How do I subtract two large numbers represented as strings in C#

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i need help with this program

subtracting 2 large numbers using string

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

namespace SUBTRACTION PROGRAM
{
class Arithmetic
{
static void Main(string[] args)
{
const int LENGTH = 1000;
String In1;
String In2;
String Out;
int[] Top = new int[LENGTH];
int[] Bottom = new int[LENGTH];
int[] Dif = new int[LENGTH - 1];
StringBuilder temp;
int borrow = 0;
int TopLen = 0;
int BottomLen = 0;
int LenDif = 0;
int SumLen = 0;
int TopIn = 0;
int BottomIn = 0;

Console.WriteLine("Doing Subtraction\n");
Console.Write("Enter the First Number: ");
In1 = Console.ReadLine();
Console.Write("Enter the Second Number: ");
In2 = Console.ReadLine();

//GETTING LARGER NUMBER ON TOP

// if(In1 < In2) //first shorter than second
{

for (int i = 0; i < TopLen; i++)
{
if (In1[i] < In2[i]) ;
Top[i] = In2[i] - '0';
}
LenDif = TopLen - BottomLen;

for (int i = 0; i < LenDif; i++)
{
Bottom[i] = 0; // pad bottom with zeros on left
}
for (int i = 0; i < BottomLen; i++)
{
Bottom[i + LenDif] = In1[i] - '0';
}
}
else

// if (In2 < In1 )
{
TopLen = In1.Length;
BottomLen = In2.Length;
for (int i = 0; i < TopLen; i++)
{
Top[i] = In1[i] - '0';
}
LenDif = TopLen - BottomLen;
for (int i = 0; i < LenDif; i++)
{
Bottom[i] = 0;
}
for (int i = 0; i < BottomLen; i++)
{
Bottom[i + LenDif] = In2[i] - '0';
}
}

// start subtraction from the left and work back to right

//---------------DOING ARITHMETIC---------------------------
for (int i = TopLen -1; i >= 0; i--)
{
//two inputs are same
if (In1[i] == In2[i])
{
Out = 0; // figure out.....
}

//one of the numbers is zero
else
{
if (BottomIn == 0)
{
Out = Top[i];
}
}
else
{
if (TopIn == 0)
{
Bottom[i] = Out;
}
}

}

for (int i = TopLen -1 -BottomLen)
{
//two digits plus borrow
if(i > TopLen -1 -BottomLen)
{
Dif[i] = Top[i] - Bottom[i];
if (Top[i] == 0)
{
borrow = 1;
Top[i] = 10;
Top[i] -= 9;
}
else
{
borrow = 0;
}
}

What I have tried:

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

namespace SUBTRACTION PROGRAM 
{
    class Arithmetic
    {
        static void Main(string[] args)
        {
        const int LENGTH = 1000;
        String In1;
        String In2;
        String Out;
        int[] Top = new int[LENGTH];
        int[] Bottom = new int[LENGTH];
        int[] Dif = new int[LENGTH - 1];
        StringBuilder temp;
        int borrow = 0;
        int TopLen = 0;
        int BottomLen = 0;
        int LenDif = 0;
        int SumLen = 0;
        int TopIn = 0;
        int BottomIn = 0;



        Console.WriteLine("Doing Subtraction\n");
        Console.Write("Enter the First Number: ");
        In1 = Console.ReadLine();
        Console.Write("Enter the Second Number: ");
        In2 = Console.ReadLine();


        //GETTING LARGER NUMBER ON TOP 

        // if(In1 < In2)    //first shorter than second
        {
            
            for (int i = 0; i < TopLen; i++)
            {
                if (In1[i] < In2[i]) ;
                Top[i] = In2[i] - '0';
            }
            LenDif = TopLen - BottomLen;

            for (int i = 0; i < LenDif; i++)
            {
                Bottom[i] = 0;               // pad bottom with zeros on left 
            }
            for (int i = 0; i < BottomLen; i++)
            {
                Bottom[i + LenDif] = In1[i] - '0';
            }
        }
        else

         //   if (In2 < In1 )
        {
            TopLen = In1.Length;
            BottomLen = In2.Length;
            for (int i = 0; i < TopLen; i++)
            {
                Top[i] = In1[i] - '0';
            }
            LenDif = TopLen - BottomLen;
            for (int i = 0; i < LenDif; i++)
            {
                Bottom[i] = 0;
            }
            for (int i = 0; i < BottomLen; i++)
            {
                Bottom[i + LenDif] = In2[i] - '0';
            }
        }

        // start subtraction from the left and work back to right

        //---------------DOING ARITHMETIC---------------------------
        for (int i = TopLen -1; i >= 0; i--)
        {
            //two inputs are same 
            if (In1[i] == In2[i])
            {
               Out = 0;       // figure out.....
            }


            //one of the numbers is zero
            else
            {
                if (BottomIn == 0)
                {
                    Out = Top[i];
                }
            }
            else
            {
                if (TopIn == 0)
                {
                    Bottom[i] =  Out;
                }
            }

        }

        for (int i = TopLen -1 -BottomLen)
        {
            //two digits plus borrow
            if(i > TopLen -1 -BottomLen)
            {
                Dif[i] = Top[i] - Bottom[i];
                if (Top[i] == 0)
                {
                    borrow = 1;
                    Top[i] = 10;
                    Top[i] -= 9;
                }
                else
                {
                    borrow = 0;                
                }            
            }

Posted 7-May-19 11:29am

Eniiyinola Okulate

Updated 7-May-19 19:14pm

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Comments

Patrice T 7-May-19 17:40pm

describe the problem you have.
Show sample input and output actual and expected.

