Using Math.Cos function in C# for value 90 degree.

[Bindul Bhowmik]

I am in a project and have to find out the value of cos of
an angle. The user will input the value of the angle only
in degrees. I am using the following expression to get the
radian equivalent of the input :
(Input Value / 180) * Math.PI

To find the Cos I am using the Function Math.Cos. But the
problem comes when I get an input value of 90 degrees, and
the cos dosent come out to be 0.

If anyone has worked on a similar problem, or has any idea
how to get around this problem, please reply.

[Jon Skeet [C# MVP]]

Re: Using Math.Cos function in C# for value 90 degree.

Bindul Bhowmik <bindul_bhowmik @mindtree.com> wrote:[color=blue]
> I am in a project and have to find out the value of cos of
> an angle. The user will input the value of the angle only
> in degrees. I am using the following expression to get the
> radian equivalent of the input :
> (Input Value / 180) * Math.PI
>
> To find the Cos I am using the Function Math.Cos. But the
> problem comes when I get an input value of 90 degrees, and
> the cos dosent come out to be 0.
>
> If anyone has worked on a similar problem, or has any idea
> how to get around this problem, please reply.[/color]

On my box it comes out to a value very, very close to 0 - namely about
6x10^-17.

The problem is that pi/2 isn't exactly representable (in either decimal
or binary). You run into the same problems as you do for any other
floating point number. See
http://www.pobox.com/~skeet/csharp/floatingpoint.html for more details
of these.

Is something being 0 to 16 significant digits really not good enough?

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Hans Kesting]

Re: Using Math.Cos function in C# for value 90 degree.


"Bindul Bhowmik" <bindul_bhowmik @mindtree.com> wrote in message
news:0a8e01c3ae 8d$f8b9e520$a40 1280a@phx.gbl.. .[color=blue]
> I am in a project and have to find out the value of cos of
> an angle. The user will input the value of the angle only
> in degrees. I am using the following expression to get the
> radian equivalent of the input :
> (Input Value / 180) * Math.PI
>
> To find the Cos I am using the Function Math.Cos. But the
> problem comes when I get an input value of 90 degrees, and
> the cos dosent come out to be 0.
>
> If anyone has worked on a similar problem, or has any idea
> how to get around this problem, please reply.[/color]

Is your "input value" an int? Then you will get unexpected results!
90/180 == 0

you could use
90/180.0 which will be (about) 0.5

Hans Kesting

[Ted Miller]

Re: Using Math.Cos function in C# for value 90 degree.

Well, since 90/180 = 1/2 which is precisely 2^-1, isn't this value precisely
representable in an x86 float or double?

Just asking.

"Hans Kesting" <news.2.hansdk@ spamgourmet.com > wrote in message
news:OhjjLcqrDH A.4056@TK2MSFTN GP11.phx.gbl...[color=blue]
>
> "Bindul Bhowmik" <bindul_bhowmik @mindtree.com> wrote in message
> news:0a8e01c3ae 8d$f8b9e520$a40 1280a@phx.gbl.. .[color=green]
> > I am in a project and have to find out the value of cos of
> > an angle. The user will input the value of the angle only
> > in degrees. I am using the following expression to get the
> > radian equivalent of the input :
> > (Input Value / 180) * Math.PI
> >
> > To find the Cos I am using the Function Math.Cos. But the
> > problem comes when I get an input value of 90 degrees, and
> > the cos dosent come out to be 0.
> >
> > If anyone has worked on a similar problem, or has any idea
> > how to get around this problem, please reply.[/color]
>
> Is your "input value" an int? Then you will get unexpected results!
> 90/180 == 0
>
> you could use
> 90/180.0 which will be (about) 0.5
>
> Hans Kesting
>
>[/color]

[Hans Kesting]

Re: Using Math.Cos function in C# for value 90 degree.


"Ted Miller" <ted@nwlink.com > wrote in message
news:vro0kmt9rg f522@corp.super news.com...[color=blue]
> Well, since 90/180 = 1/2 which is precisely 2^-1, isn't this value[/color]
precisely[color=blue]
> representable in an x86 float or double?
>[/color]

The problem is that it is: _int_ 90 divided by _int_ 180, which will lead
to an _int_ result of 0.
Change one of the operands to double (by using 180.0 for example) and you
get a double result.

Hans Kesting

[Jon Skeet [C# MVP]]

Re: Using Math.Cos function in C# for value 90 degree.

Hans Kesting <news.2.hansdk@ spamgourmet.com > wrote:[color=blue]
>
> "Ted Miller" <ted@nwlink.com > wrote in message
> news:vro0kmt9rg f522@corp.super news.com...[color=green]
> > Well, since 90/180 = 1/2 which is precisely 2^-1, isn't this value[/color]
> precisely[color=green]
> > representable in an x86 float or double?
> >[/color]
>
> The problem is that it is: _int_ 90 divided by _int_ 180, which will lead
> to an _int_ result of 0.
> Change one of the operands to double (by using 180.0 for example) and you
> get a double result.[/color]

I suspect Ted was querying the word "about" in your previous post.

--
Jon Skeet - <skeet@pobox.co m>

http://www.pobox.com/~skeet

If replying to the group, please do not mail me too

[Hans Kesting]

Re: Using Math.Cos function in C# for value 90 degree.

.... eh, you could be right (now that I re-read his post), in which case he
was right. (right? :-)

Hans Kesting

[Ted Miller]

Re: Using Math.Cos function in C# for value 90 degree.

I understand that, but one poster said essentially that 90.0 / 180.0 is
"approximat ely" .5.

--

"Hans Kesting" <news.2.hansdk@ spamgourmet.com > wrote in message
news:%230Sv8H0r DHA.2028@TK2MSF TNGP10.phx.gbl. ..[color=blue]
>
> "Ted Miller" <ted@nwlink.com > wrote in message
> news:vro0kmt9rg f522@corp.super news.com...[color=green]
> > Well, since 90/180 = 1/2 which is precisely 2^-1, isn't this value[/color]
> precisely[color=green]
> > representable in an x86 float or double?
> >[/color]
>
> The problem is that it is: _int_ 90 divided by _int_ 180, which will lead
> to an _int_ result of 0.
> Change one of the operands to double (by using 180.0 for example) and you
> get a double result.
>
> Hans Kesting
>
>[/color]