Programming Forums > Application Development > C++

Patfish · Oct 29th, 2010, 10:01 AM

Hello everyone,

i'm trying to get this program, it searches for the final 'a' in an word inputted by the user, to terminate normally with a predefined error message ("This word is too long") if a word longer than 20 chars is entered. I've used a try throw catch statement and I seem to be able to get the program to terminate but I cant get the error message to appear. Can someone help me out with where i'm going wrong please? code and output below

(Toggle Plain Text)

/* 
   
#include <iostream>
#include <cstdlib>

using namespace std;

int main( )
{
  const int WORD_LENGTH   = 20 ;
  const int INVALID_INDEX = -1 ;

  const char NULL_CHAR   = '\0' ;
  const char SEARCH_CHAR = 'a' ;

  char word[ WORD_LENGTH ] ;   

   char* word_ptr     = word ;
   int   indexOf_char = INVALID_INDEX ;

try
  {
   cout << "Input a word: " ;
   cin  >> word ;
   
   if  (word[ WORD_LENGTH] > 20 )
     { 
       throw "Incorrect Input - Word Too Long";
     }
  }

 catch ( const int exception[] )

   {
     cout << exception
          << "Array Out Of Bounds"
          << endl ;

     exit( 0 ) ;
   }
   
   for ( int i = 0 ; *word_ptr != NULL_CHAR ; word_ptr++, i++ )
   {
      if ( *word_ptr == SEARCH_CHAR )
      {
         indexOf_char = i ;
      }
   }
   
   if ( indexOf_char == INVALID_INDEX )
   {
      cout << "There is no '"
	   << SEARCH_CHAR
	   << "' in " 
	   << word 
	   << endl ;
   }
   else
   {
      cout << "Last '"
	   << SEARCH_CHAR
	   << "' in " 
	   << word 
	   << " has index " 
	   << indexOf_char 
	   << endl ;
   }
 
}

Output At Runtime:

(Toggle Plain Text)

Input a word: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
terminate called after throwing an instance of 'char const*'
Aborted

Thanks for any help!

DaWei · Oct 29th, 2010, 11:07 AM

You've only allowed 20 characters for the word. What do you think happens if you exceed that length? Bad things, some more visibly damaging than others.

Since this is C++, why don't you use the string class and avoid the problem sensibly?

Patfish · Oct 29th, 2010, 11:16 AM

Hi Dawei,

The brief i have is to alter the original program (which is the code above minus the try throw catch) so that it terminates with the error message "Array Out Of Bounds" if a word longer than 20 characters is ended. As you can tell I'm pretty new to C++ and although I've covered some of classes its not within the scope of this assignment.
A throw function with this termination statement seemed the best (in my limited knowledge) way of doing this and I have managed to make it terminate but I cant get it to display the error message

DaWei · Oct 29th, 2010, 11:23 AM

The problem is that you're invoking undefined behavior BEFORE you reach your throw. In this case you're clobbering other things on the stack.

In a typical implementation that would begin with SEARCH_CHAR and continue on, depending upon the length entered by the user. If you continue beyond your own variables and clobber the stack frame used by "main" your system is going to run off into the weeds and puke on its shoes.

If you MUST use a C-style string, then use an input function that allows you to restrict the input length, and declare enough chars to hold input of that length. See, for instance, "getline."

Patfish · Oct 29th, 2010, 11:27 AM

thanks Dawei, i'll look into getline

L7Sqr · Oct 29th, 2010, 11:50 AM

You should be catching what you throw. Consider the following:

 cpp Syntax (Toggle Plain Text) 
#include <iostream>
 
int main () {
    try {
        throw "Some error message";
    } catch ( const int msg[]) {
        std::cerr << "Got: " << msg << std::endl;
        return 0;
    }
    std::cerr << "No exception caught" << std::endl;
    return 0;
}
#include <iostream>

int main () {
    try {
        throw "Some error message";
    } catch ( const int msg[]) {
        std::cerr << "Got: " << msg << std::endl;
        return 0;
    }
    std::cerr << "No exception caught" << std::endl;
    return 0;
}

Yields

(Toggle Plain Text)

terminate called after throwing an instance of 'char const*'
Aborted

 cpp Syntax (Toggle Plain Text) 
#include <iostream>
 
int main () {
    try {
        throw "Some error message";
    } catch ( const char * msg) {
        std::cerr << "Got: " << msg << std::endl;
        return 0;
    }
    std::cerr << "No exception caught" << std::endl;
    return 0;
}
#include <iostream>

int main () {
    try {
        throw "Some error message";
    } catch ( const char * msg) {
        std::cerr << "Got: " << msg << std::endl;
        return 0;
    }
    std::cerr << "No exception caught" << std::endl;
    return 0;
}

Yields

(Toggle Plain Text)

Got: Some error message

Notice the difference.

