How to Ensure Valid Integer Input in C ?

Checking User Input for Integer Validity

In the provided C code, the goal is to develop a program that reads two integers from the user and performs basic mathematical operations on them. However, an important consideration arises: how to ensure that the user enters valid integers.

To check if the input is an integer, we can utilize the cin.fail() function. It returns false if the input is valid and true if it is invalid or contains non-numeric characters.

Checking for Valid Integers

The following code snippet demonstrates how to check the validity of the user's input for both integers:

int firstvariable, secondvariable;
cin >> firstvariable;
if (cin.fail()) {
    // Not an integer; handle appropriately
}

cin >> secondvariable;
if (cin.fail()) {
    // Not an integer; handle appropriately
}

If the input is invalid, error handling is necessary. This can involve displaying a message, clearing the input stream, and re-prompting the user to enter a correct integer.

Handling Invalid Input

To ensure continuous input until a valid integer is entered, we can implement a loop that continues until the input passes the validity check:

while (cin.fail()) {
    // Clear the input stream
    cin.clear();

    // Ignore the invalid input
    cin.ignore(256, '\n');

    // Re-prompt the user for a valid integer
    cout << "Please enter a valid integer: ";

    // Retry the input
    cin >> firstvariable;
}

In this loop, the input stream is cleared, the invalid input is ignored, and the user is prompted to enter a valid integer.

Example

The following code provides a complete example that checks for valid integer input and handles invalid input:

#include <iostream>

using namespace std;

int main() {
    int firstvariable, secondvariable;

    cout << "Please enter two integers: ";
    cin >> firstvariable;

    while (cin.fail()) {
        cin.clear();
        cin.ignore(256, '\n');
        cout << "Invalid input. Please enter a valid integer: ";
        cin >> firstvariable;
    }

    cin >> secondvariable;

    while (cin.fail()) {
        cin.clear();
        cin.ignore(256, '\n');
        cout << "Invalid input. Please enter a valid integer: ";
        cin >> secondvariable;
    }

    // Perform mathematical operations on the valid integers

    return 0;
}

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