Why Does Letter Input Cause an Infinite Loop in My C Program?

Infinite Loop Issue in C Program Accepting Letters as Integer Input

This issue arises when the program expects an integer input, but the user enters a letter instead. The program enters an infinite loop due to an unexpected character in the input stream. This results in a continuous display of the message "The number you have entered is negative. Please enter a positive number to continue." without providing the user a chance to enter a valid number.

Explanation

The underlying reason for this issue lies within the behavior of the C input stream. When the user enters a non-numeric character, the input stream's "failbit" flag is set. However, the stream is not cleared, so the program remains stuck in the same loop iteration, attempting to read the invalid character repeatedly.

Solution

To resolve this problem, it is necessary to implement proper input validation and error handling. Here is an example of how to address this issue:

#include <iostream>
#include <limits>

// ... (remaining code)

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

In this modified code:

• The while loop has been encapsulated within a do-while loop, ensuring at least one execution even if the initial input is invalid.
• Inside the inner loop, proper input validation is implemented using the cin.fail() function.
• If invalid input is detected, an error message is displayed, and the cin buffer is cleared and ignored to prevent it from affecting subsequent input attempts.
• The user is then prompted to re-enter a valid integer.
• This process continues until the user enters a positive integer.

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