This calculation is just wrong / computer can't count!

I have been debugging something for ages now. I have a method that does some
complex maths, but right at the beginning it works out a proportion and a
few ratios and the maths is simply wrong. In my code I (obviously) use
variables and the values vary each time the method is called, but there
seems to a problem with the maths. I have narrowed the problem down to the
following. Can someone else please try this simply calculation and see what
their computer gets.

Code Line 1:
double effortChangeProportion = (55.0 - 30.0) / 30.0;


This first line does 55-30 and divides the result by 30. In other words
25/30, which is 0.8333 (recurring 3s).

The computer manages to give the answer 0.83333333333333337 !!

Code Line 2:
effortChangeProportion++;

or

effortChangeProportion = effortChangeProportion + 1.0;

The second line of code (both alternatives give the same result) builds on
the first by simply adding 1 (so I can then multiply other numbers by this
proportion).

In this case 0.8333 becomes 1.8333 , but again the computer gets this wrong.
It tries to add 1 to 0.83333333333333337 and gets 1.8333333333333335.

Obviously this can easily be done in 1 line of code, but it is broken down
to demonstrate the maths going wrong twice!

Can anyone shed some light on this for me please?

GT


.

Relevant Pages

  • Re: This calculation is just wrong / computer cant count!
    ... but right at the beginning it works out a proportion and a ... few ratios and the maths is simply wrong. ... effortChangeProportion = effortChangeProportion + 1.0; ... Try the DECIMAL datatype of OLE which uses decimal arithmetics ...
    (microsoft.public.vc.mfc)
  • Re: This calculation is just wrong / computer cant count!
    ... some complex maths, but right at the beginning it works out a proportion ... and a few ratios and the maths is simply wrong. ... effortChangeProportion = effortChangeProportion + 1.0; ...
    (microsoft.public.vc.mfc)
  • Re: This calculation is just wrong / computer cant count!
    ... but right at the beginning it works out a proportion and a ... few ratios and the maths is simply wrong. ... effortChangeProportion = effortChangeProportion + 1.0; ...
    (microsoft.public.vc.mfc)
  • Re: This calculation is just wrong / computer cant count!
    ... but right at the beginning it works out a proportion and a ... few ratios and the maths is simply wrong. ... effortChangeProportion = effortChangeProportion + 1.0; ... So the thread moved on from the original question very ...
    (microsoft.public.vc.mfc)
  • Re: This calculation is just wrong / computer cant count!
    ... but right at the beginning it works out a proportion and a ... few ratios and the maths is simply wrong. ... effortChangeProportion = effortChangeProportion + 1.0; ... I tried multiplying ...
    (microsoft.public.vc.mfc)