How can I?

Why I cannot use this methode to flush out the invalid input.
I wanted to have the input part inside the while loop, unlike the one the FAQ show on the board(Inside the while condition).

Code:

#include <iostream>
#include <cctype>
#include <fstream>
#include <limits>
using namespace std;
struct inventory_items
{
	int code,quantity,reorder_level;
	char name[100], units[100];
	double price;
};

int main()
{
	int counter=0;
	inventory_items s;

	while (counter==0)
	{
  
	  cout<<"Enter the item code.\n";
	  cin>>s.code;
	  cin.get();
	  if(!isdigit(s.code))
	  {
		cout << "\nInvalid input" <<endl;
		cin.clear();
		cin.ignore(std::numeric_limits < int >::max(), '\n');
	  }
	  else
	  {
		  cout<<"You enter : "<<s.code<<endl;
		  counter=1;
	  }
	}
  return 0;
}

You're using an int into s.code using >>. This will only work if there is an actual number being inputted (sp?). Otherwise it'll go into an infinite loop searching for a number. You should use a temporary char, and check that.

Code:

char temp;
while(counter == 0)
{
   cin.get(temp);
   if(!isdigit(temp))
      cout<<"Invalid input"<<endl;
   else
   {
      s.code = atoi(temp);
      cout<<"Correct. You entered "<<s.code<<endl;
   }
}

This will only get the first digit of the input. If you want more than that you'll have to make your own function to check an array of chars.

Or, an easier way to read a whole number:

Code:

#include <iostream>
#include <cctype>
#include <fstream>
#include <limits>
using namespace std;
struct inventory_items
{
	int code,quantity,reorder_level;
	char name[100], units[100];
	double price;
};

int main()
{
	int counter=0;
	inventory_items s;

	while (counter==0)
	{
  
	  cout<<"Enter the item code.\n";
	  cin>>s.code;
	  if(!cin)
	  {
		cout << "\nInvalid input" <<endl;
		cin.clear();
		cin.ignore(std::numeric_limits < int >::max(), '\n');
	  }
	  else
	  {
		  cout<<"You entered : "<<s.code<<endl;
		  counter=1;
	  }
	}
  return 0;
}

One note, if someone enters "10254rgh", it is seen as a valid input of "10254". Also, you're always going to leave something in the input stream, so you may want to do the clear/ignore no matter what:

Code:

int main()
{
    int counter=0;
    inventory_items s;

    while (counter==0)
    {
  
      cout<<"Enter the item code.\n";
      cin>>s.code;
      if(!cin)
      {
        cout << "\nInvalid input" <<endl;
      }
      else
      {
          cout<<"You entered : "<<s.code<<endl;
          counter=1;
      }
      cin.clear();
      cin.ignore(std::numeric_limits < int >::max(), '\n');
    }
  return 0;
}