How do I check if the input is letter? - c++

Question

I'm trying to display "invalid option" when the user enters a letter instead of a number. I tried using the isalpha() function but I get an infinite loop showing 0 Invalid option!try again:. The 0 from the output is displayed when entering a letter. When I actually type in the number 0 the message is displayed and the loop is exited.
#include <iostream>
#include <cstdlib>
#include <ctime>
#include <cctype>
using namespace std;
int main() {
// Vaules do not change
const int minNum = 1;
const int maxNum = 100;
int userInput;
// Changes every second
srand(time(0));
// Random number is stored
const int answer = minNum + (rand() % maxNum);
cout << answer << endl; // For testing purposes
cout << "Guess a number 1-100: ";
cin >> userInput;
cout << endl;
if(userInput == answer) {
cout << "Correct!\n\n";
}
while(userInput != answer) {
if(userInput < 1 || userInput > 100 || isalpha(userInput)) {
cout << userInput << " Invalid option!\ntry again: ";
cin >> userInput;
cout << endl;
}
else if(userInput < answer) {
cout << userInput << " is too low!\ntry again: ";
cin >> userInput;
cout << endl;
}
else if(userInput > answer) {
cout << userInput << " is too high!\ntry again: ";
cin >> userInput;
cout << endl;
}
}
cout << userInput << " is correct!\n\n";
return 0;
}

Answer 1

When you need to deal with user input differently based on some logic, your best option is to:
Read lines of text. Figure out what to do when there is no more input.
Process each line of text using custom logic.
In your case, you could use:
#include <iostream>
#include <cstdlib>
#include <ctime>
#include <cctype>
using namespace std;
int main() {
// Vaules do not change
const int minNum = 1;
const int maxNum = 100;
int userInput;
// Changes every second
srand(time(0));
// Random number is stored
const int answer = minNum + (rand() % maxNum);
cout << answer << endl; // For testing purposes
std::string line;
cout << "Guess a number 1-100: ";
while ( getline(std:::cout, line ) )
{
// Deal with empty lines.
if ( line.size() == 0 )
{
continue;
}
// If the first character is a letter ...
if(isalpha(line[0])) {
cout << line << "\n Invalid option!\ntry again: ";
continue;
}
// Extract the number from the line using a stringstream.
// If there is a problem extracting the number ...
std::istringstream str(line);
if ( !(str >> userInput ) )
{
cout << line << "\n Invalid option!\ntry again: ";
continue;
}
cout << endl;
// Check the user input against the random answer.
if(userInput == answer) {
cout << "Correct!\n\n";
}
else if(userInput < 1 || userInput > 100 ) {
cout << userInput << " Invalid option!\ntry again: ";
}
else if(userInput < answer) {
cout << userInput << " is too low!\ntry again: ";
}
else if(userInput > answer) {
cout << userInput << " is too high!\ntry again: ";
}
cout << "Guess a number 1-100: ";
}
cout << userInput << " is correct!\n\n";
return 0;
}