Data types and IO

In the previous lesson we built the template every solution starts from. Now we fill in the middle: how to store values and how to read input and print output.

As a reminder, this is where we left off:

#include <bits/stdc++.h>

using namespace std;

int main(){

	// our logic goes here

	return 0;
}

Variables

Before a program can compute anything, it needs somewhere to keep its values. That's what a variable is - a named box that holds one value:

int main(){

	int a = 13; // a box named "a" that holds the whole number 13

	return 0;
}

Reading it left to right: int says what kind of value the box holds (a whole number), a is the name we chose, and = 13 puts the value 13 inside. We can also create the box first and fill it later:

int main(){

	int a;  // create the box (it holds garbage for now)
	a = 13; // put 13 inside

	return 0;
}

Data types

The int above is a data type - it tells C++ what kind of value the variable stores. These are the basic ones you should know:

• int - for standard integer numbers (32 bit, roughly up to ±2 billion)
• long long - for large integer numbers (64 bit) - you'll need this surprisingly often in competitive programming
• char - for a single character (for example 'a')
• string - for text (for example "hello")
• double - for decimal numbers (you may also see float, but double is more precise and is what you should use)
• bool - for booleans (they can either be true or false)

There are of course many more with specific purposes, but we will get to them later. For now: whole numbers go in an int, big whole numbers in a long long, text in a string.

Input/Output

When solving a problem, most of the time there will be a "Standard Input/Output" or "Standard I/O" tag inside of the problem. This means the program takes information from the console and writes its answer back to the console.

You already know half of this - cout from the previous lesson prints to the console. Its twin for reading is cin (console input).

Reading with cin

For input we write cin >> a:

• cin >> means we want to read something from the console
• a is the variable the value gets stored into

Note how the arrows point in the direction the data flows: out of cin, into a.

int main(){

	int a;    // first we create the variable
	cin >> a; // then we read a value from the console into it

	return 0;
}

We can also read multiple values at once: cin >> a >> b; - this reads the first number into a and the second into b.

Printing with cout

For output we write cout << a - this prints the value stored in a. And we can chain text and values together with more << arrows:

int main(){

	int a = 13;

	cout << "The value of a is: " << a << "!";

	return 0;
}

Output: The value of a is: 13!

Putting it together

Here is a full program that reads a number and echoes it back:

#include <bits/stdc++.h>

using namespace std;

int main(){

	int a;
	cin >> a;

	cout << "The value of a is: " << a << "!";

	return 0;
}

Input: 92
Output: The value of a is: 92!

Making cin/cout faster

One thing worth knowing about cin and cout is that by default they can be slow. For the problems you'll solve at the start this makes no difference at all, but on problems with huge inputs a correct solution can fail just because its input/output was too slow.

The fix is to add these two lines at the beginning of main:
cin.tie(0);
ios_base::sync_with_stdio(false);

So a "full speed" solution looks like this:

#include <bits/stdc++.h>

using namespace std;

int main(){

	cin.tie(0);
	ios_base::sync_with_stdio(false);

	int a;
	cin >> a;

	cout << "The value of a is: " << a << "!";

	return 0;
}

These two lines can look confusing, and you don't need to understand how they work yet - but make writing them a habit. Start every solution that uses cin and cout with them, and slow input/output will simply never be the reason a solution fails. You will see them in most lectures on this site for exactly that reason.

Learn more about them here: https://stackoverflow.com/questions/31162367/significance-of-ios-basesync-with-stdiofalse-cin-tienull

scanf() and printf()

cin/cout with the two speed-up lines is all you need - this older style is here mainly so you recognize it in other people's code, where you will run into it constantly.

Before cin and cout, C++ inherited a pair of functions from the C language: scanf() for input and printf() for output.

For input we use scanf("%d", &a):

• scanf() calls the input function
• %d means we want to read an integer (every data type has its own code)
• &a means the value should be stored into a

int main(){

	int a;
	scanf("%d", &a);

	return 0;
}

For output it is very similar - printf("%d", a), and we can put text around the %d:

int main(){

	int a = 13;

	printf("The value of a is %d!", a);

	return 0;
}

Output: The value of a is 13!

(Note that when printing we pass a and NOT &a.)

Here are the codes for the common data types:

• int - %d
• long long - %lld
• char - %c
• string - %s
• double - %lf