How to Validate Numeric Input in C Beyond `atoi()`?

Validating Numeric Input in C

In a program that processes integer input, ensuring that users provide valid numbers is crucial. Unfortunately, the atoi() function falls short when dealing with multi-digit integers. This article explores alternative methods to verify numeric input.

Using the failbit

C 's input stream (cin) sets the failbit when it encounters input it cannot parse. This can be leveraged to validate input as follows:

int n;
cin >> n;
if(!cin) {
    // User did not input a number
    cin.clear();
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

• if(!cin) checks if the failbit is set.
• cin.clear() resets the stream state.
• cin.ignore() skips the invalid input, allowing for re-input.

Handling Input Overflow

When integers exceed the maximum allowed value, cin also sets the failbit. To prevent this, check the stream state before reading the input:

while(!cin.eof()) {
    int n;
    cin >> n;
    if(!cin) {
        // Input error occurred
        cin.clear();
        cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }
    else {
        // Valid input
        process(n);
    }
}

This loop continues reading and validating input until the end of file (EOF) is reached.

Other Alternatives

• C 11's stringstream: Create a stringstream from the input and attempt to convert it to an integer.
• Regular Expressions (regex): Use regular expressions to enforce a specific numeric format.
• Boost Libraries: Employ Boost's boost::regex or boost::lexical_cast for input validation.

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