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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

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double comparisons within an if stat

Posted by nano (nano), 8 September 2006
Hi Graham,
trying to compare two doubles which are equal
and isn't passing within c++ like so  why is this
any ideas ,
Thanks

val = 0.005000
valb = 0.01000
if (2.*val == valb)
{
 return true;
}
valA=0.010000
valb=0.010000


Posted by admin (Graham Ellis), 8 September 2006
Yes ... you should never compare floats (or even doubles) to see if they're equal in C or C++ - because of routing errors.  Internally, floating point types are held only to a certain accuracy and so "issues" creep in.

A float example:

Take the value 1.0
Divide it by 3.0 ... and you get 0.3333333
Multiply it by 3.0  and yoiu get 0.9999999
which is NOT equal to 1.0

Posted by nano (nano), 8 September 2006
thanks for that , so whats the work around  in the above example to see if they are equal  

Posted by admin (Graham Ellis), 8 September 2006
It depends on what you're doing;  sometimes you'll find that integers rather than floats would have been better in the first place, and at other times you might subtract one value from the other and see if the absolute value of the result is less that a threshhold.



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