C#中的MergeSort算法

Question

I wrote below code for Merge Class :

class Merge
{

    public static void sort(IComparable[] a)
    {
        sort(a, 0, a.Length);
    }

    public static void sort(IComparable[] a, int low, int high)
    {
        int N = high - low;
        if (N <= 1)
            return;

        int mid = low + N / 2;

        sort(a, low, mid);
        sort(a, mid, high);

        IComparable[] aux = new IComparable[N];
        int i = low, j = mid;
        for (int k = 0; k < N; k++)
        {
            if (i == mid) aux[k] = a[j++];
            else if (j == high) aux[k] = a[i++];
            else if (a[j].CompareTo(a[i]) < 0) aux[k] = a[j++];
            else aux[k] = a[i++];
        }

        for (int k = 0; k < N; k++)
        {
            a[low + k] = aux[k];
        }
    }

    private static Boolean isSorted(IComparable[] a)
    {
        for (int i = 1; i < a.Length; i++)
            if (a[i].CompareTo(a[i - 1]) < 0) return false;
        return true;
    }

}

And below code is Implementation. i thought below code should't be wrong! but it's does't compile...

class Program
{
    static void Main(string[] args)
    {
    Merge ms = new Merge();

    Double[] MyArray = { 80,10,52,7,36,7,67,1,8,54 };
    Console.WriteLine("first array is: \n");
    for (int k = 0; k < MyArray.Length; k++)
    {
        Console.Write(MyArray[k]);
        if (k<9)
           Console.Write(" , ");
    }
    ms.sort(MyArray);  // Error is here. Does't compile !!!
    Console.WriteLine("\n");
    Console.WriteLine("\nsorted array is: \n ");
    for (int k = 0; k < MyArray.Length; k++)
    {
        Console.Write(MyArray[k]);
        if (k<9)
           Console.Write(" , ");
    }
    Console.ReadLine();
    }
}

It's does't compile. Error is in ms.sort(MyArray); . What am i doing wrong? Please lead me...

Regards

Answer 1

There are two problems with this code:

1. The signature doesn't match, IComparable[] is not directly compatible with double[] in this case
2. You cannot call the sort method through the instance directly

The minimal amount of changes to fix this would be to make the method generic, and call Merge.sort instead of ms.sort .

Here's how I would implement sort :

public static void sort<T>(T[] a)
    where T : IComparable<T>
{
    sort(a, 0, a.Length);
}

public static void sort<T>(T[] a, int low, int high)
    where T : IComparable<T>
{
    int N = high - low;
    if (N <= 1)
        return;

    int mid = low + N / 2;

    sort(a, low, mid);
    sort(a, mid, high);

    T[] aux = new T[N];
    int i = low, j = mid;
    for (int k = 0; k < N; k++)
    {
        if (i == mid) aux[k] = a[j++];
        else if (j == high) aux[k] = a[i++];
        else if (a[j].CompareTo(a[i]) < 0) aux[k] = a[j++];
        else aux[k] = a[i++];
    }

    for (int k = 0; k < N; k++)
    {
        a[low + k] = aux[k];
    }
}

Note that I changed to using T instead of IComparable , and added a constraint saying we need a T that implements IComparable<T> .

Additionally, change your call from this:

ms.sort(...);

to this:

Merge.sort(...);

Answer 2

    //merge sort recurcive

    public void MergeSort(int[] input,int startIndex,int endIndex)
    {
        int mid;

        if (endIndex > startIndex)
        {
            mid = (endIndex + startIndex) / 2;
            MergeSort(input, startIndex, mid);
            MergeSort(input, (mid + 1), endIndex);
            Merge(input, startIndex, (mid + 1), endIndex);
        }
    }

    public void Merge(int[] input, int left, int mid, int right)
    {
        //Merge procedure takes theta(n) time
        int[] temp = new int[input.Length];
        int i, leftEnd,lengthOfInput,tmpPos;
        leftEnd = mid - 1;
        tmpPos = left;
        lengthOfInput = right - left + 1;

        //selecting smaller element from left sorted array or right sorted array and placing them in temp array.
        while ((left <= leftEnd) && (mid <= right))
        {
            if (input[left] <= input[mid])
            {
                temp[tmpPos++] = input[left++];
            }
            else
            { 
                temp[tmpPos++]=input[mid++];
            }
        }
        //placing remaining element in temp from left sorted array
        while (left <= leftEnd)
        {
            temp[tmpPos++] = input[left++];
        }

        //placing remaining element in temp from right sorted array
        while (mid <= right)
        {
            temp[tmpPos++] = input[mid++];
        }

        //placing temp array to input
        for (i = 0; i < lengthOfInput;i++ )
        {
            input[right]=temp[right];
            right--;
        }
    }

     int[] input3 = { 22, 4, 6, 0, 9, 12, 156, 86,99};
     MergeSort(input3, 0, input3.Length - 1);
        for (int i = 0; i < input3.Length; i++)
            Console.Write(input3[i]+", ");
        Console.WriteLine("");

Answer 3

Your sort method is static, you shouldn't call it from an instance of your class.

