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How do I check for a missing character and throw an exception if the character is not in the string?

Posted on 2010-11-14
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Last Modified: 2012-05-10
How do I check for a missing character and throw an exception if the character is not in the string?  I have a program where i enter an expression and I want it to display and error if the end of the expression is missing a ';'

ex. (x+2), x=2

This should throw an exception when I click enter because it is missing the ';' at the end of the expression. ie. (x+2), x=2;

I have tried to account for the missing ';' in a for loop, such as:

if (cin.peek() != ';')      
{
     throw SyntaxException();
}

but, my exception is thrown even on valid expressions that have a ';' at the end.  I assume that it is because even on valid expressions that end with ';', once enter is clicked, the next character will never be ';' which is what peek() is looking for.
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Question by:NSing9
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12 Comments
 
LVL 86

Expert Comment

by:jkr
ID: 34131284
Well, I don't know how you are reading your inpur, but you could just check the last character in the input line, e.g.
#include <iostream>
#include <string>
using namespace std;

//...

string sLine;

getline(cin,sLine);

for (string::reverse_iterator ri = sLine.rbegin(); ri != sLine.rend(); ++ri)
{
  if (*ri == ' ') continue; // skip whitespace

  if (*ri != ';')  throw SyntaxException();
   else break; // found semicolon, all is fine
}

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Author Comment

by:NSing9
ID: 34131327
I think this might work.  I am entering the input from the console and it is being stored as an expression.  I have to try to figure out how to convert the expression to a string?
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LVL 86

Expert Comment

by:jkr
ID: 34131379
How are you reading the epresson?
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Author Comment

by:NSing9
ID: 34131442
cout << "Enter expression: ";

The expression is entered from the keyboard when this statement is run.  Below is the main function that I have:

int main()
{
    Expression* expression;
    char paren, comma;
      
    cout << "Enter expression: ";
    cin >> paren;
   
      try
      {
            if (cin.peek() == '(')  //mistmatch left paren exception
            {
                  throw SyntaxException();
            }

            expression = SubExpression::parse();
            
            if (cin.peek() == ')')       //mistmatch right paren exception
            {
                  throw SyntaxException();
            }

            if (cin.peek() != ',')            //missing comma exception
            {
                  throw SyntaxException();
            }

            cin >> comma;  
            parseAssignments();
            double result = expression->evaluate();
            cout << "Value = " << result << endl;
      }
      catch (DivideByZeroException &divideByZeroException)
      {
            cout << "Error: " << divideByZeroException.what() << endl;
      }
      catch (UninitializedException &uninitializedException)
      {
            cout << "Error: " << uninitializedException.what() << endl;
      }
      catch (SyntaxException &syntaxException)
      {
            cout << "Error: " << syntaxException.what() << endl;
      }
    return 0;
}
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LVL 86

Expert Comment

by:jkr
ID: 34131526
You could then just read a complete line and replace 'cin' by a stringstream, e.g.
#include <sstream>
using namepace std;

// ...

int main()
{
    Expression* expression;
    stringstream ss;
    string sLine;
    char paren, comma;
      
    cout << "Enter expression: ";
   
    ss >> paren;
   
      try
      {
            getline(cin,sLine);

            for (string::reverse_iterator ri = sLine.rbegin(); ri != sLine.rend(); ++ri)
            {
              if (*ri == ' ') continue; // skip whitespace

              if (*ri != ';')  throw SyntaxException();
               else break; // found semicolon, all is fine
            }

            ss.str(sLine.c_str());

            if (ss.peek() == '(')  //mistmatch left paren exception
            {
                  throw SyntaxException();
            }

            expression = SubExpression::parse();
            
            if (ss.peek() == ')')       //mistmatch right paren exception
            {
                  throw SyntaxException();
            }

            if (ss.peek() != ',')            //missing comma exception
            {
                  throw SyntaxException();
            }

            ss >> comma;  
            parseAssignments();
            double result = expression->evaluate();
            cout << "Value = " << result << endl;
      }
      catch (DivideByZeroException &divideByZeroException)
      {
            cout << "Error: " << divideByZeroException.what() << endl;
      }
      catch (UninitializedException &uninitializedException)
      {
            cout << "Error: " << uninitializedException.what() << endl;
      }
      catch (SyntaxException &syntaxException)
      {
            cout << "Error: " << syntaxException.what() << endl;
      }
    return 0;
}

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Author Comment

by:NSing9
ID: 34131737
I get a fatal error when I try to modfy it this way:

fatal error LNK1104: cannot open file 'C:\Documents and Settings......
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Author Comment

by:NSing9
ID: 34131797
My apologies.  I fixed the fatal error, but it appears that it throws the exception now when ';' is missing from the end of the expression, but it's not evaluating the expression and displaying the value as it did before when there is a correctly placed ';' at the end of the expression.
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LVL 86

Accepted Solution

by:
jkr earned 750 total points
ID: 34132028
Oops, sorry, little correction:
#include <sstream>
using namepace std;

// ...

int main()
{
    Expression* expression;
    stringstream ss;
    string sLine;
    char paren, comma;
      
    cout << "Enter expression: ";
   
      try
      {
            getline(cin,sLine);

            for (string::reverse_iterator ri = sLine.rbegin(); ri != sLine.rend(); ++ri)
            {
              if (*ri == ' ') continue; // skip whitespace

              if (*ri != ';')  throw SyntaxException();
               else break; // found semicolon, all is fine
            }

            ss.str(sLine.c_str());

            ss >> paren; // should be here...

            if (ss.peek() == '(')  //mistmatch left paren exception
            {
                  throw SyntaxException();
            }

            expression = SubExpression::parse();
            
            if (ss.peek() == ')')       //mistmatch right paren exception
            {
                  throw SyntaxException();
            }

            if (ss.peek() != ',')            //missing comma exception
            {
                  throw SyntaxException();
            }

            ss >> comma;  
            parseAssignments();
            double result = expression->evaluate();
            cout << "Value = " << result << endl;
      }
      catch (DivideByZeroException &divideByZeroException)
      {
            cout << "Error: " << divideByZeroException.what() << endl;
      }
      catch (UninitializedException &uninitializedException)
      {
            cout << "Error: " << uninitializedException.what() << endl;
      }
      catch (SyntaxException &syntaxException)
      {
            cout << "Error: " << syntaxException.what() << endl;
      }
    return 0;
}

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Author Comment

by:NSing9
ID: 34132133
I'm still having the same problem.  Does the fact that after I enter an expression such as:

(x+2), x=2;

I am immediately clicking <enter> after keying in the ';' at the end of the expression.  This is a valid expression and should display results but it does nothing after clicking <enter>.  It works correctly without making the modifications but then it doesn't throw the exception for the missing semicolon.
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LVL 86

Expert Comment

by:jkr
ID: 34132250
That's weird, when I strip down the code to have a test, it seems to work. Can you step through the modified code to see where it stops working?
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Author Comment

by:NSing9
ID: 34132340
There is another method that follows the main class that I didn't provide initially.  It is as follows:

void parseAssignments()
{
    char assignop, delimiter;
    string variable;
    double value;

    do
    {
        variable = parseName();
        cin >> ws >> assignop >> value >> delimiter;
            symbolTable.insert(variable, value);
    }
    while (delimiter == ',');
}

I'm not sure if or where this would affect the modifications, but I have tired it changed cin to ss and just leaving it as is, but I still get the same result.
0
 

Author Closing Comment

by:NSing9
ID: 34178903
Still couldn't get it to work.
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