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Only accept an alphabetical character

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Only accept an alphabetical character

Dec 4th, 2007

I've just completed an exercise from a book wherein the task is to display a menu of choices each associated to an alphabetical character. The program is to reject invalid characters and only finishing when a valid choice is made. Below is my solution, it achieves the task but I'd like to know if there is a completely easier approach.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
// chap05q03.cpp  
// C++ Primer Plus, Fifth Edition
// Chapter 6, Page 276.
// Programming Exercise # 3
// 4 Dec, 2007.
 
#include <iostream>
 
int main()
{
   using namespace std;
   char ch;
   char msg[] = "\nPlease enter  c, p, t or g: ";
   bool invalid = true;
 
   cout << "Please enter on of the following choices:\n\n";
   cout << "c) carnivore        p) pianist\n";
   cout << "t) tree             g) game\n";
 
   while (invalid)
   {
      cin >> ch;
      if ((ch != 'c') && (ch != 'p') && (ch != 't') && (ch != 'g'))
      {
         cin.clear();
	 while (cin.get() != '\n')
	    continue;
	 cout << msg;
      }
      else 
         invalid = false;	// we've got a correct entry, exit loop.
   }
   switch (ch)
   {
	case 'c'	: cout << "Option 'c' selected"; break;
	case 'p'	: cout << "Option 'p' selectd"; break;
	case 't'	: cout << "Option 't' selected"; break;
	case 'g'	: cout << "Option 'g' selected"; break;
	default 	: cout << "It will never get her"; break;
   }
   system("PAUSE");
   return 0;
}
// chap05q03.cpp  
// C++ Primer Plus, Fifth Edition
// Chapter 6, Page 276.
// Programming Exercise # 3
// 4 Dec, 2007.

#include <iostream>

int main()
{
   using namespace std;
   char ch;
   char msg[] = "\nPlease enter  c, p, t or g: ";
   bool invalid = true;
		
   cout << "Please enter on of the following choices:\n\n";
   cout << "c) carnivore        p) pianist\n";
   cout << "t) tree             g) game\n";
	
   while (invalid)
   {
      cin >> ch;
      if ((ch != 'c') && (ch != 'p') && (ch != 't') && (ch != 'g'))
      {
         cin.clear();
	 while (cin.get() != '\n')
	    continue;
	 cout << msg;
      }
      else 
         invalid = false;	// we've got a correct entry, exit loop.
   }
   switch (ch)
   {
	case 'c'	: cout << "Option 'c' selected"; break;
	case 'p'	: cout << "Option 'p' selectd"; break;
	case 't'	: cout << "Option 't' selected"; break;
	case 'g'	: cout << "Option 'g' selected"; break;
	default 	: cout << "It will never get her"; break;
   }
   system("PAUSE");
   return 0;
}

Any advice welcome.

Steve
Prime 357 and Improve memory and Steve's Stuff

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vmanes

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Re: Only accept an alphabetical character

Dec 4th, 2007

You could put the switch inside the loop (better as a do...while), let the default case handle the erroneous inputs. What is your if( ) condition becomes the while( ) condition.

Val

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Re: Only accept an alphabetical character

Dec 4th, 2007

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Originally Posted by vmanes

You could put the switch inside the loop (better as a do...while), let the default case handle the erroneous inputs. What is your if( ) condition becomes the while( ) condition.

Val

Thanks for the advice, very much appreciated. I've altered the code as suggested and is a lot 'cleaner' looking. Below is the result.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
// chap05q03-a.cpp  
// C++ Primer Plus, Fifth Edition
// Chapter 6, Page 276.
// Programming Exercise # 3
// 4 Dec, 2007.
 
// modified after advice from DaniWeb C++ forum.
 
#include <iostream>
 
int main()
{
using namespace std;
char ch;
char msg[] = "\nPlease enter  c, p, t or g: ";
bool invalid = false;
 
cout << "Please enter on of the following choices:\n\n";
cout << "c) carnivore        p) pianist\n";
cout << "t) tree             g) game\n";
 
do 
{
	cin >> ch;
	invalid = false;  
	switch (ch)
	{
		case 'c': cout << "Option 'c' selected\n"; break;
		case 'p': cout << "Option 'p' selectd\n"; break;
		case 't': cout << "Option 't' selected\n"; break;
		case 'g': cout << "Option 'g' selected\n"; break;
		default : cout << msg;
			cin.clear();
			while (cin.get() != '\n')
				continue;
			invalid = true;
	}
} while (invalid);
system("PAUSE");
return 0;
}
// chap05q03-a.cpp  
// C++ Primer Plus, Fifth Edition
// Chapter 6, Page 276.
// Programming Exercise # 3
// 4 Dec, 2007.

// modified after advice from DaniWeb C++ forum.

