How to Robustly Validate User Input as a Double in C ?

Validating User Input as a Double in C

One common programming task is to validate user input to ensure it meets certain criteria. One such criterion is ensuring the input is a valid floating-point number, specifically a double. Here's how you can approach this validation in C :

Basic Validation Approach

double x;

while (1) {
  cout << ">";
  if (cin >> x) {
    // valid number
    break;
  } else {
    // not a valid number
    cout << "Invalid Input! Please input a numerical value." << endl;
  }
}

This code provides basic validation by attempting to read a double from the standard input. If the input is successfully read, you continue with the program. Otherwise, an error message is displayed.

Fixing the Infinite Input Loop

However, the above code may result in the program continuously outputting the error message even for valid input. This is because the cin >> x line fails on invalid input, leaving the error flag set in the input stream. To fix this issue, you need to clear the error state after the invalid input is detected.

while (1) {
  if (cin >> x) {
    // valid number
    break;
  } else {
    // not a valid number
    cout << "Invalid Input! Please input a numerical value." << endl;
    cin.clear();
    while (cin.get() != '\n') ; // empty loop to discard invalid input
  }
}

The cin.clear() line clears the error flag, and the subsequent empty loop consumes the remaining invalid characters on the input line. This ensures the next input attempt starts with a clean slate.

By following these steps, you can effectively validate user input as a double in C , providing a robust and user-friendly input validation mechanism.

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