Trying to understand why my calculation fails

Question

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I am new to programming and trying to understand why the void method works as it does. In the below example, obtainedMarks is displayed in the console while percentage does not.

C#

class Student
    {
        string name;
        int age;
        int marksOfMath;
        int marksOfEnglish;
        int marksOfScience;
        int totalMarks = 300;
        int obtainedMarks;
        double percentage;

        void CalculateTotalMarks()
        {
            obtainedMarks = marksOfMath + marksOfEnglish + marksOfScience;
        }

        void CalculatePercentage()
        {
            percentage = obtainedMarks / totalMarks * 100;
        }

        static void Main()
        {
            Student st1 = new Student();
            st1.name = "x";
            st1.age = 20;
            st1.marksOfEnglish = 80;
            st1.marksOfMath = 99;
            st1.marksOfScience = 96;
            st1.CalculateTotalMarks();
            st1.CalculatePercentage();

            Console.WriteLine("{0} of {1} years age got {2}% marks, {3}", st1.name, st1.age, st1.percentage, st1.obtainedMarks);
        }

If I change the "void CalculatePercentage()" to this:

C#

void CalculatePercentage()
        {
            percentage = obtainedMarks + totalMarks + 100;
        }

percentage is now displayed in the console. Could anybody enlighten me as to why the first calculation gives me 0 when it should give me 91.67%?

Thanks in advance for your help.

Posted 16-Dec-10 9:43am

ShawnSapp

Updated 16-Dec-10 10:03am

Pete O'Hanlon

v2

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Toli Cuturicu 17-Dec-10 15:12pm

Never post fake answers (I deleted yours this time).

3 solutions

Solution 4

Or in C#

percentage = (Convert.ToDouble(obtainedMarks) / totalMarks) * 100.0;

Posted 16-Dec-10 22:07pm

R. Erasmus

Comments

fjdiewornncalwe 17-Dec-10 10:34am

Ummmm... The previous answers were in C# already, weren't they?

Toli Cuturicu 17-Dec-10 15:11pm

Already answered... and way better.

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Pete O'Hanlon · Accepted Answer · 2010-12-16T09:58:00

The answer is that the calculation you are performing is not the calculation you think you are performing. Here's a question for you - what do you get when you divide 275 by 300? The answer is not 0.91666 as you are expecting - it's actually 0, so 0 * 100 = 0.

I know this seems wrong, but I've divided two integers here, and the result is 0. What you are trying to do is divide 275 as a floating point number. Fortunately there's a quick fix. Change this calculation to be

C#

percentage = (double)obtainedMarks / totalMarks * 100;

Now when you run it, you get the answer you are expecting.

fjdiewornncalwe · Accepted Answer · 2010-12-16T09:52:00

Solution 1

try this...

percentage = ((double)obtainedMarks / (double)totalMarks) * 100.0;

Posted 16-Dec-10 9:52am

fjdiewornncalwe

Comments

Marc A. Brown 16-Dec-10 15:52pm

Good answer. Just beat me to it.

Pete O'Hanlon 16-Dec-10 16:04pm

Good answer Marcus, but the second (double) is redundant. You only need to cast obtainedMarks to double.

fjdiewornncalwe 16-Dec-10 16:31pm

Pete: Agreed!!! I just put both and the brackets in to make the order of precedence and the logic clearer. I was trying to say the same thing you did in your answer, but in a shorter(lazier) way.

Trying to understand why my calculation fails

3 solutions

Solution 2

Solution 1

Solution 4

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