NullReferenceException

Question

0.00/5 (No votes)

See more:

Hi,
I am writing a console program which takes an Infix expression ( such as "(a+b)" ) and displays RPN of that expression, well actually I am at the first phase of the program where I am checking if the given expression is parenthesized correctly or not.A NullRefereceException is being generated at expLength = expression.Length;. Why is the console not asking me to input my expression, thus resulting in a null value? I could not understand it from Google.
Thanks in advance!
The code is as in the following:

C#

static class Program
    {
        static void Main()
        {
            int squareB = 0, curlyB = 0, parenth = 0, expLength;
            string expression;
            //Get the Infix Expression
            System.Console.Write("Enter the Infix Expression : ");
            expression = System.Console.ReadLine();
            //Get the Length of the string
            expLength = expression.Length;
            //Checking expression
            for (int i = 0; i < expLength ; i++)
            {
                //Getting single character of the expression
                char expressionChar = expression[i];
                //Checking if Infix Expression is logically correct
                if (expressionChar == '[')
                    squareB++;
                if (expressionChar == ']')
                    squareB--;

                if (expressionChar == '{')
                    curlyB++;
                if (expressionChar == '}')
                    curlyB--;

                if (expressionChar == '(')
                    parenth++;
                if (expressionChar == ')')
                    parenth--;
            }
            if (squareB == 0)
                if (curlyB == 0)
                    if (parenth ==0)
                Console.Write("\n The Infix Expression is fully parentesized, RPN is being generated. ");
            else
                Console.Write("\n The Infix Expression is not fully parenthesized, hence RPN cannot be generated. ");

        }
    }

Posted 26-Nov-10 23:22pm

PrinceFayyaz

Updated 27-Nov-10 0:30am

#realJSOP

v2

Add a Solution

Comments

velvet7 27-Nov-10 6:13am

I tried your code, and it works perfectly for me.

2 solutions

Solution 1

C#

int squareB = 0, curlyB = 0, parenth = 0;
      string _sInputDetails; // Expression from user

      System.Console.CursorVisible = false;
      System.Console.ForegroundColor = ConsoleColor.DarkGreen;
      System.Console.BackgroundColor = ConsoleColor.Black;

   Begin: // Marker togo if we want more inputs.

      // Get the Infix Expression
      System.Console.Write("Enter the Infix Expression : ");

      _sInputDetails = System.Console.ReadLine();

      if (_sInputDetails.ToUpper().Trim() == "Q" ||
         _sInputDetails.ToUpper().Trim() == "QUIT")
      {
         Console.WriteLine("");
         Console.WriteLine("Good Bye");
         System.Threading.Thread.Sleep(1000); // Small Pause
         return;
      }

      // Checking expression

      for (int _iStepExpression = 0; _iStepExpression < _sInputDetails.Length; ++_iStepExpression)
      {
         // Research Array.FindAll();

         // Checking if Infix Expression is logically correct - More efficient as you dont bother with multiple condition validations.
         switch (_sInputDetails[_iStepExpression])
         {
            case '[':
               {
                  squareB++;

                  if(squareB > 0)
                     System.Console.Beep(squareB, 100);

                  break;
               }
            case ']':
               {
                  squareB--;

                  if (squareB > 0)
                     System.Console.Beep(squareB, 100);

                  break;
               }
            case '{':
               {
                  curlyB++;

                  if (curlyB > 0)
                     System.Console.Beep(curlyB, 100);

                  break;
               }
            case '}':
               {
                  curlyB--;

                  if (curlyB > 0)
                     System.Console.Beep(curlyB, 100);

                  break;
               }
            case '(':
               {
                  parenth++;

                  if (parenth > 0)
                     System.Console.Beep(parenth, 100);

                  break;
               }
            case ')':
               {
                  parenth--;

                  if (parenth > 0)
                     System.Console.Beep(parenth, 100);

                  break;
               }
         } // switch (_sInputDetails[_iStepExpression])
      } // for (int _iStepExpression = 0; _iStepExpression < _sInputDetails.Length; ++_iStepExpression)

      string _sOutputResult = string.Empty;

      if (squareB == 0)
         if (curlyB == 0)
            if (parenth == 0)
               _sOutputResult =
                  System.Environment.NewLine +
                  "The Infix Expression is fully parentesized, RPN is being generated.";
            else
               _sOutputResult =
                  System.Environment.NewLine +
                  "The Infix Expression is not fully parenthesized, hence RPN cannot be generated.";

      Console.WriteLine(_sOutputResult);
      Console.WriteLine("");
      goto Begin;

Posted 27-Nov-10 0:07am

xcorporation

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

#realJSOP · Accepted Answer · 2010-11-27T00:29:00

All you have to do to fix this is before using expresion, do this:

if (string.IsNullOrEmpty(expression))
{
  // parse your string here
}

And by the way, you parsing (as it stands right now) is going to give you a false valid if your brackets occur out of order, even if there are enough of them. So, this:

]bracket[ blah blah blah

will report valid, just like this will:

[bracket] blah blah blah