You should call it like this:

Merge.sort(MyArray);

not:

ms.sort(MyArray);

In fact, since you Merge class has no instance methods, there is no point creating an instance of it and you should probably just mark the whole class as static.

Answer 4

A simple C# code to do merge sort

using System;

namespace MergeSort
{
    class Program
    {
        static void Main(string[] args)
        {            
            int[] arInt = { 6, 4, 2, 8, 4, 8, 11, 1, 7, 4, 13, 5, 45, -1, 0, -7, 56, 10, 57 };
            var sortedIntAr = GenericMergeSort<int>.Sort(arInt);

            string[] arStr = { "Here", "Is", "A", "Cat", "Really", "Fast", "And", "Clever" };
            var sortedStrAr = GenericMergeSort<string>.Sort(arStr);

            Console.WriteLine(String.Join(',', sortedIntAr));
            Console.WriteLine(String.Join(',', sortedStrAr));

            Console.ReadLine();
        }

    }
    class GenericMergeSort<T> where T : IComparable
    {
        public static T[] Sort(T[] ar)
        {
            var arLen = ar.Length;
            if (arLen < 2)
                return ar;

            var arL = ar.AsSpan(0..(arLen / 2)).ToArray();
            var arR = ar.AsSpan((arLen / 2)..arLen).ToArray();

            arL = Sort(arL);
            arR = Sort(arR);

            return Merge(arL, arR);
        }

        private static T[] Merge(T[] arL, T[] arR)
        {
            var mergedArray = new T[arL.Length + arR.Length];
            int iM = 0, iL = 0, iR = 0;

            while (iL < arL.Length || iR < arR.Length)
            {
                mergedArray[iM++] = iL < arL.Length && (iR > arR.Length - 1 || arL[iL].CompareTo(arR[iR]) < 0)
                                    ? arL[iL++] : arR[iR++];
            }
            return mergedArray;
        }
    }
}

Here is the output

-7,-1,0,1,2,4,4,4,5,6,7,8,8,10,11,13,45,56,57

A,And,Cat,Clever,Fast,Here,Is,Really

Answer 5

public static int[] mergeSort(int[] ar)
{
    Func<int[], int[]> firstHalf = (array) =>
     {
         return array.Take((array.Length + 1) / 2).ToArray();
     };

    Func<int[], int[]> secondHalf = (array) =>
    {
        return array.Skip((array.Length + 1) / 2).ToArray();                
    };

    Func<int[], int[], int[]> mergeSortedArrays = (ar1, ar2) =>
    {
        int[] mergedArray = new int[ar1.Length + ar2.Length];

        int i1 = 0, i2 = 0, currentMin;

        while (i1 < ar1.Length || i2 < ar2.Length)
        {
            if (i1 < ar1.Length && i2 < ar2.Length)
            {
                if (ar1[i1] < ar2[i2])
                {
                    currentMin = ar1[i1];
                    i1++;
                }
                else
                {
                    currentMin = ar2[i2];
                    i2++;
                }
            }
            else if (i1 < ar1.Length)
            {
                currentMin = ar1[i1];
                i1++;
            }
            else
            {
                currentMin = ar2[i2];
                i2++;
            }
            mergedArray[i1 + i2 - 1] = currentMin;
        }
        return mergedArray;
    };

    int[] half1 = firstHalf(ar); //always /geq than half2
    int[] half2 = secondHalf(ar);

    if (half1.Length < 2)
        return mergeSortedArrays(half1, half2);
    else
        return mergeSortedArrays(mergeSort(half1), mergeSort(half2));

}

Answer 6

A simple C# merge sort.

using System.Collections.Generic;

namespace Sort
{
    public class MergeSort
    {
        public static List<int> mergeSort(List<int> list)
        {
            if (list.Count <= 1) return list;

            List<int> result;
            List<int> left = new List<int>();
            List<int> right = new List<int>();

            int midPoint = list.Count / 2;

            left.AddRange(list.GetRange(0, midPoint));
            right.AddRange(list.GetRange(midPoint, list.Count - midPoint));

            left = mergeSort(left);

            right = mergeSort(right);

            result = merge(left, right);
            return result;
        }

        //This method will be responsible for combining our two sorted arrays into one giant array
        public static List<int> merge(List<int> left, List<int> right)
        {
            List<int> result = new List<int>();

            int indexLeft = 0, indexRight = 0, indexResult = 0;
            while (indexLeft < left.Count || indexRight < right.Count)
            {
                //if both arrays have elements  
                if (indexLeft < left.Count && indexRight < right.Count)
                {
                    if (left[indexLeft] <= right[indexRight])
                    {
                        result.Add(left[indexLeft]);
                        indexLeft++;
                    }
                    else
                    {
                        result.Add(right[indexRight]);
                        indexRight++;
                    }
                }
                else if (indexLeft < left.Count)
                {
                    result.Add(left[indexLeft]);
                    indexLeft++;
                }
                else if (indexRight < right.Count)
                {
                    result.Add(right[indexRight]);
                    indexRight++;
                }
                indexResult++;
            }
            return result;
        }
    }
}