cognot, csk, debian-gcc, denis.nagorny, deshpand, dsell, dyang, eda-qa, gbburkhardt, gcc-bugs, gcc-erikd, Graham.Murphy, green, gsinai, gtalbot, have, hjl.tools, jmurray, konstantin, lani, mark, neff.kevin, npr1, olcios, paul_blankenbaker, petr.savicky, piaget, pr2345, preciseflight, rozenman, sk2alexa, themis_hv, u.strempel, wirawan0, xiaoyi_wu
Priority: P3    
Version: 4.1.0   
Target Milestone: ---   
Host: Target:
Build: Known to work:
Known to fail: Last reconfirmed:

Description brad 2006-10-25 21:58:38 UTC
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.
Comment 1 Andrew Pinski 2006-10-25 22:05:04 UTC
This comes from precission of the floating point.

*** This bug has been marked as a duplicate of 323 ***
Comment 2 Wolfgang Bangerth 2006-10-26 06:39:14 UTC
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.