c++ float and integer

[Geky]

Why when I multiply a float , for example X10, and than convert it as
an integer the new value is changed?

Example:

float fl = 1.8
int x;

x = int(fl * 10);

x = 17 <<<<<<<< I think this is not correct :-)

this happen a lot of time in my code with other float numbers.

is this a bug of VisualC6 or I need a solution?

[TB]

Re: c++ float and integer

Geky sade:[color=blue]
> Why when I multiply a float , for example X10, and than convert it as
> an integer the new value is changed?
>
> Example:
>
> float fl = 1.8
> int x;
>
> x = int(fl * 10);
>
> x = 17 <<<<<<<< I think this is not correct :-)
>
> this happen a lot of time in my code with other float numbers.
>
> is this a bug of VisualC6 or I need a solution?
>[/color]

Most likely because (1.8f * 10) can't be perfectly represented.
Internally it might be 17.9999999999 which when you truncate it
to an integer becomes 17.

--
TB @ SWEDEN

[Victor Bazarov]

Re: c++ float and integer

Geky wrote:[color=blue]
> Why when I multiply a float[..][/color]

Please see my answer to the same post of yours in the newsgroup
'microsoft.publ ic.vc.language' .

V

[Shark]

Re: c++ float and integer

Geky wrote:[color=blue]
> Why when I multiply a float , for example X10, and than convert it as
> an integer the new value is changed?[/color]

Apart from being a VC question (which you should ask in a relevant
group) consider what happens when you convert from a float to an int. A
float can be represented in many ways, and generally the convention is
to use IEEE754-1985 standard.

Lets say you have a number 32 bits long, and you try jamming those 32
bits into 8 bits. What happens? 24 bits are left out!!!! So 3/4 times
information about your float number is lost! Makes sense? No wonder you
are getting a wrong answer.
[color=blue]
>
> Example:
>
> float fl = 1.8
> int x;
>
> x = int(fl * 10);
>
> x = 17 <<<<<<<< I think this is not correct :-)
>
> this happen a lot of time in my code with other float numbers.
>
> is this a bug of VisualC6 or I need a solution?[/color]

[Mike Wahler]

Re: c++ float and integer


"Geky" <attackack@yaho o.com> wrote in message
news:1137438599 .417562.35500@o 13g2000cwo.goog legroups.com...[color=blue]
> Why when I multiply a float , for example X10, and than convert it as
> an integer the new value is changed?
>
> Example:
>
> float fl = 1.8
> int x;
>
> x = int(fl * 10);[/color]

std::cout << std::fixed << fl << '\n';
std::cout << std::fixed << fl * 10 << '\n';

[color=blue]
>
> x = 17 <<<<<<<< I think this is not correct :-)
>
> this happen a lot of time in my code with other float numbers.
>
> is this a bug of VisualC6[/color]

No.
[color=blue]
> or I need a solution?[/color]

http://docs.sun.com/source/806-3568/ncg_goldberg.html

-Mike