C# math, giving wrong result

Question

0.00/5 (No votes)

See more:

My code:

C#

using System;

namespace PolarToCart
{
	class Program
	{
		static void Main(string[] args)
		{
			double r;
			double θ;
			double ϕ;

			string result = "";

			r = 1.0;
			θ = 2.0;
			ϕ = 3.0;

			result = Convert.ToString(
					r * Math.Sin(θ) * Math.Cos(ϕ)
				);

			Console.WriteLine(result);
			Console.ReadKey();
		}
	}
}

Gives me the result:

-0.900197629735517

However, the correct result, when it's calculated on my calculator, (and other calculators), is:

0.03485166816

I've no idea what's going on here. Does somebody have any insights?

Thank you!

What I have tried:

I've tried to experiment with parenthesizes, and my math-wiz better half, (who has had some programming experience), is also stumped. Some googling was naturally tried

Posted 25-Feb-16 7:42am

Frank R. Haugen

Updated 25-Feb-16 8:37am

Add a Solution

3 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Jochen Arndt · Accepted Answer · 2016-02-25T08:01:00

Solution 1

The arguments to the trigonometric functions are in radians. If your input is in degrees, you must divide by 2 * Pi:
C#

result = Convert.ToString( r * Math.Sin(θ / (2 * Math.PI)) * Math.Cos(ϕ / (2 * Math.PI)) );

[EDIT]
Used thew wrong factor.

C#

result = Convert.ToString(
    r * Math.Sin(Math.PI / 180 * θ) * Math.Cos(Math.PI / 180 * ϕ)
);

[/EDIT]

Posted 25-Feb-16 8:01am

Jochen Arndt

Updated 25-Feb-16 8:43am

v2

Comments

Frank R. Haugen 25-Feb-16 14:33pm

Facepalm!, (actually a double, since the GF also felt like an idiot).

though your answer is correct, the solution is:
r * Math.Sin((Math.PI / 180) * θ) * Math.Cos((Math.PI / 180) * ϕ)

I never use radians in the math I do, (not that I do math that much), so no wonder the result was wacky.

Thank you!

Jochen Arndt 25-Feb-16 14:39pm

Off course!
Sorry for the wrong factor. I was in a hurry when answering (had to return to the cooker).

OriginalGriff · Accepted Answer · 2016-02-25T08:04:00

Solution 2

And I'd agree with it: https://www.wolframalpha.com/input/?i=1.0+*+Sin(2.0)+*+Cos(3.0)[^]
Wolfram alpha agrees.
As does my calculator (Sharp EL-5020)

Are you sure your calculators aren't assuming that θ and ϕ are in degrees, rather than radians, like my calculator, Wolfram Alpha, and .NET?

Posted 25-Feb-16 8:04am

OriginalGriff

Updated 25-Feb-16 8:05am

v3

Comments

Frank R. Haugen 25-Feb-16 14:35pm

Though technically not an answer, you found the flaw, I mixed up radians/degrees

Frank R. Haugen · Accepted Answer · 2016-02-25T08:37:00

The fixed code:

C#

using System;

namespace PolarToCart
{
	class Program
	{
		public double ConvertToRadians(double angle)
		{
			return (Math.PI / 180) * angle;
		}

		static void Main(string[] args)
		{
			double r;
			double θ;
			double ϕ;

			string result = "";

			r = 1.0;
			θ = 2.0;
			ϕ = 3.0;

			result = Convert.ToString(

				r * Math.Sin((Math.PI / 180) * θ) * Math.Cos((Math.PI / 180) * ϕ)
				);

			Console.WriteLine(result);

			Console.ReadKey();
		}
	}
}