Patfish · Nov 2nd, 2010, 10:28 AM

Thanks for the help on the above query guys, I've got that working nicely now but I also have a related problem on a similar subject.
I've another program which takes 5 in-putted numbers and displays them in reverse order. In its original state it has a cassert statement to terminate the program if anything but a digit is in-putted. My job is to get it to repeatedly ask for a digit until one is entered rather than just terminating. I have tried using another try throw catch with a for loop inside the catch statement but I cant seem to get it to work. I'd appreciate any comments/abuse on what I've got so far!

(Toggle Plain Text)

#include <iostream>

using namespace std;

int main( )
{
    const int NUMB_DIGITS = 5 ;

    int digits[ NUMB_DIGITS ]; 
   
   cout << "Enter " 
	<< NUMB_DIGITS
        << " digits, each on a new line:" 
	<< endl ;

   for ( int i = 0; i < NUMB_DIGITS ; i++ )

   {
     try
       {
         cin >> digits[ i ] ;

         if( ( digits[ i ] >= 0 ) && ( digits[ i ] <= 9 ) ) ;
	 { 
	   throw "Thats Not A Digit";
	 }
        
       }
     
     catch ( const char exception[] )

       {
	 for ( int i = 0; i < NUMB_DIGITS ; i++ );
	 {
	  cin >> digits[ i ];
	 }
       }

   }

   cout << "Digits in reverse order:" 
	<< endl ;

   for ( int j = NUMB_DIGITS - 1 ; j >= 0 ; j-- )
   {
     cout << digits[ j ] 
	  << endl ;
   }
}

DaWei · Nov 2nd, 2010, 11:38 AM

You're laboring under a misperception. If you ask the extraction operator (>>) for an integer and you give it something else it will not store that something else in some location for you to magically test for type. It will, rather, break the stream. The stream will then not function again until you have taken it down to the local repair shop and had it fixed.

Newbies don't know this st*ff, of course. They would actually have to read and understand the documentation for the functions they use.

See the methods, "good()," "bad()," "fail()," et. al.

Ancient Dragon · Nov 2nd, 2010, 6:16 PM

>>They would actually have to read and understand the documentation for the functions they use.

And that's one of the problems with today's modern programming -- no printed documentation. When you and I started program we had no choice but to buy programming books and read the printed documentation that came with our compilers -- Microsoft provided a shelf full of small to large programming technical books with their compilers, which is probably one reason their compilers were always to expensive. We did not have an internet on which we could post our problems -- we had to do all the research and tedious analysis ourselves. We quickly learned how to read documentation and knew where to find answers to questions. Today's new programmers normally don't know diddly shit about those things. Instant gratification is all they want.

