Why Do Letters Cause Infinite Loops in C Number Input Programs?

Understanding Infinite Loops When Entering Letters Instead of Numbers

When executing a program that prompts for integer input, it's common to encounter an infinite loop if a user enters a letter instead of a number. This issue arises due to how input handling works in C .

The Root Cause:

In C , the cin function is used to read input. However, if non-numeric characters are entered, cin fails to extract a valid integer. As a result, the failbit flag is set within the cin stream object, indicating an error.

Fixing the Infinite Loop:

To resolve the infinite loop, we need to detect and handle the invalid input scenario. Here's a modified portion of the code that corrects this issue:

#include <limits> // Includes numeric_limits for input validation

// (...) Existing code

// user enters a number
cout << "\nPlease enter a positive number and press Enter: \n";
do {
    while (!(cin >> num1)) {
        cout << "Incorrect input. Please try again.\n";

        // Clear the failbit and ignore the remaining input
        cin.clear();
        cin.ignore(numeric_limits<streamsize>::max(), '\n');
    }

    if (num1 < 0)
        cout << "The number you entered is negative. Please enter a positive number to continue.\n";
} while (num1 < 0);

Explanation:

• while (!(cin >> num1)) checks if cin successfully read an integer. If it didn't, it enters the inner do-while loop to handle the error.
• cin.clear(); resets the failbit flag, allowing cin to read input again.
• cin.ignore(numeric_limits::max(), 'n'); reads and discards all characters until a newline character is encountered. This ensures that any invalid input entered before the newline is discarded.

With this correction, the program will now loop only when a valid positive integer is entered, preventing an infinite loop due to invalid input.

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