#include <iostream>

int main()
{
using namespace std;
char ch;
char msg[] = "\nPlease enter  c, p, t or g: ";
bool invalid = false;
		
cout << "Please enter on of the following choices:\n\n";
cout << "c) carnivore        p) pianist\n";
cout << "t) tree             g) game\n";
	
do 
{
	cin >> ch;
	invalid = false;  
	switch (ch)
	{
		case 'c': cout << "Option 'c' selected\n"; break;
		case 'p': cout << "Option 'p' selectd\n"; break;
		case 't': cout << "Option 't' selected\n"; break;
		case 'g': cout << "Option 'g' selected\n"; break;
		default : cout << msg;
			cin.clear();
			while (cin.get() != '\n')
				continue;
			invalid = true;
	}
} while (invalid);
system("PAUSE");
return 0;
}

Last edited by superjacent; Dec 4th, 2007 at 3:41 am. Reason: Formatting was all out of whack.

Steve
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Re: Only accept an alphabetical character

Dec 4th, 2007

You can also let 'c','p' & 't' fall through to 'g'

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Originally Posted by superjacent

Help with Code Tags

C++ Syntax (Toggle Plain Text)
	switch (ch)
	{
		case 'c': 
		case 'p': 
		case 't': 
		case 'g': cout << "Option '" << ch << "' selected\n";
                             break;
 
		default : cout << msg;
			cin.clear();
			while (cin.get() != '\n')
				continue;
			invalid = true;
	}
switch (ch) { case 'c': case 'p': case 't': case 'g': cout << "Option '" << ch << "' selected\n"; break; default : cout << msg; cin.clear(); while (cin.get() != '\n') continue; invalid = true; }

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Re: Only accept an alphabetical character

Dec 4th, 2007

•

Originally Posted by Tight_Coder_Ex

You can also let 'c','p' & 't' fall through to 'g'

Thanks, very much appreciated.

Steve
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Re: Only accept an alphabetical character

Dec 4th, 2007

•

Originally Posted by superjacent

Thanks for the advice, very much appreciated. I've altered the code as suggested and is a lot 'cleaner' looking.

Not bad, although it's still not formatted properly.
By the way, what does cin.clear(); do? You might want to look it up.
Another thing to look at is system("PAUSE"); See this

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Re: Only accept an alphabetical character

Dec 4th, 2007

•

Originally Posted by WaltP

Not bad, although it's still not formatted properly.
By the way, what does cin.clear(); do? You might want to look it up.
Another thing to look at is system("PAUSE"); See this

My understanding re cin.clear() is that if the cin fails (eof or mismatch of data) then further access or use of cin is prevented; to avoid this use the clear() method.

I guess you're probably suggesting that in this particular case it's not needed. Looking at the code I can't see how a mis-match of data cin >> ch could occur and I don't think EOF is an issue either. I hope I haven't embarrassed myself with this answer or response.

Taking this a step further in relation to cin.clear(). If the receiving variable is of a numeric type is that when cin.clear() would be considered in fixing up erroneous data entries.

Steve
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Re: Only accept an alphabetical character

Dec 4th, 2007

•

Originally Posted by superjacent

My understanding re cin.clear() is that if the cin fails (eof or mismatch of data) then further access or use of cin is prevented; to avoid this use the clear() method.

True. So what error are you expecting that needs to be cleared at this point?

•

Originally Posted by superjacent

I guess you're probably suggesting that in this particular case it's not needed. Looking at the code I can't see how a mis-match of data cin >> ch could occur and I don't think EOF is an issue either. I hope I haven't embarrassed myself with this answer or response.

That's what I'm suggesting, yes. And you have definitely NOT embarrassed yourself. It's a correct analysis.

•

Originally Posted by superjacent

Taking this a step further in relation to cin.clear(). If the receiving variable is of a numeric type is that when cin.clear() would be considered in fixing up erroneous data entries.

Actually, no. All cin.clear() does is clear the error flags. If they were set, it does not fix any data entries. The way you're using it, the error is simply cleared and ignored. You never test for one, so you don't even know it happened.

What you suggested above is "If the receiving variable is of a numeric type" and you type in a letter instead, cin.clear() magically changes the letter to a digit. What in fact happens is the error flags are set, nothing is loaded into the variable (previous value remains unchanged), and the letter stays in the input stream for the next cin .

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Re: Only accept an alphabetical character

Dec 4th, 2007

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Originally Posted by WaltP

Actually, no. All cin.clear() does is clear the error flags. If they were set, it does not fix any data entries. The way you're using it, the error is simply cleared and ignored. You never test for one, so you don't even know it happened.

What you suggested above is "If the receiving variable is of a numeric type" and you type in a letter instead, cin.clear() magically changes the letter to a digit. What in fact happens is the error flags are set, nothing is loaded into the variable (previous value remains unchanged), and the letter stays in the input stream for the next cin .

The above makes sense and is 100% consistent with what I've read though I know I wasn't too clear with my explanation.

Thanks for the advice, and everyone else as well, it's really helped.

Steve
Prime 357 and Improve memory and Steve's Stuff