Fixed-size arrays

In this lesson we'll briefly explain what is std::array. Later in the course you'll learn more about it.

Motivation

We already have vectors, which represent a dynamic array, and we can use them to store any number of elements. You may ask:

Why do we even need arrays of fixed size?

The short answer is: performance.

array vs vector vs C-style arrays

std::array is a more performant alternative to std::vector, but with a few limitations. It is also a much safer alternative to C-style arrays.

We, the creators of this course are fully aware that for beginners a tiny performance improvement over vector is not worth the extra effort. This container will be covered more in-depth later, when you'll be more familiar with C++.

Using std::array

The first thing we have to do is to include the <array> header file:

#include <array>

Creating an array

There are quite a few ways to create an instance of std::array. Remember: we're talking about an array of fixed-size that we have to know in advance. We can't change the size of the array after it's created. A common practice is to estimate the largest number of elements we'll use and then create an array of that size.

Allow up to 10 players in our game

std::array<std::string, 10> player_names;

In the previous lesson we used similar notation to create vector instance. This time we have to provide both:

• the type of the elements (Type)
• the size of the array (N)

and we do that by separating them with a comma:

// prism-push-types:Type
std::array<Type, N>

It's quite usual to initialize an array with some predefined values. We can do that by using the curly braces syntax:

Allow up to 10 players, provide three initial names

std::array<std::string, 10> player_names = {
	"HappyBanana", "AngryCrab", "SadWolf",
};

Alternative syntax

You can omit the = between the player_names and the curly braces then wrap the list of elements in an additional {} to achieve the same result:

std::array<std::string, 10> player_names {{
	"HappyBanana", "AngryCrab", "SadWolf",
}};

Elements we left unset in { }

If you don't provide initial values for all elements, the remaining ones will be initialized with a default or zero value. In simple terms, depending on the type inside an array: numeric values will be set to 0, booleans to false, and strings will be empty.

This is the content of the array after the previous code snippet:

Index	Value
0	"HappyBanana"
1	"AngryCrab"
2	"SadWolf"
3	empty
...	...
9	empty

Accessing elements

We can access elements of an array the same way we do with vectors:

Display the first player's name

#include <iostream>
#include <string>
#include <array>

int main()
{
    std::array< std::string, 10 > player_names = {
		"HappyBanana", "AngryCrab", "SadWolf"
	};

    // Setting names of the players
    player_names[0] = "SadBanana";
    player_names[2] = "HappyWolf";

    // Printing the name of the first player:
    std::cout << "Name of the first player: " << player_names[0];
}

We changed the first and the third element of the array. The output of the program is:

Name of the first player: SadBanana

This is the content of the array after the previous code snippet:

Index	Value
0	"SadBanana"
1	"AngryCrab"
2	"HappyWolf"
3	empty
...	...
9	empty

Reading the size of an array

We can use the size() method to get the size of an array:

std::array<std::string, 10> player_names = {
	"HappyBanana", "AngryCrab", "SadWolf",
};

std::cout << "Number of players: " << player_names.size();

The output of the program is:

Number of players: 10

Size of std::array

As you can see from the example above, all of array "slots" are used regardless of the number of elements we set manually. The rest are initialized with default values, which in this case are empty strings.

Displaying elements

Just like we did with vectors, we can use a for loop to display all elements of an array:

Display all player names

for (std::string name : player_names)
{
	std::cout << "Player: " << name << '\n';
}

Zobacz pełen kod

Display all player names

#include <iostream>
#include <string>
#include <array>

int main()
{
	std::array< std::string, 10 > player_names = {
		"HappyBanana", "AngryCrab", "SadWolf"
	};

	// Printing all player names
	for (std::string name : player_names)
	{
		std::cout << name << '\n';
	}	
}

The output of the program is:

Player: HappyBanana
Player: AngryCrab
Player: SadWolf
Player:
Player:
Player:
Player:
Player:
Player:
Player:

Summary

To sum up, we've learned that there is a fixed-size array in C++ called std::array. In many cases we can use it similar to a vector, however it has some limitations and it's not as flexible. Our recommendation for you is to stick with std::vector for now.

We'll leave it here for now, but we'll come back to it later in the course.