Inserting element in sorted Generic list (List) using binary search


I got a mail asking:

"How we could insert an item in sorted generic list such that after insertion list would be remaining sorted?"

The answer to this is using Binary Search 

As we know Binary search works on a Divide and conquer algorithm. And running time using Binary search is very efficient. 

So we need to follow below algorithm 

Step 1 

Sort the list 

Step 2

Save the value to be inserted in a variable 

Step 3

Do the binary search of the inserted variable in the list.

1.gif 

Step 4

Insert the item at the compliment of the number returned by the binary search. 

Example #1: Inserting a string in sorted list of string.

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections.ObjectModel;

namespace ConsoleApplication21
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> lstMyString = new List<string>();
            lstMyString.Add("Apple");
            lstMyString.Add("Mango");
            lstMyString.Add("Banana");
            lstMyString.Add("papya ");
            lstMyString.Sort();
            foreach (var r in lstMyString)
            {
                Console.WriteLine(r);
            }
            Console.ReadKey(true);
        }
    }
}
            
Output 

2.gif
 
Now if in above sorted list we need to add one more string item cashew. 

3.gif 

So first we need to perform the binary search and then find the index of the next top element by negating the integer returned by the Binary search. 

Program.cs

namespace ConsoleApplication21
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> lstMyString = new List<string>();
            lstMyString.Add("Apple");
            lstMyString.Add("Mango");
            lstMyString.Add("Banana");
            lstMyString.Add("papya ");
            lstMyString.Sort();
            string strToInsert = "Cashew";
            int binraySearchIndex =  lstMyString.BinarySearch(strToInsert);
            if (binraySearchIndex < 0)
            {
                lstMyString.Insert(~binraySearchIndex, strToInsert);
            }
            foreach (var r in lstMyString)
            {
                Console.WriteLine(r);
            }
            Console.ReadKey(true);
        } 
    }
}

Output 

4.gif

Example #2:  Inserting a custom class in list of custom class.

I have a class called Product 

Product.cs

class Product
{
    public string ProductName { get; set; }
    public int ProductPrice { get; set; }
}

And List of Product as below, 

List<Product> prdList = new List<Product>()
            {
                new Product {ProductName = "Apple",ProductPrice = 101},
                new Product {ProductName = "Apple",ProductPrice = 99},
                new Product {ProductName = "Pen",ProductPrice = 99},
                new Product {ProductName = "Pencil", ProductPrice = 100},
                new Product {ProductName ="Apple", ProductPrice = 100},
                new Product { ProductName = "Mango", ProductPrice = 35},
                new Product {ProductName = "Shirt", ProductPrice=200}
            }; 

Now first let us sort this list. 

5.gif
 
How to sort a Generic List? Read here 

Now in sorted list of Product we need to insert one more product such that it should be inserted in sorted order in the list. 

6.gif
 
Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Collections.ObjectModel;

namespace ConsoleApplication21
{
    class Program
    
        static void Main(string[] args)
        {
            int tempPrevious = 0;
            int tempcurrent = 0;
            IComparer<Product> compare = new CompareProduct();
            List<Product> prdList = new List<Product>()
            {
                new Product {ProductName = "Apple",ProductPrice = 101},
                new Product {ProductName = "Apple",ProductPrice = 99},
                new Product {ProductName = "Pen",ProductPrice = 99},
                new Product {ProductName = "Pencil", ProductPrice = 100},
                new Product {ProductName ="Apple", ProductPrice = 100},
                new Product { ProductName = "Mango", ProductPrice = 35},
                new Product {ProductName = "Shirt", ProductPrice=200}
            };
            prdList.Sort(compare.Compare);
            int search = 0;
            Product productToInsert = new Product {
                                        ProductName = "Ball",
                                        ProductPrice = 30 };
            search = prdList.BinarySearch(productToInsert, (IComparer<Product>)compare);
            if (search < 0)
            {
                prdList.Insert(~search, productToInsert);
            }
            foreach (Product p in prdList)
            {
                tempcurrent = p.ProductPrice;
                if (tempcurrent != tempPrevious)
                {
                    Console.WriteLine("**********************");
                    Console.WriteLine("Price = " + p.ProductPrice);
                }
                Console.WriteLine(p.ProductName);
                tempPrevious = p.ProductPrice;
            }
            Console.ReadKey(true);
        }
    }
}

Output 

7.gif 

For reference generic sort class for list of Product is as below 

class CompareProduct : IComparer<Product>
{
    public   int Compare(  Product p1,   Product p2)
    {
        int result;
        if (Product.ReferenceEquals(p1, p2))
        {
            result = 0;
        }
        else
        {
            if (p1 == null)
            {
                result = 1;
            }
            else if (p2 == null)
            {
                result = -1;
            }
            else
            {
                result = NumberCompare(p1.ProductPrice, p2.ProductPrice);
                //result = StringCompare(p1.ProductName, p2.ProductName);
                if (result == 0)
                {
                   // result = NumberCompare(p1.ProductPrice, p2.ProductPrice);
                    result = StringCompare(p1.ProductName, p2.ProductName);
                }
            }
        }
        return result;
    }
    int StringCompare(string strFirstString, string secondString)
    {
        int result;
        if (strFirstString == null)
        {
            if (secondString == null)
            {
                result = 0;
            }
            else
            {
                result = 1;
            }
        }
        else
        {
            result = strFirstString.CompareTo(secondString);
        }
        return result;
    }
    int NumberCompare(int number1, int number2)
    {
        int result;
        if (number1 > number2)
        {
            result = 1;
        }
        else if (number1 < number2)
        {
            result = -1;
        }
        else
        {
            result = 0;
        }
        return result;
    }
}

Similar Articles