Ruby-Code

def dec_to_x(decimal, base)
=begin
 In:
  - Decimal number preentered by the user
  - Base of the numbersystem the decimal has to be converted to (2, 8, 16)
 
 Out:
  - converted decimal number
  
=end

#Section - CODE

 #Section - Declaration
    start = decimal
    exponent = 0
    result = ""
    numsys = ""

    hex = {10 => "A", 11 => "B", 12 => "C", 13 => "D", 14 => "E", 15 => "F"}
    
 #Section - Naming Numbersystem
    case base
    when 2
        numsys = "Binary"
    
    when 8
        numsys = "Octal"
        
    when 16
        numsys = "Hexadecimal"
        
    else
        return
        
    end
 #Section - Converting
    if decimal == 0
        result = "0"
        
    elsif decimal > 0
        #calculating the highest fitting exponent for decimal >= base ** exponent
        exponent = Math.log(decimal, base).to_i
        
        loop do
            fits = 0
            
            # how often does the base**exponent fit in the decimal?
            fits += decimal / base ** exponent
            
            # reducing decimal
            decimal -= fits * base ** exponent
            
            #in case of hexadecimal numbersystem converting fits
            if base == 16 && fits >= 10
                fits = hex[fits]
                
            end
            
            #filling result
            result += "#{fits}"
            
            #reducing the exponent by 1
            exponent -= 1
            
            #setting break-condition
            break if exponent < 0
            
        end
        
    else
        return
        
    end
    
 #Section - Output
    puts "#{numsys}: #{result}"
    
end

#getting the decimal number
dec = gets.to_i

#outputting the decimal number
puts "Decimal: #{dec}"

#running method for each numbersystem
dec_to_x(dec , 2)
dec_to_x(dec , 8)
dec_to_x(dec , 16)