Array Basics

Array Basics

Definition

An array is a series of elements of the same type placed in contiguous memory locations that can be individually referenced by adding an index to a unique identifier.

That means that, for example, five values of type int can be declared as an array without having to declare 5 different variables (each with its own identifier). Instead, using an array, the five int values are stored in contiguous memory locations, and all five can be accessed using the same identifier, with the proper index.
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).
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:

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.

• 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:

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.
 

Examples

• example of array declarations and initializer sets
  

  // Example of array declarations and initializer sets
  
  #include 
  #include 
  using namespace std;
  
  int main()
  {
     int i;    // loop counter
  
     int array1[10];   // uninitialized array
     int array2[5] = {2, 4, 6, 8, 10}; // initializer set used
     int array3[5] = {2, 4};  // last three elements default to 0
     int array4[] = {10, 20, 30};  // array is size 3
  
     /* Initializing arrays of other types */
     double nums[5] = {1.1, 2.2, 3.3, 4.4, 5.5};
     char vowels[5] = {'a', 'e', 'i', 'o', 'u'};
  
     /* print out the array contents, for each array, using for-loops */
  
     cout << "\nContents of uninitialized array:\n";
     for (i = 0; i < 10; i++)
        cout << array1[i] << ' ';
  
     cout << "\n\nArray2:   ";
     for (i = 0; i < 5; i++)
        cout << array2[i] << ' ';
  
     cout << "\nArray3:   ";
     for (i = 0; i < 5; i++)
        cout << array3[i] << ' ';
  
     cout << "\nArray4:   ";
     for (i = 0; i < 3; i++)
        cout << array4[i] << ' ';
  
     cout << fixed << setprecision(1);
  
     cout << "\nArray of doubles:   ";
     for (i = 0; i < 5; i++)
        cout << nums[i] << ' ';
  
     cout << "\nArray of vowels:   ";
     for (i = 0; i < 5; i++)
        cout << vowels[i] << ' ';
  
     cout << '\n';
  
     return 0;
  } 
  

• initializing and printing c-style strings
  

  // Example of initializing C-style strings.
  // Example also illustrates output of strings.
  
  #include 
  using namespace std;
  
  int main()
  {
     int i;
     char input[30];
     char input2[30];
  
     char greeting[6] = "Hello";  // leaving room for the null char
     char name[20] = "Marvin Dipwart"; // initializer string is smaller 
       // than allocated space
  
     /* Now who really wants to count up the letters?? */
     char str1[] = "The quick brown fox jumped over the lazy platypus";
  
     /* Printing the strings.  A couple ways */
  
     cout << "\ngreeting = ";
     for (i = 0; i < 5; i++)   // not preferred
   cout << greeting[i];
  
     cout << "\nname = " << name << '\n';
     cout << "str1 = " << str1 << '\n';
  
     cout << "\nType an input string: ";
     cin >> input;
  
     cout << "\nType another input string: ";
     cin >> input2;
  //   cin.getline(input2, 30); // a call that can read a whole sentence
  
     cout << "\nThe input string you typed: " << input << '\n';
     cout << "\nThe another input string you typed: " << input2 << '\n';
  
     return 0;
  } 
  

• a few examples of array usage
  

  // Examples of array use
  
  #include 
  using namespace std;
  
  int main()
  {
     const int SIZE = 15;
     int i;   // loop counter
     int sum = 0;   // accumulator variable
     int list[SIZE];
  
     /* initialize to a pattern, with a for-loop */
     for (i = 0; i < SIZE; i++)
   list[i] = 3 * i + 1;
  
  
     /* print contents */
     cout << "\nThe list: ";
     for (i = 0; i < SIZE; i++)
   cout << list[i] << ' ';
  
     /* Let user type new values */
     cout << "\nPlease enter " << SIZE << " new array elements: ";
     for (i = 0; i < SIZE; i++)
   cin >> list[i];
  
     /* print contents */
     cout << "\nThe list: ";
     for (i = 0; i < SIZE; i++)
   cout << list[i] << ' ';
  
     /* print contents backwards */
     cout << "\nThe list backwards: ";
     for (i = SIZE-1; i >= 0; i--)
   cout << list[i] << ' ';
  
     /* Now add the array contents */
     for (i = 0; i < SIZE; i++)
   sum = sum + list[i];
  
     cout << "\nThe sum of the array elements is: " << sum << '\n';
  
     return 0;
  }