pelicin › prog › C++

Arrays

Arrays are sequences of items of the same type. An array has:

  • a type — the type of items in it
  • a size — the number of items in it

Declaring an array

One-dimensional array

Specify the array type and the array size (the number of items in the array):

int my_array[5];

You can also initialize the content of the array directly. (If you do this, specifying the size becomes optional because the compiler can infer it.)

// You can also write the size explicitly,
// e.g. `int myArray[5]`
int my_array[] = { 1, 2, 3, 4, 5 };

Multi-dimensional array

An array could also be an array of array ("multi-dimensional array").

To define one, specify the array type and the size of each "dimension".

// Two-dimensional array
int array_2d[10][20];

// Three-dimensional array
int array_3d[10][20][50];

You can initialize multi-dimensional array in two ways:

// The "nested" way
int arr_a[2][4] = {
  { 1, 2, 3, 4 },
  { 5, 6, 7, 8 },
};

// The "flat" way
int arr_b[2][4] = { 1, 2, 3, 4, 5, 6, 7, 8 };

Accessing array elements

Specify the array name, and the index that you want to access in [].

Array indexing are zero-based. So, the first element is at index 0, the tenth element is at index 9, and so on.

int arr[] = { 1, 2, 3 };
printf("%d\n", arr[2]); //=> 3

Accessing the element of a multi-dimensional array works similarly:

int arr[2][4] = {
  { 1, 2, 3, 4 },
  { 5, 6, 7, 8 },
};
printf("%d\n", arr[1][2]); //=> 7

Getting the size of an array

Using the sizeof operator

You can use the sizeof operator (which returns the total size of the array in bytes), then divide it by the size of one element in bytes.

(Note that sizeof returns a size_t type, whose actual type depends on the compiler, but usually it's an unsigned long long.)

short arr[] = { 4, 5, 6, 7, 8, 9 };
size_t arr_length = sizeof(arr) / sizeof(short);
printf("%zu\n", arr_length); //=> 6

Using std::size()

You can #include <iterator> and use the std::size() function.

#include <cstdio>
#include <iterator>

int main() {
  short arr[] = { 4, 5, 6, 7, 8, 9 };
  size_t arr_length = std::size(arr);
  printf("%zu\n", arr_length); //=> 6
}

References