Your First Program

This lesson assumes you have never written a line of C++ before.

What are we even doing?

In competitive programming, every task follows the same pattern:

The program reads some input (usually numbers or text)
It computes something
It prints the answer

The programs are short, so we don't need most of what "real" software development involves. We just need a small set of tools, and this lesson introduces the very first ones.

The smallest possible program

Every C++ program needs a starting point - a place where the computer begins executing your code. That starting point is a function called main:

int main(){

}

This is a whole program that runs, it does nothing, but it runs. Let's break down the parts:

main - the name of the function. The computer always looks for a function with exactly this name and starts there.
(){} - the parentheses and curly braces. Everything between the curly braces {} is the code that will run.
int - this says that main will give back (return) an integer when it finishes. By convention, that number tells the operating system how the program ended.

return 0

Since we promised that main returns an integer, it is good practice to actually return one. Returning 0 means "everything went fine":

int main(){

	return 0;
}

return 0 is really important in competitive programming: some online judges will reject your solution without it, even if it is correct. Make it a habit to always write it as the last line of main.

Printing something

An empty program is boring, let's make it say hello. To print text, we use cout.

cout << "Hello!";

The << arrows point into cout - you can think of it as "send "Hello!" to the console output".

But there is a catch: cout doesn't exist on its own. It lives inside the standard library - a big collection of ready-made tools that comes with C++. To use it, we have to add two lines above our main function:

#include <bits/stdc++.h> - this includes the standard library into our program. Think of it as bringing the toolbox to the desk.
using namespace std; - the tools in the toolbox have the label std (standard) on them, and would normally be called like std::cout. This line lets us drop the label and just write cout.

Putting all of that together, we get your first real program:

Solution.cpp

#include <bits/stdc++.h> // bring in the standard library

using namespace std; // let us write cout instead of std::cout

int main(){

	cout << "Hello!"; // print Hello! to the console

	return 0; // tell the judge everything went fine
}

Output: Hello!

This is your template

Those few lines are the skeleton of every solution you will ever submit:

Solution.cpp

#include <bits/stdc++.h>

using namespace std;

int main(){

	// your logic goes here

	return 0;
}

Don't worry if #include and namespace still feel a bit like magic words. For now it is completely fine to treat them as "the two lines at the top" - the details behind them will make more sense as you write more programs.