While loop meant to verify user input constantly loops - c++

Question

I am trying to verify the user input, but I have tried two compilers and I either have one of two things happen. Either it will:
-Constantly loop the error message without asking for user input
OR
-Wait for user input, and if the input is incorrect, will constantly loop the error message.
Here is the code:
cout << "Input number of the equation you want to use (1,2,3): " ;
cin >> userInput;
cout << endl;
while (userInput <= 0 || userInput >= 4)
{
cout << "Please enter a correct input (1,2,3): " ;
cin >> userInput;
cout << endl;
}
if (userInput == 1)
{
userInput is declared as an integer. Is there an easier way to verify user input, or a while loop is necessary? I am still very new to coding.

Answer 1

While using int userInput seems straight forward, it fails when the user inputs non-numeric values. You can use a std::string instead and check, if it contains a numeric value
std::string userInput;
int value;
std::cout << "Input number of the equation you want to use (1,2,3): " ;
while (std::cin >> userInput) {
std::istringstream s(userInput);
s >> value;
if (value >= 1 && value <= 3)
break;
std::cout << "Please enter a correct input (1,2,3): " ;
}
std::istringstream is similar to other input streams. It provides input from an internal memory buffer, in this case the value provided by userInput.

Answer 2

I would add an additional check to make sure that if the user enters non-integral input, the stream is cleared before attempting the next read.
cout << "Input number of the equation you want to use (1,2,3): " ;
cin >> userInput;
cout << endl;
while (userInput <= 0 || userInput >= 4)
{
if ( !cin.good() )
{
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
cout << "Please enter a correct input (1,2,3): " ;
cin >> userInput;
cout << endl;
}

Answer 3

I would suggest using a do loop instead so you have less repeated lines
int userInput = 0;
do
{
cout << "Input number of the equation you want to use (1,2,3): " ;
cin >> userInput;
cout << endl;
if ( !cin.good() )
{
cin.clear();
cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}
} while (userInput <= 0 || userInput >= 4);

Answer 4

You don't want to cin >> int, if you are to perform any error checking. If the user enters a non-integer, you'll end up in difficult-to-recover-from situations.
Rather, cin into a string, perform any error checking you want and convert the string to a integer:
long x;
string sx;
cin >> sx;
x = strtol(sx.c_str(), NULL, 10);