How To Output Error When Input Isnt A Number In C++

The type of data to be retrieved from the input is automatically determined by std::cin, which writes it to the specified variable. We can use the (!) NOT operator to check the overloaded cin. If the variable type and input data type differ, std::cin is in an unstable state. This feature allows us to determine whether the given data is a number or not.

Input validation and error handling can be used together in C++ to generate an error when the input is not a number. To determine whether the input was successful, one popular option is to utilize the fail function in combination with the std::cin input stream.

Approach:

• ✅ To get started, use std::cin to read the input and check for input errors.
• ✅ Use std::cin.clear function to clear the error condition and use std::cin.ignore to ignore the rest of the input line if an error is found.
• ✅ Finally, if an error is found, print an error message.

Example 1:

Example
Copy
// C++ program to Output Error When Input Isn't a Number 
#include <iostream> 
#include <limits> 
using namespace std; 
int main() 
{ 
	// Prompt the user to enter a number 
	cout << "Please enter a number: "; 
	int num; 
	// Keep reading input until a valid number is entered 
	while (!(cin >> num)) { 
		// Clear the error state 
		cin.clear(); 
		// Ignore the rest of the incorrect input 
		cin.ignore(numeric_limits<streamsize>::max(), '\n'); 
		// Prompt the user again 
		cerr << "Error: That was not a number. Please "
				"enter a number: "; 
	} 

	// Output the entered number 
	cout << "You entered the number: " << num << endl; 
	return 0; 
}
Try it Yourself

Output:

Complexity Analysis:

Time Complexity: O (1)

Space Complexity: O (1)

Example 2:

#include <iostream>
#include <limits>
int main() {
    double number;
    std::cout << "Enter a number: ";
    std::cin >> number;
    if (std::cin.fail()) {
        std::cin.clear(); // Clear the fail flag
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n'); // Discard the input
        std::cerr << "Error: Input is not a number." << std::endl;
    } else {
        std::cout << "You entered: " << number << std::endl;
    }
    return 0;
}

Output:

Complexity Analysis:

Time Complexity: O (1)

Space Complexity: O (1)