CODEWITHSUNDEEP
c#floating-point
Thomas Andreè Wang
Thomas Andreè Wang

Reputation: 3429

double equals to 0 on 9999999 / 10000000

I have a problem, im doing some personal research into how this works http://www.codeproject.com/Articles/17480/Optimizing-integer-divisions-with-Multiply-Shift-i but i did not get far until i got stumped, so the code atm is supposed to return roughly 100% error

        double sum = 0, div = 3;
        DateTime t1 = DateTime.Now;
        List<double> l1 = new List<double>();
        for (int a = 0; a < 10000000; a++)
        {
            sum = a / div;
            l1.Add(sum);
        }
        Console.WriteLine("Milisecond: " + DateTime.Now.Subtract(t1).TotalMilliseconds);
        t1 = DateTime.Now;
        List<double> l2 = new List<double>();
        for (int a = 0; a < 10000000; a++)
        {
            sum = a * div;
            l2.Add(sum);
        }
        Console.WriteLine("Milisecond: " + DateTime.Now.Subtract(t1).TotalMilliseconds);
        int c=0;
        int err = 0;
        for (int a = 0; a < (l1.Count < l2.Count ? l1.Count : l2.Count); a++)
        {
            c++;
            err += l1[a] != l2[a] ? 1:0;
        }
        Console.WriteLine("Error: " + 100*(err / c) + "%");

but that last line returns 0 apparently 9999999 / 10000000 = 0

any ideas to how i can work around the problem? my best guess is that its a float problem. (but now i know its a doh error on my part)

Upvotes: 0

Views: 718

Answers (2)

YoryeNathan
YoryeNathan

Reputation: 14532

The variables of the operation are declared as int and therefore the operation is done on integers and the result is only then converted to double.

To get the double result, you could do:

dRes = (double)x / y;

By the way, there is a better way to test the running time of code:

Stopwatch sw = Stopwatch.StartNew();

// ... Some code

sw.Stop();
Console.WriteLine(sw.Elapsed);

Upvotes: 8

Douglas
Douglas

Reputation: 54917

It's not a float problem; it's an int problem. You need to cast err to double before dividing it.

Console.WriteLine("Error: " + 100 * ((double)err / c) + "%");

Upvotes: 4

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