C++ - Basic Input/Output

Basic Input/Output in C++

C++ provides built-in functionality for handling input and output operations through streams. These operations allow us to interact with the user, read data from the keyboard, and display information on the screen. The primary header file used for these operations is #include <iostream>.

1. Standard Input and Output Streams

C++ uses two standard streams for input and output:

• cin: The standard input stream used to accept data from the user (usually from the keyboard).
• cout: The standard output stream used to display data to the user (usually on the screen).

Syntax for Input and Output

The input and output operations in C++ are done using the stream insertion operator (<<) for output and the stream extraction operator (>>) for input.

Example

#include 
using namespace std;

int main() {
    int age;
    cout << "Enter your age: ";  // Output message to user
    cin >> age;  // Read input from user and store in 'age'
    cout << "Your age is " << age << endl;  // Output the entered age
    return 0;
}

2. Output with cout

The cout object is used for output in C++. It is followed by the insertion operator (<<) to display data to the console.

Example of Output

#include 
using namespace std;

int main() {
    cout << "Hello, World!" << endl;  // Output text with a newline at the end
    return 0;
}

In the example above, the text "Hello, World!" will be displayed on the screen. The endl is used to insert a newline after the output.

Commonly Used Output Modifiers

• endl: Outputs a newline character and flushes the output buffer.
• \n: A newline character that can also be used in place of endl, but it doesn't flush the buffer.
• setw: Sets the width for the next output value. It's defined in the iomanip header file.

Example with setw

#include 
#include   // For setw
using namespace std;

int main() {
    cout << setw(10) << 42 << endl;  // Set width of 10 for output
    cout << setw(5) << 123 << endl;  // Set width of 5 for output
    return 0;
}

3. Input with cin

The cin object is used for input in C++. It is followed by the extraction operator (>>) to take input from the user.

Example of Input

#include 
using namespace std;

int main() {
    int number;
    cout << "Enter a number: ";
    cin >> number;  // Read the input from user and store it in 'number'
    cout << "You entered: " << number << endl;
    return 0;
}

In this example, the program asks the user to enter a number. The cin >> number; statement takes the user input and stores it in the variable number.

Reading Multiple Values

We can use cin to read multiple values in a single statement by separating them with the extraction operator (>>).

Example

#include 
using namespace std;

int main() {
    int x, y;
    cout << "Enter two numbers: ";
    cin >> x >> y;  // Read two integers from user
    cout << "You entered: " << x << " and " << y << endl;
    return 0;
}

4. Handling Strings with cin

When reading strings, we can use cin to read individual words (until a space is encountered). However, if we need to read a full line, including spaces, we should use the getline() function.

Example with cin for Strings

#include 
using namespace std;

int main() {
    string name;
    cout << "Enter your first name: ";
    cin >> name;  // Reads a single word
    cout << "Hello, " << name << "!" << endl;
    return 0;
}

Using getline()for Full Line Input

If you want to read a full line of text, use thegetline() function, which reads input until it encounters a newline character.

Example with getline()

#include 
#include 
using namespace std;

int main() {
    string full_name;
    cout << "Enter your full name: ";
    getline(cin, full_name);  // Reads a full line including spaces
    cout << "Hello, " << full_name << "!" << endl;
    return 0;
}

5. Input/Output with fstream (File I/O)

C++ also provides functionality to read from and write to files using the fstream library. You can use the ifstream (input file stream) and ofstream (output file stream) objects for file I/O operations.

Syntax

#include 
using namespace std;

int main() {
    ofstream outfile("output.txt");  // Open file for writing
    outfile << "This is a test." << endl;  // Write to file
    outfile.close();  // Close the file
    
    ifstream infile("output.txt");  // Open file for reading
    string line;
    getline(infile, line);  // Read a line from the file
    cout << "File contents: " << line << endl;
    infile.close();  // Close the file
    return 0;
}

6. Error Handling in I/O

C++ provides a way to handle errors during input and output operations. For example, you can check if an I/O operation was successful using the fail() function.

Example of Error Handling

#include 
using namespace std;

int main() {
    int number;
    cout << "Enter a number: ";
    cin >> number;

    if (cin.fail()) {
        cout << "Error: Invalid input!" << endl;  // Error message for invalid input
    } else {
        cout << "You entered: " << number << endl;
    }
    return 0;
}

7. Summary of C++ Input/Output

cout << "Hello, World!" << endl;

cin >> age;

ofstream outfile("file.txt");