Input and Output

In the "Hello, World!" lesson, you learned how to make your program display a simple message using cout. That's one half of interacting with your program: output. The other half is input, which allows your program to receive data from the user or other sources.

This lesson will dive deeper into both output and input, teaching your programs how to talk to the outside world and listen back.

Output

You've already met cout, the standard output stream. It's used to send data to the console. We use the insertion operator << to send data to cout.

You can chain multiple << operators to print several things on one line:

#include <bits/stdc++.h>
using namespace std;

int main() {
    int age = 25;
    int month = 3;

    cout << "Hello! You are " << age << " years and " << month << " months old." << endl;
    // Output: Hello! You are 25 years and 3 months old.

    return 0;
}

Input

To get input from the user, we use cin, the standard input stream, along with the extraction operator >>. When cin reads input, it typically stops at whitespace (spaces, tabs, newlines).

#include <bits/stdc++.h> 
using namespace std;

int main() {
    int number;
    cout << "Enter a number: ";
    cin >> number; // Read an integer from the user

    // Example of reading multiple values
    int a, b;
    cout << "Enter two numbers (separated by space): ";
    cin >> a >> b; // Read two integers
    cout << "You entered: " << a << " and " << b << endl;

    return 0;
}

Example Interaction:

Enter a number: 42
You entered: 42
Enter two numbers (separated by space): 10 20
You entered: 10 and 20

This lesson covers the basics of how your C++ programs can communicate with the user, taking input and providing output. Mastering these operations is fundamental for any interactive program, especially in competitive programming where you'll constantly be reading problem inputs and printing answers.

endl vs '\n'

You've seen using endl to end a line. There is another way to do this, by using the newline character '\n'.

cout << "Hello" << endl;
cout << "World" << '\n';

Both lines will move the cursor to the next line in the output. So what's the difference?

• '\n' (Newline Character): This is a simple character that just represents a line break.
• endl (End Line): This does two things: it inserts a newline character (' ') and it flushes the output buffer.

An output buffer is a temporary storage area. When you use cout, your output might be held and pilled up in this buffer for a short time before being sent to the console. This is done for efficiency, because sending to console each time takes a little bit of time. Flushing the buffer forces the program to write everything in the buffer to the console immediately.

Why does this matter for Competitive Programming?

In competitive programming, you often have to print a very large amount of output (e.g., thousands of lines inside a loop). Forcing a flush on every single line with endl can be slow and inefficient. Using ' ' allows the output to be buffered and written in larger, more efficient chunks. This can sometimes be the difference between a program that is "Accepted" and one that gets a "Time Limit Exceeded" error.

Rule of Thumb:

• In most competitive programming problems, prefer ' ' for better performance, especially when printing many lines.
• Use endl only when you need to guarantee that the output is displayed on the screen immediately. An example of this is interactive problems, where the judge waits for your output before sending the next input, so if don't flush, both your program and the judge program will get stuck in a cycle of waiting for each other, ultimately causing your program to receive "Idleness Limit Exceeded" verdict.