C#•4w ago

Truncating 1 and 0 and returning as a double

I'm having trouble with the Cars, Assemble exercise on the C# path on Exercism. I'm calculating the success rate based on the speed parameter and no matter how I round or truncate, whenever I try to return 0 or 1, my code only returns 0.9000000000002.

public static double SuccessRate(int speed)
    {
        double rate = Math.Round(0.0);
        
        if (speed >=1 || speed <= 4)
        {
            rate = Math.Round(1.0, 0);
        }
        if (speed >=5 || speed <=8)
        {
            rate = Math.Round(0.9, 1);
        }
        if (speed == 9)
        {
            rate = Math.Round(0.8, 1);
        }
        if (speed == 10)
        {
            rate = Math.Round(0.77, 2);
        }

        return rate;
    }

public static double SuccessRate(int speed)
    {
        double rate = Math.Round(0.0);
        
        if (speed >=1 || speed <= 4)
        {
            rate = Math.Round(1.0, 0);
        }
        if (speed >=5 || speed <=8)
        {
            rate = Math.Round(0.9, 1);
        }
        if (speed == 9)
        {
            rate = Math.Round(0.8, 1);
        }
        if (speed == 10)
        {
            rate = Math.Round(0.77, 2);
        }

        return rate;
    }

14 Replies

Angius•4w ago

Welcome to floating point rounding errors 0.9 cannot be represented in binary, the closest number is 0.9000000000002

Pobiega•4w ago

you also have a bug - your conditions are a bit off if speed is greater than or equal to 1, or speed is lower than or equal to 4.. that is true for all numbers

Cykotech•4w ago

So I should change it to speed > 0 and speed < 4

Pobiega•4w ago

what does || mean in C#?

Cykotech•4w ago

I thought it meant or?

Pobiega•4w ago

it does lets take some numbers as an example thou -50, is this above 1 or below 4? yes it is 353214, is this above 1 or below 4? yes it is 0, is this above 1 or below 4? yes it is see my point? 🙂

Cykotech•4w ago

Oh... Lol Im dumb. That should fix it

Pobiega•4w ago

well, yes and no because of how your code is written, it checks all the cases, and potentially "overwrites" the former answer it would be better to return once you know what case you are in in fact, your entire method can actually be written as..

static double SuccessRate(int speed)
{
    return speed switch
    {
        10 => 0.77,
        9 => 0.8,
        >= 5 and <= 8 => 0.9,
        >= 1 => 1,
        _ => 0.0
    };
}

static double SuccessRate(int speed)
{
    return speed switch
    {
        10 => 0.77,
        9 => 0.8,
        >= 5 and <= 8 => 0.9,
        >= 1 => 1,
        _ => 0.0
    };
}

Cykotech•4w ago

I think I had it set up like that because I was having trouble with the floating points I was trying to log things to find the problem

Pobiega•4w ago

what problem? as ZZZZZZ explained, a float/double simply can't represent 0.9

Cykotech•4w ago

The problem that was fixed by switching from || to &&

Pobiega•4w ago

right debugging should help with that

Cykotech•4w ago

But I switched from a direct return to a variable so that I had something to log.

Pobiega•4w ago

right

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Truncating 1 and 0 and returning as a double