C arrays (groups of variables)

• Arrays and indexes
• A video that covers the fundamentals in more depth

Concepts

• Indexing
• Getting the address of the first element

Examples to aid understanding

1. Array

int arr[2]{};
arr[0] = 1;
arr[1] = 2;

std::cout << arr[0];
std::cout << std::endl;

std::cout << arr[1];
std::cout << std::endl;

What does int arr[2]{} mean?

It declares an array of 2 elements, all initialized to zero by {}.

What is an array?

An array is like several variables combined into one. In this case, there are effectively 2 variables: arr[0] and arr[1].

The phrase “effectively” is intentional here, because in fact arr[0] and arr[1] are objects, not variables, but that is covered in another lesson.

Answer

arr[0] and arr[1] can effectively be treated like variable names. So 1 and 2 will be printed.

2. Array initialization

int arr1[2]{ 21, 32 };

std::cout << arr1[0];
std::cout << std::endl;

std::cout << arr1[1];
std::cout << std::endl;

What does this syntax mean?

Here, the array elements are initialized when the array is created, in index order (0, 1).

3. Array with an unspecified length

int arr1[3]{ 1, 2, 3 };
int arr2[]{ 1, 2, 3, 4 };

std::cout << sizeof(arr1);
std::cout << std::endl;

std::cout << sizeof(arr2);
std::cout << std::endl;

What is sizeof?

It is an operator evaluated at compile time that gives the size of the entire array in bytes. In this example, the arr1 array has 3 ints, each 4 bytes, so the total size is 12.

What is the size of arr2?

arr2 has no explicitly specified size; its size is determined automatically from its elements.

What does [] mean?

In this context, the array length is determined at compile time by the number of elements on the right (4 in this example).

Answer

3 ints of 4 bytes each make 12.

4 ints of 4 bytes each make 16.

4. Size of an array of size_t

size_t arr[3]{ 1, 2, 3 };

std::cout << sizeof(arr);
std::cout << std::endl;

Answer

sizeof gives the size of the entire array in bytes, not the number of elements. Here, each element has type size_t, which is 8 bytes on a 64-bit system, so 3 elements of 8 bytes each make 24.

5. Computing the number of elements from sizeof

int arr[]{ 1, 2, 3, 4 };

std::cout << sizeof(arr);
std::cout << std::endl;

std::cout << sizeof(arr[0]);
std::cout << std::endl;

std::cout << sizeof(arr) / sizeof(arr[0]);
std::cout << std::endl;

How to compute the number of items using sizeof?

Divide the size of the whole array by the size of one element.

Answer

Here, sizeof(arr) is 16 (4 ints of 4 bytes each), sizeof(arr[0]) is 4 (one int), so 16 / 4 gives 4 elements.

This only works while arr is still an array. Once it decays to a pointer (see below), sizeof gives the pointer size instead.

6. Reading by index

int arr[3]{ 1, 2, 3 };
size_t index { 2 };
int it { arr[index] };
std::cout << it;
std::cout << std::endl;

Answer

You can use a variable or expression to specify an index.

The answer is 3.

7. Writing by index

int arr[3]{};
size_t index { 2 };
arr[index] = 5;
std::cout << arr[2];
std::cout << std::endl;

Answer

You can also write by index using an index produced by an expression.

8. An expression as an index

int arr[3]{};
size_t index { 1 };
arr[index + 1] = 5;
std::cout << arr[2];
std::cout << std::endl;

Answer

This demonstrates using a more complex expression to obtain an index.

9. Copying an array element into a variable

int arr[3]{ 0, 2, 1 };
size_t index { 2 };
int it { arr[index] };
arr[index] = 5;
std::cout << it;
std::cout << std::endl;

Answer

On the line int it { arr[index] }, the value 1 is copied into it, not a reference to an array element, because the type of it is int. Since it is just an int, changing the array element from which its value originated after assignment does not affect it.

1 will be printed.

10. A pointer as the element type

int a = 1;
int b = 2;
int* arr[]{ &a, &b };
*arr[0] = 3;
*arr[1] = *arr[0];

std::cout << arr[0] << std::endl;
std::cout << arr[1] << std::endl;

std::cout << *arr[0] << std::endl;
std::cout << *arr[1] << std::endl;

Answer

Arrays can store data other than int values. In this example, the array stores pointers to int values (int*).

• arr[0] contains the address of variable a.
• arr[1] contains the address of variable b.
• a, equivalent to *arr[0], equals 3.
• b, equivalent to *arr[1], equals 3.

11. Using an array as a pointer

int arr[2]{};
int* p = arr;
*arr = 1;

std::cout << *p;
std::cout << std::endl;

std::cout << arr[0];
std::cout << std::endl;

std::cout << arr[1];
std::cout << std::endl;

Answer

When arr is used as an expression in int* p = arr, it decays into a pointer to the first element of the array. Here, arr is equivalent to &arr[0] or &(arr[0]).

*p, *arr, and arr[0] all access the same array element.

1 will be printed twice.

12. Printing an array

int arr[3]{ 1, 2, 3 };
std::cout << arr;
std::cout << std::endl;

Answer

When arr is used as an expression in this context (as an argument to <<), it decays into a pointer to the first element, equivalent to &arr[0].

std::cout doesn’t know how to print C arrays of a certain size — it only sees a pointer, so what gets printed is that address, not the elements 1, 2, 3. To print the elements, print each one by index, as in the first examples.