hello, My program works fine but I want every time that the user add a character int the calculation section the program will print an error message and will ask the user again... I have try a lot of different things but nothing helps me... for example static_cast() and other fnc that I found and convert the stings inti double but nothing works, so If you have any ideas please let me know!! Below the code of my program....Thank you very much!!

Best Regards!!!!!!

```
//============================================================================
// Name : CalculatorProject.cpp
// Author :
// Version :
// Copyright : Your copyright notice
// Description :
//============================================================================
#include <iostream>
#include <stdio.h>
#include <stdlib.h>//all the libraries that the program used some of them are not required but I used it
#include <cmath> // since the program output an error message
#include <unistd.h>
using namespace std;
double total = 0.0; // variables of the whole program
double sum = 0.0;
int quit()
{
char option2;
cout <<"Are you sure you want to quit from the program? (Y/N): "<<endl;
cin >> option2; // the program ask from user if he/she wants to quit or not
if ((option2=='Y') || (option2=='y')) // if statement that check the input of the user
{ //start brackets of if statement
cout << "GoodBye!!! C ya later!!!.. "<<endl; // goodbye message if the user wants to quit end the program exit at next line
return(1);
}//end brackets of if statement
else // else of if statement
if ((option2=='N') || (option2=='n')) //if statement check if the user select N/n and continues
{ //start bracket of if statement
cout <<"You will now return to main menu!"<<endl; //print message that the program will return to main menu
}// end bracket of if statement
else //else of the second if statement
cout << "Error input..."<<endl; // print an error message and inform the user that the program
cout << endl;
return(0);
}
void addition() //function for the addition operation...
{
double num1,num2;
cout << "**********"<<endl;
cout << "*Addition*"<<endl;
cout << "**********"<<endl; // welcome screen
cout << "Enter 2 numbers to calculate: "<<endl;
cin >> num1>>num2; // ask from the user to at 2 numbers
total = num1 + num2; // the program makes the calculation of the two numbers
sum += total; // save the value and keep
cout <<"The total sum of "<<num1<<" + "<<num2<<" = "<<total<<endl; //prints out the equation
cout << "The total of sum is now "<<sum<<endl;
} // end brackets of addition function....
void subtraction() // function of subtraction...
{
double num1,num2; // all the variables of the fuctions
cout <<"*************"<<endl;
cout <<"*Subtraction*"<<endl;
cout <<"*************"<<endl; // welcome screen
cout <<"Enter 2 number to calculate: "<<endl;
cin >> num1>>num2; //ask from user to enter 2 numbers
total = num1 - num2; // the program make the calculation of subtraction
sum +=total; //save the value
cout <<"The subtraction of "<<num1<<" - "<<num2<<" = "<<total<<endl; //prints out the equation
cout << "The total of sum is now "<<sum<<endl;
}
void multiplication() // function of multiplication
{
double num1,num2; // all the variables of the function
cout <<"****************"<<endl;
cout <<"*Multiplication*"<<endl;
cout <<"****************"<<endl; //welcome screen
cout <<"Enter 2 number to calculate: "<<endl;
cin >> num1>>num2; // ask from user to enter 2 numbers
total = num1 * num2; // the program make the multiplication
sum += total; //save the value
cout << "The total multiplication of "<<num1<<" * "<<num2<<" = "<<total<<endl; //prints out the equation
cout << "The total of sum is now "<<sum<<endl;
}
void division() // function of division
{
double num1,num2; // all the variables of the function
cout <<"**********"<<endl;
cout <<"*Division*"<<endl;
cout <<"**********"<<endl; //welcome screen of the function
cout <<"Enter 2 number to calculate: "<<endl;
cin >> num1>>num2; //ask from user to enter 2 numbers
total = num1 / num2; // the program make the calculation
sum += total; //the program save the value
cout << "The total division of "<<num1<<" / "<<num2<<" = "<<total<<endl; //prints out the equation
cout << "The total of sum is now "<<sum<<endl;
}
void sum_squared() // function of the sum squared
{
double num1,num2; //all the variables of the function
double displayedVal;
displayedVal = 0.0;
cout <<"*************"<<endl;
cout <<"*Sum squared*"<<endl; // welcome screen of function
cout <<"*************"<<endl;
cout <<"Enter 2 number to calculate: "<<endl;
cin >> num1>>num2; //ask from user to enter 2 numbers
displayedVal = pow (num1, num2); // the program make the calculation
sum += displayedVal;
cout << "The total sum squared of "<<num1<<" ^ "<<num2<<" is "<<displayedVal<<endl; // the program prints out the equation
cout << "The total of sum is now "<<sum<<endl;
}
void less_than_relation() // function of less then relation
{
double num1,num2; //all the variables of the function
cout <<"********************"<<endl;
cout <<"*Less than relation*"<<endl; // welcome screen
cout <<"********************"<<endl;
cout <<"Enter 2 number to calculate: "<<endl;
cin >> num1>>num2; // ask from user to add 2 numbers
if (num1<num2) // if statement to check the numbers if it's less
{ //start bracket of if statement
total = num2; // change the value of total into num2 if it's biggest than num1
sum += total;
cout <<"The "<<num1<<" is smaller (<) than "<<num2<<endl;