Patfish · Nov 2nd, 2010, 6:25 PM

/me hides

Oct 29th, 2010, 10:01 AM	#1
Patfish Newbie Join Date: Oct 2010 Posts: 8 Rep Power: 0	C++ Exception Handling Hello everyone, i'm trying to get this program, it searches for the final 'a' in an word inputted by the user, to terminate normally with a predefined error message ("This word is too long") if a word longer than 20 chars is entered. I've used a try throw catch statement and I seem to be able to get the program to terminate but I cant get the error message to appear. Can someone help me out with where i'm going wrong please? code and output below (Toggle Plain Text) /* #include <iostream> #include <cstdlib> using namespace std; int main( ) { const int WORD_LENGTH = 20 ; const int INVALID_INDEX = -1 ; const char NULL_CHAR = '\0' ; const char SEARCH_CHAR = 'a' ; char word[ WORD_LENGTH ] ; char* word_ptr = word ; int indexOf_char = INVALID_INDEX ; try { cout << "Input a word: " ; cin >> word ; if (word[ WORD_LENGTH] > 20 ) { throw "Incorrect Input - Word Too Long"; } } catch ( const int exception[] ) { cout << exception << "Array Out Of Bounds" << endl ; exit( 0 ) ; } for ( int i = 0 ; word_ptr != NULL_CHAR ; word_ptr++, i++ ) { if ( word_ptr == SEARCH_CHAR ) { indexOf_char = i ; } } if ( indexOf_char == INVALID_INDEX ) { cout << "There is no '" << SEARCH_CHAR << "' in " << word << endl ; } else { cout << "Last '" << SEARCH_CHAR << "' in " << word << " has index " << indexOf_char << endl ; } } /* #include <iostream> #include <cstdlib> using namespace std; int main( ) { const int WORD_LENGTH = 20 ; const int INVALID_INDEX = -1 ; const char NULL_CHAR = '\0' ; const char SEARCH_CHAR = 'a' ; char word[ WORD_LENGTH ] ; char* word_ptr = word ; int indexOf_char = INVALID_INDEX ; try { cout << "Input a word: " ; cin >> word ; if (word[ WORD_LENGTH] > 20 ) { throw "Incorrect Input - Word Too Long"; } } catch ( const int exception[] ) { cout << exception << "Array Out Of Bounds" << endl ; exit( 0 ) ; } for ( int i = 0 ; word_ptr != NULL_CHAR ; word_ptr++, i++ ) { if ( word_ptr == SEARCH_CHAR ) { indexOf_char = i ; } } if ( indexOf_char == INVALID_INDEX ) { cout << "There is no '" << SEARCH_CHAR << "' in " << word << endl ; } else { cout << "Last '" << SEARCH_CHAR << "' in " << word << " has index " << indexOf_char << endl ; } } Output At Runtime: (Toggle Plain Text) Input a word: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa terminate called after throwing an instance of 'char const' Aborted Input a word: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa terminate called after throwing an instance of 'char const' Aborted Thanks for any help!

Oct 29th, 2010, 11:07 AM	#2
DaWei Resident Grouch Join Date: Jun 2005 Posts: 8,368 Rep Power: 17	Re: C++ Exception Handling You've only allowed 20 characters for the word. What do you think happens if you exceed that length? Bad things, some more visibly damaging than others. Since this is C++, why don't you use the string class and avoid the problem sensibly? __________________ Contributor's Corner: Politically Incorrect DaWei on Pointers

Oct 29th, 2010, 11:16 AM	#3
Patfish Newbie Join Date: Oct 2010 Posts: 8 Rep Power: 0	Re: C++ Exception Handling Hi Dawei, The brief i have is to alter the original program (which is the code above minus the try throw catch) so that it terminates with the error message "Array Out Of Bounds" if a word longer than 20 characters is ended. As you can tell I'm pretty new to C++ and although I've covered some of classes its not within the scope of this assignment. A throw function with this termination statement seemed the best (in my limited knowledge) way of doing this and I have managed to make it terminate but I cant get it to display the error message

Oct 29th, 2010, 11:23 AM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 8,368 Rep Power: 17	Re: C++ Exception Handling The problem is that you're invoking undefined behavior BEFORE you reach your throw. In this case you're clobbering other things on the stack. In a typical implementation that would begin with SEARCH_CHAR and continue on, depending upon the length entered by the user. If you continue beyond your own variables and clobber the stack frame used by "main" your system is going to run off into the weeds and puke on its shoes. If you MUST use a C-style string, then use an input function that allows you to restrict the input length, and declare enough chars to hold input of that length. See, for instance, "getline." __________________ Contributor's Corner: Politically Incorrect DaWei on Pointers

Oct 29th, 2010, 11:27 AM	#5
Patfish Newbie Join Date: Oct 2010 Posts: 8 Rep Power: 0	Re: C++ Exception Handling thanks Dawei, i'll look into getline

Oct 29th, 2010, 11:50 AM	#6
L7Sqr Professional Programmer Join Date: Jun 2005 Location: here Posts: 440 Rep Power: 8	Re: C++ Exception Handling You should be catching what you throw. Consider the following: cpp Syntax (Toggle Plain Text) #include <iostream> int main () { try { throw "Some error message"; } catch ( const int msg[]) { std::cerr << "Got: " << msg << std::endl; return 0; } std::cerr << "No exception caught" << std::endl; return 0; } #include <iostream> int main () { try { throw "Some error message"; } catch ( const int msg[]) { std::cerr << "Got: " << msg << std::endl; return 0; } std::cerr << "No exception caught" << std::endl; return 0; } Yields (Toggle Plain Text) terminate called after throwing an instance of 'char const' Aborted terminate called after throwing an instance of 'char const' Aborted cpp Syntax (Toggle Plain Text) #include <iostream> int main () { try { throw "Some error message"; } catch ( const char * msg) { std::cerr << "Got: " << msg << std::endl; return 0; } std::cerr << "No exception caught" << std::endl; return 0; } #include <iostream> int main () { try { throw "Some error message"; } catch ( const char * msg) { std::cerr << "Got: " << msg << std::endl; return 0; } std::cerr << "No exception caught" << std::endl; return 0; } Yields (Toggle Plain Text) Got: Some error message Got: Some error message Notice the difference. __________________ "...and though our kids are blessed their parents let them shoulder all the blame." - The Quiet Things That No One Ever Knows [BrandNew] My Blog

