Inconsistency With the Value

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Hi all,

In a code for exactly same line of formula,my C++ gives me one answer and excel gives another.
The formula:

C++

double val ;

val = cos(-0.00623381826089208+90/180*3.14159265358979)*12.85+cos(-0.00623381826089208)*(-49.72)+6767250.28012434;

The c++ result is 6767200.5623347033,  but the excel result shows:  6767200.641. SO I split it and compared a part of the formula,
val = cos(-0.0062338182608920800000+90/180 *3.14159265358979000);
 but again, this shows 0.006233778 in excel and 0.99998056981786743 in c++

This makes a huge difference in further calculations. Another thing I observed was the line val = cos(20) or val = cos(10) gives compiler error and cos can take only small numbers like 0.2, 0.9. The error I get is "more than one instance of overloaded function "cos" matches the argument list"

Need some help please..

Posted 31-Jul-12 22:54pm

Sumal.V

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3 solutions

Solution 3

Watch the operator precedence and type casting rules! The term

90/180

delivers in C++ the value of 0, as it is an int division. Excel will probably convert both 90 and 180 to double first and comes to a result of 0.5.

Try writing 90./180. in your C++ expression and see the difference.

Also: Your expression is numerically not very stable. You arr adding something in the range of 50 to a value of 6767250. That's 5 orders of magnitude difference. In type float you would probably get a totally erratic result. In double things run somewhat better, but don't expect the result to have more than 15 - 5 = 10 significant digits.

Posted 31-Jul-12 23:29pm

nv3

Comments

Sumal.V 1-Aug-12 6:06am

Yippeeee....Thank u soooooooooooooo much. The 90.0/180.0 instead of 90/180 made a big difference :)

Solution 2

I believe Excel considers argument of cos() function to be in radians while C++ treats it as degrees. Also, as Richard noted, check operator precedence in complex expressions like 90/180*3.14159265358979000, and make sure you divide float values, no ints

Posted 31-Jul-12 23:27pm

Timberbird

Comments

Sumal.V 1-Aug-12 6:14am

Yup thanks, including brackets helped :)

nv3 1-Aug-12 7:23am

No, the C++ cos function also works in radians! The real reason lay in the integer division 90/180!

Timberbird 1-Aug-12 9:11am

Yes, my bad, sorry for misleading answer. Well, at least part about int/floating point values division was relevant, though it definitely should have been explained, not just mentioned :)

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Richard MacCutchan · Accepted Answer · 2012-07-31T23:19:00

You need to make sure your code is evaluating in the correct order, see the rules of Operator precedence in C++[^] for guidance. You also need to check which version of the cos() function you are using, the compiler messages suggest it has a number of overloads. You may also find discrepancies owing to the way that floating point values are represented in computing and how accuracy can be a problem; a Google search will get you lots of articles on the subject.