使用TryParse C＃检查负值

Question

i know there are several threads about this topic, but I didn't find an answer for me. So I check for correct values with double.TryParse , but I also have to check if they're negative:

static void Main(string[] args)
{
    double radius, hoehe, umfang, volumen, oberfl;

    Console.WriteLine("Radius des Zylinders eingeben: ");

    while (!double.TryParse(Console.ReadLine(), out radius))
    {
        Console.WriteLine("Wert ist ungültig! Bitte erneut versuchen.");
    }

    Console.WriteLine("Hoehe des Zylinders eingeben: ");

    while (!double.TryParse(Console.ReadLine(), out hoehe))
    {
        Console.WriteLine("Wert ist ungültig! Bitte erneut versuchen.");
    }

    umfang = 2 * Math.PI * radius;
    volumen = Math.PI * Math.Pow(radius, 2) * hoehe;
    oberfl = umfang * (radius + hoehe);

    Console.WriteLine("Umfang: {0:f2}", umfang);
    Console.WriteLine("Volumen: {0:f2}", volumen);
    Console.WriteLine("Oberfläche: {0:f2}", oberfl);

    Console.ReadKey();
}

First I thought just add it to the while statement, like

while (!double.TryParse(Console.ReadLine(), out radius) && radius <=0)

but this doesn't work. So can you please tell me another possibility, maybe another if statement?

Answer 1

Change && to || . As || stops after first expression to be evaluated as true then:

• If the input is not a double then it will iterate again (and won't check second condition as && would - which was the problem)
• If the input is a double but is <= 0 it will iterate again.

Therefore it will stop iterating iff input is a positive double.

while (!double.TryParse(Console.ReadLine(), out radius) || radius <=0)

---

Also as you are using this code twice (at least) maybe encapsulate it in a function?

public double GetPositiveDouble()
{
    double result;
    while (!double.TryParse(Console.ReadLine(), out result))
    {
        Console.WriteLine("Wert ist ungültig! Bitte erneut versuchen.");
    }
    return result;
}

Answer 2

You can use an overload of double.TryParse that accept NumberStyles flags. If you expect a floating point number without a sign (eg only non-negative) you can specify that:

double.TryParse(
    Console.ReadLine(),
    NumberStyles.AllowDecimalPoint,
    CultureInfo.CurrentCulture,
    out var radius)

This will return false if the number has a sign (both - or + ) and you know that the specified number is always non-negative.

Notice that this return true if the number 0.0 which is not exactly what you want.

If you also want to allow whitespace and exponential notation etc. you have to provide additional NumberStyles flags.

Answer 3

You condition is wrong. It should read:

while (!(double.TryParse(Console.ReadLine(), out radius) && radius > 0))

Answer 4

您的代码可以工作...应该是OR，而不是AND：

while (!double.TryParse(Console.ReadLine(), out radius) || radius <=0)