Nov 2nd, 2010, 10:28 AM	#7
Patfish Newbie Join Date: Oct 2010 Posts: 8 Rep Power: 0	Re: C++ Exception Handling Thanks for the help on the above query guys, I've got that working nicely now but I also have a related problem on a similar subject. I've another program which takes 5 in-putted numbers and displays them in reverse order. In its original state it has a cassert statement to terminate the program if anything but a digit is in-putted. My job is to get it to repeatedly ask for a digit until one is entered rather than just terminating. I have tried using another try throw catch with a for loop inside the catch statement but I cant seem to get it to work. I'd appreciate any comments/abuse on what I've got so far! (Toggle Plain Text) #include <iostream> using namespace std; int main( ) { const int NUMB_DIGITS = 5 ; int digits[ NUMB_DIGITS ]; cout << "Enter " << NUMB_DIGITS << " digits, each on a new line:" << endl ; for ( int i = 0; i < NUMB_DIGITS ; i++ ) { try { cin >> digits[ i ] ; if( ( digits[ i ] >= 0 ) && ( digits[ i ] <= 9 ) ) ; { throw "Thats Not A Digit"; } } catch ( const char exception[] ) { for ( int i = 0; i < NUMB_DIGITS ; i++ ); { cin >> digits[ i ]; } } } cout << "Digits in reverse order:" << endl ; for ( int j = NUMB_DIGITS - 1 ; j >= 0 ; j-- ) { cout << digits[ j ] << endl ; } } #include <iostream> using namespace std; int main( ) { const int NUMB_DIGITS = 5 ; int digits[ NUMB_DIGITS ]; cout << "Enter " << NUMB_DIGITS << " digits, each on a new line:" << endl ; for ( int i = 0; i < NUMB_DIGITS ; i++ ) { try { cin >> digits[ i ] ; if( ( digits[ i ] >= 0 ) && ( digits[ i ] <= 9 ) ) ; { throw "Thats Not A Digit"; } } catch ( const char exception[] ) { for ( int i = 0; i < NUMB_DIGITS ; i++ ); { cin >> digits[ i ]; } } } cout << "Digits in reverse order:" << endl ; for ( int j = NUMB_DIGITS - 1 ; j >= 0 ; j-- ) { cout << digits[ j ] << endl ; } }

Nov 2nd, 2010, 11:38 AM	#8
DaWei Resident Grouch Join Date: Jun 2005 Posts: 8,368 Rep Power: 17	Re: C++ Exception Handling You're laboring under a misperception. If you ask the extraction operator (>>) for an integer and you give it something else it will not store that something else in some location for you to magically test for type. It will, rather, break the stream. The stream will then not function again until you have taken it down to the local repair shop and had it fixed. Newbies don't know this st*ff, of course. They would actually have to read and understand the documentation for the functions they use. See the methods, "good()," "bad()," "fail()," et. al. __________________ Contributor's Corner: Politically Incorrect DaWei on Pointers

Nov 2nd, 2010, 6:16 PM	#9
Ancient Dragon Achieved Level 70 Join Date: Jun 2005 Location: near St Louis, MO. (USA) Posts: 4,091 Rep Power: 10	Re: C++ Exception Handling >>They would actually have to read and understand the documentation for the functions they use. And that's one of the problems with today's modern programming -- no printed documentation. When you and I started program we had no choice but to buy programming books and read the printed documentation that came with our compilers -- Microsoft provided a shelf full of small to large programming technical books with their compilers, which is probably one reason their compilers were always to expensive. We did not have an internet on which we could post our problems -- we had to do all the research and tedious analysis ourselves. We quickly learned how to read documentation and knew where to find answers to questions. Today's new programmers normally don't know diddly shit about those things. Instant gratification is all they want. __________________ PFO's FAQ is here Forum Rules There is no cow level (Diablo III)

Nov 2nd, 2010, 6:25 PM	#10
Patfish Newbie Join Date: Oct 2010 Posts: 8 Rep Power: 0	Re: C++ Exception Handling /me hides

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C++ Exception Handling