Use of Curly Brackets (Braces) Around a variable C++

Question

I'm learning 2D arrays in my programming class. My teacher used something without explaining it and I was curious why we used it. Since it has to do with a symbol I'm not sure how to google or search for it, as these symbols are used in the search itself. Anyways the code was this:

int small[26]= {0}, large[26]={0}, i;

Why are the curly braces needed around the 0's?

The program this code is a part of examines a file and looks for each letter of the alphabet and counts them individually.

Answer 1

It could be written even simpler

int small[26]= {}, large[26]={}, i;

The curly brackets means an initializer list in this case of arrays.

Let assume for example that you want to define an array with elements 1, 2, 3, 4, 5.

You could write

int a[5];

a[0] = 1;
a[1] = 2;
a[2] = 3;
a[3] = 4;
a[4] = 5;

However C++ allows to assign elements of an array when it is defined. The equivalent record will look

int a[5] = { 1, 2, 3, 4, 5 };

If there are initializers less than the size of the array then remaining elements will be initialized by zeroes. For example

int a[5] = { 1, 2 };

In this case a[0] will be equal tp 1 a[1] will be equal to 2 and all other elements will be equal to 0.

You may omit the size of an array. For example

int a[] = { 1, 2, 3, 4, 5 };

In this case the compiler will allocate as many elements of the array as there are initializers in the initializer list.

Record (valid only in C++. In C it is not allowed)

int a[5] = {};

is equivalent to

int a[5] = { 0 };

that is all elements of the array will be initialized by 0. In the last record the first element is initialized explicitly by zero and all other elements are also initialized by zero because their initializers in the initializer list were not specified.

The same way you can initialize also scalar objects. For example

int x = { 10 };

The only difference that for scalar objects you can specify only one initializer. You even may write without the assignment operator

int x { 10 };

You also can write

int x {};

In this case x will be initialized by 0.

Answer 2

int Array[26]= {0};

Initializes all elements in that array to 0.

As BryanChen mentions below:

Note: int Array[26]= {1}; will initialize first element to 1 and others to 0. 

Answer 3

By default, regular arrays of local scope (for example, those declared within a function) are left uninitialized. This means that none of its elements are set to any particular value; their contents are undetermined at the point the array is declared.

But the elements in an array can be explicitly initialized to specific values when it is declared, by enclosing those initial values in braces {}.

Answer 4

Note that

int array [constant] = {};

Default initializes all values. The default initializer for int is 0.

int small[26]= {0};

Sets the first number to 0 and default initializes the rest.

In general:

int array [constant] = {value initialize all indices listed here, 
                        the rest are default initialized};