Validate user input in C++

[promiscuoustx]

Hi again all!! I am trying to write my program below to validate user input. In other words, if the user inputs something like "1a2.34", I need to return a message that states invalid data and for them to retry. I am really lost on this one, and I really need some help. Thanks so much!!

#include <iostream>		
#include <iomanip>
#include <cmath>	
#include <cctype>
#include <string>
#include <sstream>	
		

using namespace std;
int main()
				
{
     double a; //This is the amount of the mortgage the user must enter
     double i; //This is the interest rate the user must enter
     int y; //This is the amount of years for the mortgage the user must enter			
     double mPayment; //This is a variable for ouputting the payment			
     char YesNo = 'Y';
	
	
do //In order to allow the user to be able to re-enter data, we must have a loop

{
	cout << "What is the amount of the mortgage?  For example 200000" << endl;
	cout << "Press enter." << endl;
    cin	>>	a;	
	cout << endl;
	cout << "What is the amount of years the mortgage will be financed? For example 30" << endl;
	cout << "Press enter." << endl;
    cin >>	y;
	cout << endl;
	cout << "What is the interest rate? For example 5.75" << endl;
    cout << "Press enter." << endl;
    cin >>	i;
	cout << endl;

//These are the variables required to calculate the information the user inputs
double monInterest = i / 12 / 100; //Calcualtes the interest monthly		
int t = y * 12; //This is the loan term in the amount of months

//This is the actual formula for calculating the mortgage payment amount 
mPayment = (a * monInterest) / (1-pow((1+monInterest),-t));

//This allows the user to view what they entered and also what the monthly payment would be
	cout << "Amount of mortgage = $" << a << endl;
	cout << "Year financed = " << i << "%" << endl;
	cout << "Interest Rate = " << y << " years" << endl;
	cout << endl;
	cout << "Monthly Payment Amount = $" << mPayment << endl;
	cout << endl;


//This allows the user to either enter in new information, or exit the program
	cout << "If you would like to enter different information, please press Yes." << endl;
	cout << "If you would like to exit this program, please press No." << endl;
    cout << endl;
	cin >> YesNo;
}

while ((YesNo == 'Y') || (YesNo == 'y')); //End of the loop	

	return 0;

}

[Banfa]

If you are trying to validate user input then you can not use code like this

  double a;

  cin >> a;

because you have let C++ do all the data handling for you internally, if you input something like

12b7

a will take the value 12

The only real way to properly validate user input is to read the whole line into a string and then parser that string for the data you expect and produce an error if you find anything different.

[promiscuoustx]

So should I change double a; to string a;? I am really lost on this.