how to reject bad input?

[fmedina151]

this is part of a function that I am working on I am getting it to work but now I need to make it display "Error" if user enters a non numeric value for both Miles and Gallons but I have no idea where to start please help.

  void FindMilesPerGallon(void)
{
	
	float Miles, Gallons, MPG;

	MPG = 0;

  cout << "Enter miles driven ==> ";
  cin >> Miles;
  cout << "Enter gallons consummed ==> ";
  if(cin >> Gallons)
  { MPG = Miles/ Gallons;
  cout << "Total miles per gallon :" << MPG;
}
	  
	 

[MiiNiPaa]

check cin for fail status after getting input:

if (!(cin >> Miles)) {
    std::cout <<  "Error";
    return 1;
}
//or
cin >> Miles;
if (cin.fail()) {//Or !cin, however they not actually the same thing
    //...
}

[booradley60]

To expand on MiiNiPaa's response, look at this page:
http://www.parashift.com/c++-faq/istream-and-ignore.html

It'll show you how to detect bad input, tell the user it was bad, clear cin's 'fail' state, and prompt the user again.

[andywestken]

See:

Trying to limit input to int type
http://www.cplusplus.com/forum/beginner/108849/

in particular cire's post:
http://www.cplusplus.com/forum/beginner/108849/#msg592118

And:

how to deny non-numeric input?
http://www.cplusplus.com/forum/beginner/109874/

Andy

PS Regarding:

however they not actually the same thing

Actually, using

if (!cin)

is the same as

if (cin.fail())

At least if basic_ios::operator!() follows the C++ standard and returns fail();

(same kind of deal for basic_ios::operator void*() -- it returns a null pointer if fail(), otherwise "some" non-null pointer.)

[fmedina151]

Now I have an infinite loop every time I enter a number where do I change the loop control variable to break out of the loop?

void FindMilesPerGallon(void)
{

	float Miles, Gallons, MPG;

	MPG = 0;

	cout << "Enter miles driven ==> ";
	cin >> Miles;

	cout << "Enter gallons consummed ==> ";
	if (cin >> Gallons)
	{
		MPG = Miles / Gallons;
		cout << "Total miles per gallon :" << MPG;
	}
	if (!(cin >> Miles) || !(cin >> Gallons)) {
		cout << "Error";
		Miles = 0;
		Gallons = 0;
		MPG = 0;
	}

}