Re: This calculation is just wrong / computer can't count!
If 5 decimal fraction positions are OK for your task, do the following:
a = floor(a * 100000 + 0.5) / 100000 +5e-6;
Then you won't get -0.00000
"GT" <ContactGT_remove_@hotmail.com> wrote in message
news:017d82e6$0$20675$c3e8da3@news.astraweb.com...
"Norbert Unterberg" <nunterberg@newsgroups.nospam> wrote in message
news:OGHe4DqCIHA.1164@TK2MSFTNGP02.phx.gbl...
David Wilkinson schrieb:
Norbert Unterberg wrote:
If you think it has a meaning for the end result, maybe you can explain
again *why*, because I think this information has been lost in this
long thread.
Norbert:
It is not altogether so simple. If I do
double x = 25.0/30.0;
double y = (1.0 + x)*30.0;
double z = y - 55.0;
then I get z = 7.1054273576010019e-015. None of the digits in this
result is correct. "Within x%" has no meaning when the correct answer is
zero.
It has. Try this:
#include <stdio.h>
int main(int argc, char* argv[])
{
double a = (55.0 - 30.0) / 30.0 + 1.0;
printf("result 1 is: %.5f\n", a);
double b = a * 30.0 - 55;
printf("result 2 is: %.5f\n", b);
return 0;
}
On my PC it prints:
result 1 is: 1.83333
result 2 is: 0.00000
So what are we all talking about?
Change your 3rd line of code to read:
double b = 55 - (a * 30.0);
Then run it again and you will see -0.00000. This is the problem starting
to propagate. Obviously, I have changed my display code to detect this
problem and worked around it, but I was looking for a solution that means
I don't have to work round the problem. I just want the problem to go
away!
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