Conversion from binary to octal in swift

Swift program for Conversion from binary to octal. Here mentioned other language solution.

import Foundation
//  Swift program for
//  Convert Binary number into Octal number
class Convert
{
    // Convert decimal number into octal
    func octal(_ num : Int) -> Int
    {
      	var number = num;
        var result : Int = 0;
        var multiplier : Int = 1;
        var remainder : Int = 0;
        while (number != 0)
        {
            remainder = number % 8;
            result = (remainder * multiplier) + result;
            multiplier *= 10;
            number = number / 8;
        }
        return result;
    }
    func binaryToOctal(_ num : String)
    {
        if (num.count == 0)
        {
            // When empty binary number
            return;
        }
        // Some useful variable
        var flag : Bool = false;
        var decimalNo : Int = 0;
        var counter : Int = 0;
        var index : Int = num.count - 1;
        // We Assume that given binary number is valid
        // Here - indicates negative binary number
        // First convert binary to decimal
        // Example = 10111 => 23
        while (index >= 0)
        {
            if (Array(num)[index] == "1")
            {
                decimalNo += (1 << counter);
            }
             else
                if (Array(num)[index] != "0")
                {
                    if (index == 0 && Array(num)[index] == "-")
                    {
                        // When get negative number
                        flag = true;
                    }
                     else
                    {
                        // Not a valid binary number
                        return;
                    }
                }
            counter += 1;
            index -= 1;
        }
        // When given number is 
        var output : Int = self.octal(decimalNo);
        if (flag == true)
        {
            output = -output;
        }
        // Display given number
        print("Binary : " + num,terminator: "");
        // Display result
        print(" Octal : " + String(output));
    }
}
let task : Convert = Convert();
// Test Case
task.binaryToOctal("1111");
task.binaryToOctal("10111");
task.binaryToOctal("101011");
task.binaryToOctal("11011");
task.binaryToOctal("-1000110");

Output

Binary : 1111 Octal : 17
Binary : 10111 Octal : 27
Binary : 101011 Octal : 53
Binary : 11011 Octal : 33
Binary : -1000110 Octal : -106