cout << "The total of sum is now "<<sum<<endl;
} // end bracket of if statement
else // else of if statement
total = num1; // change the value of total into num1
sum+= total;
cout << "The first number "<<num1<<" is not smaller than "<<num2<<endl; //prints the result if the num1 is smallest than num2
cout << "The total of sum is now "<<sum<<endl;
}
void greater_than_relation() // function of greater than relation
{
double num1,num2; //all variables of the function
cout <<"***********************"<<endl;
cout <<"*Greater than relation*"<<endl; //welcome screen of the function
cout <<"***********************"<<endl;
cout <<"Enter 2 number to calculate: "<<endl;
cin >> num1>>num2; // ask from user to enter 2 numbers
if (num1>num2) // if statement check if the num1 is greater of num2
{ //start bracket of if statement
total = num1; // change the value of total into num1 if the num1 is greater than num2
sum +=total;
cout <<"The "<<num1<<" is greater than (>) "<<num2<<endl; // prints the results if the num2 is greater tha num2
cout << "The total of sum is now "<<sum<<endl;
} //end bracket of the if statement
else //else of if statement
total = num2; // change the value of total into num2
sum += total;
cout << "The first number "<<num1<<" is not greater than the second number "<<num2<<endl; // prints out the results
cout << "The total of sum is now "<<sum<<endl;
}
void clear()
{
char option3;
cout <<"Are you sure you want to clear the sum?(y/n) "<<endl;;
cin >> option3;
if ((option3 == 'y') || (option3 == 'Y'))
{
total = 0.0;
sum = 0.0;
cout <<"Your sum now is clear!!!"<<endl;
cout <<"The total is now "<<sum<<endl;
}
else
if ((option3 == 'n') || (option3 == 'N'))
{
cout <<"Your sum now is not clear. You will return into main menue!!"<<endl;
}
else
cout <<"Error!!!You have enter an invalid input.Please enter a valid input"<<endl;
}
void calculator() // function of the calculator
{
char opt; // variables of calculation function
cout <<endl;
cout <<"*************************************"<<endl;
cout <<"********Welcome To Calculator********"<<endl; //welcome screen of the void function......
cout <<"*************************************"<<endl; //****************************************
cout <<"Please chose an operator you want to used: "<<endl;
cout <<"+: Addition" <<endl;
cout <<"-: Subtraction" <<endl;
cout <<"*: multiplication"<<endl;
cout <<"/: division"<<endl; //All the operators options that user has available to used....
cout <<"^: sum squared"<<endl;
cout <<"<: less than relation"<<endl;
cout <<">: greater than relation"<<endl;
cout <<"c: clear sum"<< endl;
cout <<"q: quit"<< endl;
cin >>opt; //reads the opt input from the user
switch (opt) //switch statement for the previous option that user enter
{ // start bracket of the switch statement
case '+': // case 1 for addition
addition(); //called the function addition...
break; //break of the case addition
case '-': // case 2 for subtraction
subtraction(); // called the subtraction function....
break; // break of the case subtraction
case '*': // case 3 for multiplication
multiplication(); // called the multiplication function...
break; // break of the case multiplication
case '/': // case 4 for division
division(); // called the division function...
break; // break of the case division
case '^': // case 5 for sum squared...
sum_squared(); // called the sum_squared function
break; // break of the case sum squared
case '<': // case 6 for the less_than_relation function
less_than_relation(); // called the less_than_relation function
break; // break of the case less than relation
case '>': // case 7 for the greater_than_relation function
greater_than_relation(); // called the greater_than_relation function
break; // break of the case greater than relation
case 'c': // case 8 for the clear function
clear();
break; // break of the case clear
case 'q': // case 9 for the quit
if (quit() == 1)
{
exit(0); // called the quit function
}
break; // break of the case quit
default:
cout <<"Error input..Please choose an option from the above menu!!!!"<<endl;
}//end bracket of the switch case
}
int main() // int main of the program include the total retrun
{ // start bracket of int main
char option1;
while (1) // while loop for int main... every time that the program makes a calculation the program ask again from user if wants a new operator
{ // start bracket of while loop
cout <<"********Welcome To Calculator********"<<endl;// welcome screen of the program......
cout <<"*************************************"<<endl;//****************************************
cout <<"Please choose one of the following: ";
cout <<endl;
cout <<"1. Calculator "<<endl;
cout <<"2. Quit "<<endl;
cin >> option1; //ask from user to choose one of the option that are available
if (option1 == '1') // if statement of the option1
{ // start bracket of if statement
calculator(); // called the calculator function if the option1 is 1
} // end bracket of if statement
else // else of if statement
if (option1 == '2') // if statement of option1 check if the user choose the second option
{ //start bracket of if statement
if (quit() == 1)
{
break; // break of the while loop to stop the while
}
} // end of if statement
else // else of if statement
// if statement... check if the input of user is not 1 or 2
// start bracket of if statement
cout <<"Error Input......"<<endl; // prints out an error message if the user add a wrong input
}
return 0; // return to 0
} //bracket for int main
```