C++ User Input

Introduction

Handling user input is a fundamental aspect of interactive programming in C++. It allows programs to accept data from users and process it accordingly. C++ provides various ways to capture user input, primarily using the cin object from the iostream library. Understanding how to take user input effectively is crucial for building interactive applications.

Using cin for Basic Input

The cin object is used to capture input from the standard input device, usually the keyboard. The input is stored in the variables provided to cin.

Example: Reading an Integer

#include <iostream>
using namespace std;

int main() {
    int age;
    cout << "Enter your age: ";
    cin >> age; // Capture user input and store it in 'age'
    cout << "You entered: " << age << endl;
    return 0;
}

Output

Enter your age: 25
You entered: 25

Explanation

• cin >> age; captures the input from the user and stores it in the variable age.
• The value entered by the user is then printed using cout.

Reading Multiple Values

You can use cin to read multiple values separated by spaces or newline characters.

Example: Reading Multiple Values

#include <iostream>
using namespace std;

int main() {
    int a, b;
    cout << "Enter two integers separated by space: ";
    cin >> a >> b; // Capture two integers
    cout << "You entered: " << a << " and " << b << endl;
    return 0;
}

Output

Enter two integers separated by space: 10 20
You entered: 10 and 20

Explanation

• cin >> a >> b; captures two integers from the user and stores them in variables a and b.
• The values entered by the user are then printed using cout.

Reading Strings

To read strings, you can use cin. However, cin reads only until the first whitespace character. To capture an entire line, you need to use the getline function.

Example: Reading a Single Word

#include <iostream>
using namespace std;

int main() {
    string name;
    cout << "Enter your name: ";
    cin >> name; // Capture user input and store it in 'name'
    cout << "Hello, " << name << "!" << endl;
    return 0;
}

Output

Enter your name: John
Hello, John!

Explanation

• cin >> name; captures a single word input from the user and stores it in the variable name.

Example: Reading a Line of Text

#include <iostream>
#include <string>
using namespace std;

int main() {
    string fullName;
    cout << "Enter your full name: ";
    cin.ignore(); // Ignore any leftover newline characters in the input buffer
    getline(cin, fullName); // Capture the entire line of input
    cout << "Hello, " << fullName << "!" << endl;
    return 0;
}

Output

Enter your full name: John Doe
Hello, John Doe!

Explanation

• cin.ignore(); is used to ignore any leftover newline characters in the input buffer.
• getline(cin, fullName); captures the entire line of input, including spaces, and stores it in the variable fullName.

Handling Different Data Types

You can use cin to read different data types by specifying the appropriate variable type.

Example: Reading Different Data Types

#include <iostream>
using namespace std;

int main() {
    int age;
    double height;
    char grade;

    cout << "Enter your age: ";
    cin >> age; // Capture integer input

    cout << "Enter your height in meters: ";
    cin >> height; // Capture double input

    cout << "Enter your grade: ";
    cin >> grade; // Capture char input

    cout << "You entered:\n";
    cout << "Age: " << age << "\n";
    cout << "Height: " << height << " meters\n";
    cout << "Grade: " << grade << endl;

    return 0;
}

Output

Enter your age: 20
Enter your height in meters: 1.75
Enter your grade: A
You entered:
Age: 20
Height: 1.75 meters
Grade: A

Explanation

• cin >> age; captures an integer input and stores it in age.
• cin >> height; captures a double input and stores it in height.
• cin >> grade; captures a character input and stores it in grade.

Error Handling

When capturing user input, it’s essential to handle errors gracefully, such as when the user enters an invalid data type.

Example: Handling Input Errors

#include <iostream>
using namespace std;

int main() {
    int age;
    cout << "Enter your age: ";
    cin >> age;

    if (cin.fail()) { // Check if the input is valid
        cout << "Invalid input. Please enter a number." << endl;
        cin.clear(); // Clear the error flag
        cin.ignore(numeric_limits<streamsize>::max(), '\n'); // Ignore the invalid input
    } else {
        cout << "You entered: " << age << endl;
    }

    return 0;
}

Output

Enter your age: abc
Invalid input. Please enter a number.

Explanation

• cin.fail() checks if the input operation failed.
• cin.clear() clears the error flag on cin.
• cin.ignore(numeric_limits<streamsize>::max(), '\n'); discards the invalid input from the input buffer.

Conclusion

Capturing user input is a fundamental aspect of interactive programming in C++. This chapter covered how to use the cin object to read different types of input, including integers, doubles, characters, and strings. Additionally, it addressed how to handle multiple inputs and input errors. Understanding how to handle user input effectively will help you create interactive and user-friendly programs. In the next chapter, we will explore pointers in C++.