Re: strange casting problem

Hi Nevzat,

Nevzat Guler wrote:
> Hello,
> I apologize for bringing here non-root problem but I discovered it in my root
> program and also tested with a stand alone C Program.
> I attached the simple program to the email and printed at the end ... The
> problem is, when I convert a float to int, I have value offset.

The problem is that doubles are stored on computers in base 2, so the computer cannot internally represent the exact value of decimal 0.01 which you are dividing by (nor of the numbers 1.19, etc. which you divide). So even though you think you are dividing 1.19/0.01, instead of 119 exactly you end up with 118.99999999999997 or some such. Note that "cout << aa;" prints the value of aa rounded to (I believe) six decimal places by default.

If you know that your results really should be integers, you could round them to the nearest integer with int(floor(aa + 0.5)) -- some math libraries also include functions like round() or rint() to do this for you.

> Below you can see some examples. The problem appears between 113 and 119.
> I used g++ -o tt tt.cc to complie the attached program and run it.
> I am very puzzled and thought maybe you can help on this.

[...]
> aa=119 int(aa)=118 ceil(aa)=119 floor(aa)=118
[...]

Here's where you divide 1.19 (I assume) passed in as W, by 0.01 passed in as W_width:

> int GetWIndex(double W, double W_start, double W_width)
> {
> double aa = W/W_width ;

best regards,

-- 
Kevin B. McCarty <kmccarty_at_gmail.com>
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