C# program to convert decimal to Octal number

A decimal number can be converted to an octal number using the division method. Octal is a base-8 number system that uses digits 0-7. To convert from decimal to octal, we repeatedly divide the decimal number by 8 and collect the remainders.

How It Works

The conversion process involves dividing the decimal number by 8 repeatedly and storing the remainders. The octal equivalent is formed by reading the remainders in reverse order −

Using Manual Division Method

Example

using System;

class Demo {
   public static void Main() {
      int decVal, quot, i = 1, j;
      int[] octalVal = new int[80];
      
      decVal = 40;
      quot = decVal;
      
      Console.WriteLine("Decimal Number: {0}", decVal);
      
      while (quot != 0) {
         octalVal[i++] = quot % 8;
         quot = quot / 8;
      }
      
      Console.Write("Octal Number: ");
      for (j = i - 1; j > 0; j--) {
         Console.Write(octalVal[j]);
      }
      Console.WriteLine();
   }
}

The output of the above code is −

Decimal Number: 40
Octal Number: 50

Using Convert.ToString() Method

C# provides a built-in method to convert decimal to octal using Convert.ToString() with base 8 −

Example

using System;

class Program {
   public static void Main() {
      int decimalNumber = 40;
      string octalNumber = Convert.ToString(decimalNumber, 8);
      
      Console.WriteLine("Decimal Number: " + decimalNumber);
      Console.WriteLine("Octal Number: " + octalNumber);
      
      // Converting multiple numbers
      int[] numbers = {15, 64, 255};
      Console.WriteLine("\nMultiple conversions:");
      
      foreach(int num in numbers) {
         Console.WriteLine("{0} (decimal) = {1} (octal)", 
                          num, Convert.ToString(num, 8));
      }
   }
}

The output of the above code is −

Decimal Number: 40
Octal Number: 50

Multiple conversions:
15 (decimal) = 17 (octal)
64 (decimal) = 100 (octal)
255 (decimal) = 377 (octal)

Using Custom Method with String Builder

Example

using System;
using System.Text;

class OctalConverter {
   public static string DecimalToOctal(int decimalNum) {
      if (decimalNum == 0) return "0";
      
      StringBuilder octal = new StringBuilder();
      
      while (decimalNum > 0) {
         octal.Insert(0, decimalNum % 8);
         decimalNum /= 8;
      }
      
      return octal.ToString();
   }
   
   public static void Main() {
      int[] testNumbers = {0, 8, 40, 100, 512};
      
      Console.WriteLine("Decimal to Octal Conversion:");
      Console.WriteLine("----------------------------");
      
      foreach(int num in testNumbers) {
         string octalResult = DecimalToOctal(num);
         Console.WriteLine("{0,3} decimal = {1} octal", num, octalResult);
      }
   }
}

The output of the above code is −

Decimal to Octal Conversion:
----------------------------
  0 decimal = 0 octal
  8 decimal = 10 octal
 40 decimal = 50 octal
100 decimal = 144 octal
512 decimal = 1000 octal

Comparison of Methods

Conclusion

Converting decimal to octal in C# can be done using manual division by 8, the built-in Convert.ToString() method, or custom implementations. The Convert.ToString(decimalNumber, 8) method is the most practical approach for most applications, while manual methods help understand the underlying mathematical process.