Error checking for beginner program (problem with ASCII subtraction of two-digit numbers)

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5.00/5 (1 vote)

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Hey guys, I'm really new to C++ and I'm trying to figure out how to validate a char var--to see if it's an int between 1 and 6, basically. The program I have so far catches all the exceptions I want it to except for numbers over 9 and negative numbers.
When the number entered is 9 or higher it somehow (through the magic of ASCII, I guess) selects whatever the first number is (ie. 10 = Maimi, 122 = Miami, 25 = Denver, 220 = Denver, etc). Negative numbers act the same way (ideally it should allow for no negative numbers). I looked in my book and on the web but I found nothing.
Does my program suck...yes, do I feel like a retard for asking...yes. Please help! Thanks in advance.

conditions:

Must re-prompt user for input if the input isn't an int from 1 to 6
Must catch non-integer input

using namespace std;
void ShowMenu();
int main(int argc, char *argv[])
{
      char choice;
      int ch;
      int go = 1;
      ShowMenu();           
     
      while (go == 1)   //while and if loops to detect exceptions
      {
            cout<<"enter a choice: ";
            cin>>choice;
            
            if (!isdigit(choice))
            {
               cout<< "Enter an integer! \n";
               continue;
            } 
               
                  int ch = choice - '0'; //changes "char choice" to "int ch"
                  
                  if (ch >6 || ch <1)
                  {
                     cout<< "Enter a number between 1 and 6! \n";
                     continue;
                  }                  
                  if (ch <= 6 && ch >= 1)
                  {
                      go = go - 1;
                      
        switch(ch)   //the choices I want to limit the user to
        {
        case 1:   cout<<"You selected Miami. \n";
                  break;
        case 2:   cout<<"You selected Denver. \n";
                  break;
        case 3:   cout<<"You selected New York. \n";
                  break;
        case 4:   cout<<"You selected Buffalo. \n";
                  break;
        case 5:   cout<<"You selected Los Angles. \n";
                  break;
        case 6:   cout<<"Exiting program... \n";
                  break;                                      
        }
                      
                      
                  }
               }                      
                                                                                                                         
                    
    system("PAUSE");
    return EXIT_SUCCESS;
}       
    void ShowMenu()  //shows the menu
    {
     cout<<"Please select a destination by selecting a number, or enter 6 to exit: \n"
           "1) Miami   2) Denver  3) New York \n"
           "4) Buffalo 5) Los Angles 6) Exit Program \n";      
    }

Posted 8-Feb-11 10:15am

thomas struss

Updated 8-Feb-11 10:56am

v5

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Comments

Sergey Alexandrovich Kryukov 8-Feb-11 16:20pm

Where is "catches all errors"? (You catch exceptions, not errors!) What's the problem, anyway? (Yes, it sucks, now what's the problem?)
--SA

thomas struss 8-Feb-11 16:27pm

Fine then, I want to catch the exceptions. Is that better? The problem is that it won't catch EXCEPTIONS like: the number being two digit, and the number being negative.

3 solutions

Solution 3

There's a simple C++ idiom you can use to read an object of any type with an extraction operator and check to see if it's been read successfully. It's only a couple of lines:

int n = 0;

if( std::cin >> n )
{
    std::cout << "I've read a number, it's a " << n << std::endl;
}
else
{
    std::cout << "Ooops, whatever you typed it wasn't a number..." << std::endl;

    std::cin.clear();
    std::string junk_line;
    std::getline( std::cin, junk_line );
}

The important bit is the line that does the read. It relies on two things:

- extraction operators return the stream they're operating on after the operation, this allows you to string extractions together.

- stream objects have a conversion operator to bool [1]. If this is false the stream is jammed up for some reason, usually because a conversion has failed or because the stream has hit an end of file condition. [2]

So when you do the read the one of the following things happen:

- the next characters out of the stream can represent an integer. Yay! The stream is in a good state and the condition is true.

- the next characters out of the stream can't represent an integer. Boo! The stream is clagged up and can't be read from until the error state is cleared. To do that you clear the stream state and then read and discard the line that's caused the problem - which the block of code after the error message.

Anyway, I hope this helps you write C++ that looks like C++ and not the slightly wonky C you've ended up using. If you're interested in C++ streams then I'd recommend reading "Standard C++ IOStreams and locales: Advanced programmer's guide and reference" By Angelika Langer and Klaus Kreft. It's a dry but informative read that will tell you everything you want to know about streams and loads more you don't.

Cheers,

Ash

[1] Actually it's not straight to bool, it's to a type of function pointer which is convertible to bool. Don't worry about details like that for now though.

[2] Again, this aint the whole story but enough to get you through your current problem.

Posted 10-Feb-11 3:12am

Aescleal

Solution 2

I ended up going away from the "cin" route; I would still like to know if it's possible though.

C#

char choice[10];
   int intchoice, len;
   int go = 1;
   ShowMenu();
   while (go == 1)
   {
   printf ("Enter a choice: ");
   fgets (choice, 10, stdin);
   len = strlen (choice) -1;
   if (len > 1)
   {
      cout<< "Enter a choice below...only one number. \n";
      continue;
   }
   intchoice = atoi(choice);
      if (intchoice >6 || intchoice <1)
      {
          cout<<"Enter a choice between 1 and 6! \n";
          continue;
      }
      go = go -1;
   }

Posted 9-Feb-11 16:23pm

thomas struss

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Yusuf · Accepted Answer · 2011-02-08T10:31:00

Solution 1

The easiest way is to change your input to int. After that life will be a whole lot easier

MIDL

int myInt;
myInt = atoi (choice);

SEE Cplusplus[^] and MSDN[^] for details

Posted 8-Feb-11 10:31am

Yusuf

v2

Comments

thomas struss 8-Feb-11 16:41pm

If I do that can I still catch non-digit characters? Is there something like "isdigit" for int?

Yusuf 8-Feb-11 16:49pm

You bet:
http://www.cplusplus.com/reference/clibrary/cctype/isdigit/

Try Google, I am sure you will be amazed as to what you will find

thomas struss 8-Feb-11 17:30pm

Cool, I'll try it out, thanks.