[no name] 7-May-19 18:20pm

Looks almost like assembler. I think you'll do well. (but you didn't initialize "TopLen")

4 solutions

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MadMyche · Answer 1 · 2019-05-07T14:24:00

The best thing to do is work with numbers as numbers. There are a few methods available such as int.Parse() or int.TryParse() which can assist.

I prefer to use the latter as it will return a boolean indicating if the operation was successful.

The way I rewrote a chunk of your code is using 2 instances TryParse() method to not only retrieve the values but to also return an error message if either value cannot be parsed.

C#

Console.WriteLine("Doing Subtraction\n");
Console.Write("Enter the First Number: ");
In1 = Console.ReadLine();
Console.Write("Enter the Second Number: ");
In2 = Console.ReadLine();

// new variables to store the actual values entered as numbers
int Input1 = 0;
int Input2 = 0;
int SubtractionValue = 0;
int AdditionValue = 0;

if ((!int.TryParse(In1, out Input1)) || (!int.TryParse(In2, out Input2))) {
	Console.WriteLine("You didn't enter 2 numbers");
} else {
	// do normal math for the rest of program
}

The other numeric types (float, double, Int64...) have similar functionality also.

Reference: Int32.TryParse Method (System) | Microsoft Docs[^]

Christian Graus · Answer 2 · 2019-05-07T11:37:00

You can compare strings. As a string, 1 is less than 22, because the leftmost character is most significant. What you're looking for is int.TryParse. It returns a bool. You can keep asking for a number until the user enters one.

 In1 = Console.ReadLine();
int n = 0;

if (int.TryParse(In1, out n)
{
// n is now the number version of In1
}

andres_fprado · Answer 3 · 2019-05-07T17:45:00

The following program does what you asked for, if I understood it correctly. The code could be simplified and optimized, but I went for readability. Hope it helps!

C#

using System;
using System.Linq;
using System.Text;

namespace substraction_program
   {
   class Arithmetic
      {
      static void Main(string[] args)
         {
         String In1;
         String In2;
         String Out;

         // Get and validate numbers
         Console.WriteLine("Doing Subtraction\n");
         do
            {
            Console.Write("\nEnter the First Number: ");
            In1 = Console.ReadLine();
            } while(!NumberValid(In1));

         do
            {
            Console.Write("\nEnter the Second Number: ");
            In2 = Console.ReadLine();
            } while(!NumberValid(In2));

         int maxLength = Math.Max(In1.Length, In2.Length);
         int[] Num1 = new int[maxLength];
         int[] Num2 = new int[maxLength];
         int[] Diff = new int[maxLength];

         // Load numbers from last to first char, completing with leading zeroes
         int i = maxLength - 1;

         foreach(var c in In1.Reverse())
            {
            Num1[i--] = c - '0';
            }
         while(i >= 0)
            {
            Num1[i--] = 0;
            }

         i = maxLength - 1;
         foreach(var c in In2.Reverse())
            {
            Num2[i--] = c - '0';
            }
         while(i >= 0)
            {
            Num2[i--] = 0;
            }

         // Determine largest number
         int largest = 0;     // -1 if Num1, 0 if equal, 1 if Num2
         for(i = 0; i < maxLength; ++i)
            {
            if(Num1[i] == Num2[i])
               continue;
            else
               {
               if(Num1[i] < Num2[i])
                  largest = 1;
               else
                  largest = -1;
               break;
               }
            }

         // If numbers are equal, fill result with zeroes
         if(largest == 0)
            {
            for(i = 0; i < maxLength; ++i)
               Diff[i] = 0;
            }
         else
            {
            // Start subtraction from RIGHT to left. 
            int borrow = 0;
            for(i = maxLength - 1; i >= 0; --i)
               {
               int n1 = largest > 0 ? Num2[i] : Num1[i]; // Digit from largest
               int n2 = largest > 0 ? Num1[i] : Num2[i]; // Digit from smallest
               n1 -= borrow;
               borrow = 0;

               while(n1 < n2)
                  {
                  borrow += 1;
                  n1 += 10;
                  }
               Diff[i] = n1 - n2;
               }
            if(borrow != 0)
               throw new InvalidOperationException("This should not happen!");
            // Note: this exception means the numbers were not correctly ordered.
            }

         // Store result in string
         StringBuilder sb = new StringBuilder();
         bool leading = true;
         for(i = 0; i < maxLength; ++i)
            {
            if(leading && Diff[i] == 0)
               continue;   // Skip leading zeroes.
            leading = false;
            sb.Append((char)(Diff[i] + '0'));
            }
         if(sb.Length == 0)   // If difference is zero!
            sb.Append('0');
         Out = sb.ToString();

         // Show result
         Console.WriteLine("\nThe result is:");
         Console.WriteLine(Out);
         }

      private static bool NumberValid(string str)
         {
         foreach(var c in str)
            if(c < '0' || c > '9')
               return false;     // Not a valid number!
         return true;            // All numbers -> valid!
         }
      }
   }

Patrice T · Answer 4 · 2019-05-07T19:15:00

Quote:
i need help with this program

I suspect the first problem is that your program do not compile because of this line:

C#

// if(In1 < In2)    //first shorter than second

because the if is needed.

When you ask for help, it is a good idea to tell what is the problem and give error messages with position.

How do I subtract two large numbers represented as strings in C#

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