How can I find the largest number?

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Hello, I am learning c# and I am just a beginner.
In this code, my function gets an array of strings as the parameter then inside the function I have a variable named

C#

result

which in every iteration a new string will be concatenated to it. The problem that I have is that if I set

C#

digits = {"21" , "2"}

then the output will be

C#

but it must be

C#

but I do not know how to change my

C#

if

statement to correct the code now the code works fine for an array like

C#

{"3" , "23"}

And here is my full code :

using System;
using System.Collections.Generic;
using System.Linq;

namespace codeproject
{

    class Program
    {


        /// <summary>
            // LargestNumber(Digits):
            // answer ← empty string
            // while Digits is not empty:
                // maxDigit ← −∞
                // for digit in Digits:
                    // if digit ≥ maxDigit:
                        // maxDigit ← digit
                // append maxDigit to answer
                // remove maxDigit from Digits
            // return answer
        /// </summary>
        static string LargestNumber(string[] digits)
        {
            List<string> mylist = digits.ToList();
            string result="";
            while(mylist.Count != 0)
            {
                string max = "       ";
                foreach(var d in mylist)
                {
                    if(d[0] - '0' > max[0] - '0')
                    {
                        max = d;
                    }
                }
                result+=max;
                mylist.Remove(max);
            }
            return result;
        }

        static void Main(string[] args)
        {
            
            string[] myarray = new string[]{"23" , "3"};
            System.Console.WriteLine(LargestNumber(myarray));
        }
    }
}

What I have tried:

First, this code did not give me a correct output when I set

C#

digits={"23", "3"}

so I compare the first digit of every

C#

string

in

C#

if

statement. I will be grateful for your help and advice.

Posted 29-Aug-21 12:46pm

Aylin Naebzadeh

Updated 30-Aug-21 15:37pm

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4 solutions

Solution 4

Assuming you want the largest number one can obtain by reshuffling the elements of the array (without breaking them up to individual digits), the job isn't straightforward.

Try an array holding 6 and 65; it should yield 665.
Now consider 6 and 67; it should yield 676.
The different precedence is caused by the second digit in 67 being larger than the first one, which isn't the case in 65.

The correct solution requires a special comparer that basically tries both orderings of two strings, like so:

C#

		public int ConcatComparer(string s1, string s2) {
			//long diff = long.Parse(s2+s1)-long.Parse(s1+s2);
			//if (diff>0) return 1;
			//if (diff<0) return -1;
			//return 0;

			// no need for numeric values, textual string.compare is
			// equivalent when strings have equal length; it is faster
			// and isn't limited to the int/long range

			return string.Compare(s2+s1, s1+s2);

}

and it gets used like this:

C#

				Array.Sort(myarray, ConcatComparer);
				s=string.Join("", myarray);
}

This yields the correct result even for

C#

string[] myarray = new string[] { "23", "1", "101", "3", "6", "67", "65" };

:)

Posted 30-Aug-21 15:37pm

Luc Pattyn

Updated 31-Aug-21 1:55am

v13

Comments

Maciej Los 31-Aug-21 7:57am

5ed!

Aylin Naebzadeh 31-Aug-21 11:13am

Thank you, Luc Pattyn. Yes, when I have tried the solution of Wendelius it was a bit slow I think that was because of the recursive strategy. Now if I give an array to the input of this function I should compare the elements two by two using a for loop block am I right?

Luc Pattyn 31-Aug-21 11:30am

The built-in Array.Sort() method sorts the array, which means it rearranges the content of the array according to the result of the comparer given to it; everything you need has been shown, it is just one method plus two lines of code!

Aylin Naebzadeh 31-Aug-21 11:41am

Yes, thank you again.

Solution 2

The simplest solution is to sort them - there are Linq methods to do that, and combined with the string.Join method it's one line of code:

C#

string[] myarray = new string[] { "23", "1", "101", "3" };
string s = string.Join("", myarray.OrderBy(a => int.Parse(a)));

Posted 29-Aug-21 19:57pm

OriginalGriff

Comments

CPallini 30-Aug-21 2:08am

5.

Maciej Los 30-Aug-21 3:19am

5ed!

Aylin Naebzadeh 30-Aug-21 3:30am

I have tried your code but after sorting this array and when I run the code the output is: 1323101, but the largest that can be made is 3231101🤔am I right?

BillWoodruff 30-Aug-21 16:44pm

the largest is 3321110: see my post below.

Aylin Naebzadeh 30-Aug-21 3:38am

Maybe I did not ask my question in the post clearly. But something that I want from my code is to return the largest number that can be made by concatenating the elements in the array.

BillWoodruff 30-Aug-21 20:18pm

My vote of #1.

imho, this is so far below the usual high standard of responses by OriginalGriff, I have trouble believing my eyes ;)

Solution 3

A word of caution: the technique I'll show you here is how I would do this, but, if this is homework, and the assignment requires you use a while loop, and, you turn this in: it's going to be obvious you didn't write this, and, I'd guess you would not be able to explain how this works.

// required: using System.Linq;

string[] myarray = new string[] { "23", "1", "101", "3" };
string s = string.Join("", myarray);

UTF8Encoding utf8 = new UTF8Encoding();

byte[] ba = utf8.GetBytes(s).OrderByDescending(b => b).ToArray();

string s2 = utf8.GetString(ba); // => 3321110

Posted 30-Aug-21 10:42am

BillWoodruff

Comments

Luc Pattyn 31-Aug-21 7:57am

hmm.
you might simplify that using String.ToCharArray()...

:)

Maciej Los 31-Aug-21 7:57am

5ed!

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Wendelius · Accepted Answer · 2021-08-29T17:42:00

Why do you do this with strings? It would be easier if all the elements would be numbers. Consider the following example

C#

double[] digits = new double[]{ 3, 23 };
double newNumber;

newNumber = digits.Max() + (digits.Min() * System.Math.Pow(10, digits.Max().ToString().Length));

Note that I've used Linq to fetch the max and min values from the array. For more information, see Enumerable.Max Method (System.Linq) | Microsoft Docs[^]

--------------------------------------------
Added based on the updated requirement
--------------------------------------------

This could of course be a standard method but with an extension it could look something like

C#

/// <summary>
/// Class for extensions
/// </summary>
public static class Extensions {
   /// <summary>
   /// Finds the greatest number that can be found by concatenating the string in different orders
   /// </summary>
   /// <param name="values">List of strings</param>
   /// <returns>The concatenation that results to the greatest number</returns>
   public static string MaxCombination(this List<string> values) {
      double max = 0;
      List<string> newList;
      string combination;

      if (values.Count() == 1) {
         return values[0];
      }
      foreach (string value in values) {
         newList = new List<string>();
         newList.AddRange(values);
         newList.Remove(value);
         combination = newList.MaxCombination();
         combination = value + combination;
         if (double.Parse(combination) > max) {
            max = double.Parse(combination);
         }
      }

      return max.ToString();
   }
}

So calling it could look like

C#

List<string> theList = new List<string> { "3", "23", "101", "1" };
string max = theList.MaxCombination();
Trace.WriteLine(max);

And the output would be

How can I find the largest number?

4 solutions

Solution 4

Solution 1

Solution 2

Solution 3

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