Getting an infinite loop in MS Visual Basic

[Tipycol]

I'm trying to learn C++, and right now I'm trying to make an idiot proof program (ie: enter in all the wrong values) so I use while loops. But I have a problem. If I input a letter where a number should go, the program enters an infiniteloop.

Here's an exmaple of the code I'm using:

double Water;

cout<<"Enter the amount of water used: ";
cin>>Water;

while (Water<0){
cout<<"Value must be zero, or a positive number"<<endl;
cout<<"Enter the amount of water used (in gallons): ";
cin>>Water;
}

When run and a letter is entered for Water, this is what gets displayed:

Enter the amount of water used: Value must be zero, or a positive number
Enter the amount of water used: Value must be zero, or a positive number
Enter the amount of water used: Value must be zero, or a positive number
etc.

What I should do to prevent the loop from happening?

Thanks
Tipycol

 

[seadave77]

Can you use an if statement to check whether the input is a letter or number. You could check using ascii values. Why is the title "Getting an infinite loop in MS Visual Basic but you're using C++. Just wondering.

Clint

 

[VeryFirstSMP]

I've had this problem before... the problem is that after cin>>Water there is still some input left in the input buffer (such as a carriage return or something similar) and subsequent calls to cin>>Water will not actually wait for input but get the data that is already in the buffer.

Here is an example of how to clear the input buffer. Put a call to the function after both cin>>Water statements and you should be fine.

//----------------------------------------------------------------
// ClearError
//----------------------------------------------------------------
void ClearError(istream& istr)
{
	//--------------------------------------------------------
	// This code was taken from Visual Studio (VC++) online 
	// help files and modifed to B209 conventions. This 
	// function fully resets anything left in the input buffer.
	//--------------------------------------------------------
	const int	BUFFER_LENGTH = 20;

	streambuf*  buffer;
	char        tempBuffer[BUFFER_LENGTH];
	int         count;


	istr.clear();					// Clear error flags
	buffer = istr.rdbuf();			// Get streambuf pointer
	count = buffer->in_avail();		// Number of characters in buffer

	while(count)					// Extract them to tempBuffer
	{
		if(count > BUFFER_LENGTH)
		{
			buffer->sgetn(tempBuffer, BUFFER_LENGTH);
			count -= BUFFER_LENGTH;
		}
		else
		{
			buffer->sgetn(tempBuffer, count);
			count = 0;
		}
	}
}

//----------------------------------------------------------------

for example,

double Water;

cout<<"Enter the amount of water used: ";
cin>>Water;
ClearError(cin);

while (Water<0){
   cout<<"Value must be zero, or a positive number"<<endl;
   cout<<"Enter the amount of water used (in gallons): ";
   cin>>Water;
   ClearError(cin);
}

NOTE: There are other ways of clearing errors in cin, however I have found this method to be best for all situations so far.

 

[Tipycol]

...Why is the title "Getting an infinite loop in MS Visual Basic but you're using C++. Just wondering.

Clint

Whoops, sorry bout that. I've been used to saying MS Visual Basic


Thanks for your help VeryFirstSMP, the ClearError() function did the trick.

Tipycol