Conditional Statements

**strokebow** · 11-22-2006

nice 1 happy_reaper

much appreciated

**vart** · 11-22-2006

((a < 0.00001 || a > -0.000001) && (b < 0.00001 || b > -0.000001))

always true
Should be

Code:

((a < 0.00001 && a > -0.000001) && (b < 0.00001 && b > -0.000001))

**Happy_Reaper** · 11-22-2006

hehe...right
The funny thing is that I remember correcting someone on that a couple weeks back...

**dbnelson2403** · 11-22-2006

this...

Code:

if ( !a && !b )

...seems a lot more intuitive to me.

**Happy_Reaper** · 11-22-2006

But that would also fall prey to the same floating point problems as the OP was having.

**dbnelson2403** · 11-22-2006

Originally Posted by Happy_Reaper

But that would also fall prey to the same floating point problems as the OP was having.

no it wouldn't

**Happy_Reaper** · 11-22-2006

no it wouldn't

Explain, please.

**Salem** · 11-22-2006

> if ( !a && !b )
Considering this is just
if ( !(a!=0.0) && !(b!=0.0) )

you still haven't solved the problem of comparing floats for exact equality or inequality, as described in the first link posted by andor.

**codepoet** · 11-22-2006

Originally Posted by Salem

> if ( !a && !b )
Considering this is just
if ( !(a!=0.0) && !(b!=0.0) )

you still haven't solved the problem of comparing floats for exact equality or inequality, as described in the first link posted by andor.

the original poster wanted to compare variables to absolute zero, not zero within a tolerance (at least that's how i interpreted his code), so i don't understand why you guys are providing solutions that compensate for fuzz... explain, please.

**Happy_Reaper** · 11-22-2006

Namely, he's trying to compare floats to absolute zero. That's not the same as comparing ints with 0.

**codepoet** · 11-22-2006

Originally Posted by Happy_Reaper

Namely, he's trying to compare floats to absolute zero. That's not the same as comparing ints with 0.

yes, i know. however, zero of any magnitude is still zero, and unary negation should still fit the bill in this case (!0, !0.0, !0.0000, etc should all still return 1).

**dwks** · 11-22-2006

Floating point numbers aren't exact. Assigning "1.23" might result in "1.229999999" being stored in reality. When you're comparing floating point numbers, you have to take this into account.

Take this loop, for example.

Code:

float x;

for(x = 0.0; x != 1.0; x += 0.1) {}

It might run infinitely; 1.0 can't be represented exactly on some computers at least.

This is a good read: http://en.wikipedia.org/wiki/Floating_point

Admittedly, zero is probably different. I imagine that floating point numbers can usually represent zero.

**Salem** · 11-22-2006

You're missing the point.

Saying x == 0.0 is just as broken as saying x != 0.0
If x is even off by some very small fraction then the logic of the test is completely broken.

If you want repeatability with floating point comparisions, then you have to implement some kind of approximately equal to test.

Sure it will always work if you just assign a constant, but if you're doing something involving calculations like
for ( x = 0.0 ; x < 1.0 ; x += 0.1 )
don't expect x to be reliably 1.0 when the loop finishes.

**codepoet** · 11-22-2006

no, i'm not missing the point; i'm afraid both of you are. you can't input 0 into x and have it come out 0.0000001. the inaccuracy of floating point numbers doesn't apply to zero, and the computer doesn't have any problem storing zero, regardless of the magnitude.

and if x is guaranteed to be zero, then negating it is guaranteed to be 1 and the code snippet i offered will work.