Array Basics


An array is an indexed collection of data elements of the same type.
1)  Indexed means that the array elements are numbered (starting at 0).
2)  The restriction of the same type is an important one, because arrays are stored in consecutive memory cells.  Every cell must be the same type (and therefore, the same size).

Declaring Arrays:

An array declaration is similar to the form of a normal declaration (typeName variableName), but we add on a size:
  typeName variableName[size]; 

This declares an array with the specified size, named variableName, of type typeName.  The array is indexed from 0 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).


  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:

With normal variables, we could declare on one line, then initialize on the next:
  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

Arrays of type char are special cases.

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:

Once your arrays are declared, you access the elements in an array with the array name, and the index number inside brackets [ ]. If an array is declared as: typeName varName[size], then the element with index n is referred to as varName[n]. Examples:
  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:

If we have these two arrays, how do we copy the contents of list2 to list1?
  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:

In the special case of strings (null-terminated character arrays), they can be used like normal arrays.  Accessing a single array element means accessing one character.
  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; 
Be careful to only use this on char arrays that are being used as C-style strings. (This means, only if the null character is present as a terminator).

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.