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

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.
NSing9Asked:
Who is Participating?

Improve company productivity with a Business Account.Sign Up

x
 
jkrConnect With a Mentor Commented:
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;
}

Open in new window

0
 
jkrCommented:
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
}

Open in new window

0
 
NSing9Author Commented:
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?
0
What Kind of Coding Program is Right for You?

There are many ways to learn to code these days. From coding bootcamps like Flatiron School to online courses to totally free beginner resources. The best way to learn to code depends on many factors, but the most important one is you. See what course is best for you.

 
jkrCommented:
How are you reading the epresson?
0
 
NSing9Author Commented:
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;
}
0
 
jkrCommented:
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;
}

Open in new window

0
 
NSing9Author Commented:
I get a fatal error when I try to modfy it this way:

fatal error LNK1104: cannot open file 'C:\Documents and Settings......
0
 
NSing9Author Commented:
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.
0
 
NSing9Author Commented:
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.
0
 
jkrCommented:
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?
0
 
NSing9Author Commented:
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
 
NSing9Author Commented:
Still couldn't get it to work.
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

All Courses

From novice to tech pro — start learning today.