how to stop letters from getting through a IF statement in c++? [duplicate]

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• How to make cin take only numbers 2 answers

The c++ compiler I've been using crashes whenever a letter is put into a equation. it'd be so much easier if an IF statement would just stop letters from getting to the equation. i tried this as a test from a program that I thought that would stop letters but for some reason letters still get through.

#include <iostream>
#include <sstream>
#include <string>
#include <cmath>
using namespace std;
int
main ()
{
  char choice;
  bool run = true;
  while (run)
    {

      do
    {
      int plz;
      bool done = false;


      {         
        while (!done)
          {
        cout << "work plz ";
        cin >> plz;
        if (plz = < 0 || plz > 0)
        cout << "work";
        done = true;
        else
          {
              cout << "error " << plz << " is not a number" << endl; 
            done = false;
          }
          }
      }
      cout << "Would you like to try again?(Y/N)";
      cin >> choice;     
      cout << endl;
      choice = tolower (choice);
    }
      while (choice != 'n' && choice != 'y');
      if (choice == 'n')
    {
      run = false;
    }
      else if (choice == 'y')
    {
      run = true;
    }
      else
    {
      choice != 'n' && choice != 'y';
    }
    }
  return 0;
}