validation entry

hello all,
Im having some problems doing this simple program.

Code:

#include <iostream>
using namespace std; 

void input();
float result,firstNum, secondNum;
 
int main(){
	
	input();
	
	result = firstNum + secondNum;
	cout << "\n\t\t\t\t" << result << endl;
	return 0;
}

void input(){
	
	char casting[255];
	
	cout << "\n\t\t\tEnter First Number: ";
	cin >> casting;
	firstNum = atof(casting);
	
	cout << "\n\t\t\tEnter Second Number: ";
	cin >> casting ;
   	secondNum = atof(casting);
}

the reason Im using atof is because the program crashed if I typed anything else other than a number. Now it doesnt crash it just returns 0.

What I'd like is actually validate the entry and return an error message if the entry is not a number.
I've tried using something like...

Code:

while (strcmp(casting, "@*!#&")==1);
	{
		cout << "ERROR...only numbers\n";
	}

...but maybe is not what I need to use (it doesnt work anyway).

Is there any simple way to do it? thanks in advance.

>Is there any simple way to do it?
There is indeed.

Code:

#include <limits> // numeric_limits
#include <ios>    // streamsize

double get_double(){
  double d;

  while (!(cin>> d)) {
    cerr<<"Invalid number, try again: ";
    cin.clear();
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
  }

  return d;
}

void input(){
  cout << "\n\t\t\tEnter First Number: ";
  firstNum = get_double();
	
  cout << "\n\t\t\tEnter Second Number: ";
  secondNum = get_double();
}

thanks for the reply

a bit complicated too me; a few unfamiliar words.
interesting though.

Code:

//until input into d is valid, keep doing what's between the curly braces
while (!(cin>> d)) 
{
	//let the user know there is a problem.
	//could replace cerr with cout, but cerr is 
	//technically better
	cerr<<"Invalid number, try again: ";
 
	//clear the fail bit of the state member of the cin object
	cin.clear();
 
	//ignore any remaining characters in the input 
buffer or until newline char is found, whichever 
comes first
   cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
 
  //another approach would be to do something like this:
 //cin.ignore(8000);
}

Prelude has given you the best approach. Even if you don't understand it all, you can use her approach as needed for data input validation until you learn what all the different parts mean. Alternatively, if you don't want to look that sophisticated, you can use the "alternatives" listed above and they should work the majority of the time, though Prelude's code will work at times when the slimmed down version won't.

thanks

shouldnt I put a cin >> firstNum ; somewhere?
Like here maybe?

Code:

void input(){
  cout << "\n\t\t\tEnter First Number: ";
  cin >> firstNum ; 
  firstNum = get_double();

>shouldnt I put a cin >> firstNum ; somewhere?
get_double takes care of reading the value for you, then it returns it so that you can assign it to firstNum. Everything you need is done here:

Code:

firstNum = get_double();

But don't forget to define get_double as I did or it won't work as well.