What is an array size Declarator?

Definition

Declaring Arrays:

  typeName variableName[size]; 

This declares an array with the specified size, named variableName, of type typeName.  The array is indexed from0 to size-1. The size (in brackets) must be an integer literal or a constant variable. The compiler uses the size to determine how much space to allocate (i.e. how many bytes).

Examples:

  int list[30];            // an array of 30 integers 
  char name[20];           // an array of 20 characters 
  double nums[50];         // an array of 50 decimals 
  int table[5][10];        // a two dimensional array of integers 

The last example illustrates a two dimensional array (which we often like to think about as a table). We usually think of the first size as rows, and the second as columns, but it really does not matter, as long as you are consistent! So, we could think of the last declaration as a table with 5 rows and 10 columns, for example.
 

Initializing Arrays:

  int x; 
  x = 0; 

Or, we could simply initialize the variable in the declaration statement itself:

  int x = 0; 

Can we do the same for arrays? Yes, for the built-in types. Simply list the array values (literals) in set notation { } after the declaration. Here are some examples:

  int list[4] = {2, 4, 6, 8}; 
  char letters[5] = {'a', 'e', 'i', 'o', 'u'}; 
  double numbers[3] = {3.45, 2.39, 9.1}; 
  int table[3][2] = {{2, 5} , {3,1} , {4,9}}; 

C-style strings

• We use strings frequently, but there is no built-in string type in the language
• A C-style string is implemented as an array of type char that ends with a special character, called the "null character".
  • The null character has ASCII value 0
  • The null character can be written as a literal in code this way: '\0'
• Every string literal (something in double-quotes) implicitly contains the null character at the end

Since character arrays are used to store C-style strings, you can initialize a character array with a string literal (i.e. a string in double quotes), as long as you leave room for the null character in the allocated space.

  char name[7] = "Johnny";

Notice that this would be equivalent to:

  char name[7] = {'J', 'o', 'h', 'n', 'n', 'y', '\0'};

Variations in initializing

Array declarations must contain the information about the size of the array. It is possible to leave the size out of the [ ] in the declaration as long as you initialize the array inline, in which case the array is made just large enough to capture the initialized data. Examples:

  char name[] = "Johnny";        // size is 7
  int list[] = {1, 3, 5, 7, 9};  // size is 5

Another shortcut with initializer sets is to use fewer elements than the size specifies. Remaining elements will default to 0. It is illegal to use a set containing more elements than the allocated size.

  int list[5] = {1, 2};         // array is {1, 2, 0, 0, 0}
  int nums[3] = {1, 2, 3, 4};   // illegal declaration.

Note: Using initializers on the declaration, as in the examples above, is probably not going to be as desirable with very large arrays.
Another common way to initialize an array -- with a for loop:
This example initializes the array numList to {0, 2, 4, 6, 8, 10, 12, 14, 16, 18}.

  int numList[10];
  int i;
  for (i = 0; i < 10; i++)
     numList[i] = i * 2;

Using Arrays:

  int x, list[5];        // declaration 
  double nums[10];       // declaration 

  list[3] = 6;           // assign value 6 to array item with index 3 
  cout << nums[2];       // output array item with index 2 
  list[x] = list[x+1]; 

It would not be appropriate, however, to use an array index that is outside the bounds of the valid array indices:

  list[5] = 10;        // bad statement, since your valid indices are 0 - 4. 

The statement above is syntactically legal, however. It is the programmer's job to make sure that out of bounds indices are not used. Do not count on the compiler to check it for you -- it will not!
 

Copying arrays:

  int list1[5]; 
  int list2[5] = {3, 5, 7, 9, 11}; 

With variables, we use the assignment statement, so this would be the natural tendency -- but it is wrong!

  list1 = list2;  // this does NOT copy the array contents 

We must copy between arrays element by element. A for loop makes this easy, however:

  for (int i = 0; i < 5; i++) 
    list1[i] = list2[i]; 

Simple I/O with strings:

  char greeting[] = "Hello"; 
  char word1[20]; 

  cout << greeting[1];  // prints the letter 'e' 
  cout << greeting[4];  // prints the letter 'o' 

Strings can also be output and input in their entirety, with the standard input and output objects (cin and cout):
The following line outputs the word "Hello":

  cout << greeting; 

The following line allows the entry of a word (up to 19 characters and a terminating null character) from the keyboard, which is stored in the array word1:

  cin >> word1;

Characters are read from the keyboard until the first "white space" (space, tab, newline, etc) character is encountered.  The input is stored in the character array and the null character is automatically appended.
 

Examples

• arrayinit.cpp -- example of array declarations and initializer sets
• strings.cpp -- initializing and printing c-style strings
• arrayuse.cpp -- a few examples of array usage

What is a size of array?

What is the difference between an array size Declarator and a subscript?

What happens if you leave out the size Declarator in an array definition?

How do you find the size of an array?