CSCI 161 - Lecture 11 - Array Basics, Array Traversal Algorithms

Instructor's Class Notes

1. Arrays Basics
• Array - An indexed structure that holds multiple values of the same type.
• Index - An integer indicating the position of a particular value in a data structure.
• Element - Each item within the array is called an element.
• Zero-Based Indexing - A numbering scheme used throughout Java (and many other languages) in which a sequence of values is indexed starting with 0 (element 0, element , element 2, and so on).

   

2. Why Use Arrays?
• Why use arrays? Well, consider a program that must keep track of many temperatures, maybe two, three, or more…maybe dozens.
• Example using variables:
  

                  
  double temperature1;            
  double temperature2;            
  double temperature3;            
                  
                  

• Example using an array:
  

                  
  double[] temperature = new double[3];
                  
                  

• Syntax notation:
  

                  
  <element type>[] <name> = new <element type>[<length>];                
                  
                  

3. Auto-Initialization and Initialization with Known Values
• Auto-Initialization - The initialization of variables to a default value, such as on an array's elements when it is constructed.
• Zero-Equivalent Auto-Initialization Values:

Type	Value
int	0
double	0.0
char	'\0'
boolean	false
objects	null

• Initializing with Known Values - Shorthand for initializing
  Syntax:
  

                  
  <element type>[] <name> = {<value>, <value>, ..., <value>};
                  
                  

  Example:
  

                  
  int[] daysIn = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};                
                  
                  

4. Accessing a Specfic Element
• Any element of an array can be accessed using an index into the array.
• For example, to get the value associated with the third element of the grades array and use that value to set a new variable, myGrade, execute the following:

  
  int myGrade = grades[2];                
                  

• IMPORTANT: Note that the index of the third element is two (2). Remember, this is because of zero-based indexing.

   

5. Array Traversal
• The For-Each Loop - Loop construct specifically for array loops.
  Syntax:

                  
  for (<type> <name> : <array>) {
      <statement>
      <statement>
      <statement>
      ...
  }                
                  
                  

  Example:
  

                  
  for (int x: temps) {
      if (x > average) {
          above++;
      }
  }                
                  
                  

• For loop - Or use a For loop along with .length property.
  
  Example:
  

                  
  for (int idx=0; idx<temps.length; idx++) {
      if (temp[idx] > average) {
          above++;
      }
  }                
                  
                  

  NOTE: .length is a property of an array, not a method (no parameters).

   

6. Printing an Array
• Printing an Array - cannot just use println as that prints out "garbage."
  
• Arrays Class - Similar to Integer, Double, and Character this class as static methods that are useful in performing array operations, manipulations. See book for list of useful methods including: copyOf, equals, fill, sort, toString
• Example of good and bad:

                
System.out.println(list);  // Bad, prints garbage                

System.out.println(Arrays.toString(list));  // Better
                                            // [17, -3, 42, 8, 12, 2, 103]
                
                

7. Other Array Algorithms
• Look in the book for more examples:
• Search and Count - example
• Search and Replace - example
• Reverse - example
• String Traversal - example