C# find the greatest common divisor

cmath

"The greatest common divisor of two integers is the largest integer that evenly divides each of the two numbers. Write method Gcd that returns the greatest common divisor of two integers. Incorporate the method into an app that reads two values from the user and displays the result."

(this is not homework, just an exercise in the book I'm using)

can you help me solve this? Here's what I've got so far.

(edit – I can submit the two numbers but it won't calculate the Gcd for me)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Greatest_Common_Divisor
{
class Program
{

    static int GetNum(string text)
    {
        bool IsItANumber = false;
        int x = 0;
        Console.WriteLine(text);

        do
        {
            IsItANumber = int.TryParse(Console.ReadLine(), out x);

        } while (!IsItANumber);

        return x;
    }
    static void Main(string[] args)
    {
        string text = "enter a number";
        int x = GetNum(text);
        text = "enter a second number";
        int y = GetNum(text);


        int z = GCD(x, y);
        Console.WriteLine(z);
    }

    private static int GCD(int x, int y)
    {
        int v = 0;
        int n = 0;

        v = GetGreatestDivisor(x, y);


        return v;

    }

    static int GetGreatestDivisor(int m, int h)
        {

            do
            {
                for (int i = m; i <= 1; i--)



                    if (m%i == 0 && h%i == 0)
                    {
                        int x = 0;
                        x = i;

                        return x;
                    }
            } while (true);
            return m;
        }

  }
}

Best Answer

Here's an implementation of the Euclidean algorithm that returns the greatest common divisor without performing any heap allocation.

You can substitute ulong for uint if needed. An unsigned type is used, as the technique does not work for signed values. If you know your a and b values are not negative, you can use long or int instead.

private static ulong GCD(ulong a, ulong b)
{
    while (a != 0 && b != 0)
    {
        if (a > b)
            a %= b;
        else
            b %= a;
    }

    return a | b;
}

This method is used in my metadata-extractor library, where it has associated unit tests.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – Should ‘using’ directives be inside or outside the namespace

There is actually a (subtle) difference between the two. Imagine you have the following code in File1.cs:

// File1.cs
using System;
namespace Outer.Inner
{
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now imagine that someone adds another file (File2.cs) to the project that looks like this:

// File2.cs
namespace Outer
{
    class Math
    {
    }
}

The compiler searches Outer before looking at those using directives outside the namespace, so it finds Outer.Math instead of System.Math. Unfortunately (or perhaps fortunately?), Outer.Math has no PI member, so File1 is now broken.

This changes if you put the using inside your namespace declaration, as follows:

// File1b.cs
namespace Outer.Inner
{
    using System;
    class Foo
    {
        static void Bar()
        {
            double d = Math.PI;
        }
    }
}

Now the compiler searches System before searching Outer, finds System.Math, and all is well.

Some would argue that Math might be a bad name for a user-defined class, since there's already one in System; the point here is just that there is a difference, and it affects the maintainability of your code.

It's also interesting to note what happens if Foo is in namespace Outer, rather than Outer.Inner. In that case, adding Outer.Math in File2 breaks File1 regardless of where the using goes. This implies that the compiler searches the innermost enclosing namespace before it looks at any using directive.

Best Answer

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C# – the difference between String and string in C#

This is the style that Microsoft tends to use in their examples.

C# – Should ‘using’ directives be inside or outside the namespace

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