Restricting character input in C++ [closed] - c++

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I'm really curious as how to restrict character input in a program like a calculator can anyone teach me how to do it, I'm really new to this. And if someone answers could you please explain every line of code in your answer. Thanks in advance. :)
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int choice;
double add, sub, mul, div, rem, square, power, v1, v2;
long double sine, cosine, tangent;
do{
cout << "Choose an operation." << endl << "\n";
cout << "[1] Addition." << endl;
cout << "[2] Subtraction." << endl;
cout << "[3] Multiplication." << endl;
cout << "[4] Division." << endl;
cout << "[5] Exit." << endl;
cin >> choice;
while (choice < 1 || choice > 5){
cout << "Please Choose from 1 to 5" << endl;
cin >> choice;
}
if (choice == 1){
cout << "Please Enter 1st value: ";
cin >> v1;
cout << "Please Enter 2nd value: ";
cin >> v2;
add = v1 + v2;
cout << "The answer is: " << fixed << setprecision(2) << add << endl << endl;
}
else if (choice == 2){
cout << "Please Enter 1st value: ";
cin >> v1;
cout << "Please Enter 2nd value: ";
cin >> v2;
sub = v1 - v2;
cout << "The answer is: " << fixed << setprecision(2) << sub << endl << endl;
}
if (choice == 3){
cout << "Please Enter 1st value: ";
cin >> v1;
cout << "Please Enter 2nd value: ";
cin >> v2;
mul = v1 * v2;
cout << "The answer is: " << fixed << setprecision(2) << mul << endl << endl;
}
if (choice == 4){
cout << "Please Enter 1st value: ";
cin >> v1;
cout << "Please Enter 2nd value: ";
cin >> v2;
div = v1 / v2;
rem = fmod (v1,v2);
cout << "The answer is: " << fixed << setprecision(2) << div << " remainder " << rem << endl << endl;
}
if (choice == 5)
break;
}
while (choice != 5);
{
cout << endl << "Goodbye.";
}
return 0;
}

Answer 1

Check the documentation of the functions you are using.
Simplified code:
double v1;
std::cin >> v1;
This is attempting to extract a double from std::cin, skipping leading whitespace.
If extraction fails -- e.g. because somebody entered something that isn't a representation of a double value -- a zero is written (since C++11, previously the value was left unchanged), and the failbit set on the stream.
This can be checked, and cleared:
if ( std::cin.fail() )
{
// handle error
std::cin.clear();
}
Actually the page on clear() has a fully functional example of reading a number, checking for failure, and repeating the input.
That being said, I second #Maikel's comment. If you want to take this beyond the experimentation stage, you will want to write a true grammar parser, for which Boost.Spirit is a strong choice.