Outline of Class 13: The Vector

Held: Monday, February 9, 1998

Administrivia

• At noon today, Randy Smith of Torrent Systems, Inc., a "Parallel Software Infrastructure Company", will give a talk entitled The Programming They Don't Teach You In College.
• Barry McKeown and Charlotte Abe of Towers Perrin will speak on "What Does An Actuary Actually Do" Thursday, February 12, 4:30 p.m. in Science 2413. Refreshments at 4:15.
• Our Convocation speaker, Prof. Doris Schattschneider, will be having lunch with students at Grinnell House on Wednesday, February 25. Please send a note to Prof. Herman if you would like to join her for lunch.
• Today we'll be returning to the topic of data structures, the primary focus of this course.

Vectors

• One of the simplest data types is the vector.
  • Different people define vectors somewhat differently.
  • Many people use the term array to refer to structures like Vectors.
  • Bailey's vector is but one version of what a vector can be; it is designed to match the built-in Java Vector.
  • Java also provides an array class.
• The primary intent of vectors and arrays is that you have a collection of objects of similar types (sometimes the same type) that you can reference by number (using numbers in some reasonable range).
• What questions might you ask in changing that vague definition to a more concrete list of methods?
• What constructors should we support?
  • Create new vectors?
  • Create new vectors of specified size?
  • Create new vectors of specified size with the same element in each position?
  • Create new vectors of specified size with different elements in each position?
• Do we differentiate between the number of elements in a vector, the number of valid indices, and the number of elements the vector can hold?
• Can vectors grow in size? Can they shrink?
• Can we change an assigned element?
• What happens if we can delete elements from the vector?
  • Do the remaining elements change position?
  • Do we simply get a hole in the vector?
  • Do we get a default value at that position?
  • ...
• Do we have a default/required lower-end of the range of indices?
• What happens if an illegal request is made?
  • And what are potentially illegal requests?
• What syntax should we use for making the requests?
• You may be surprised by some of the answers the designers of Java chose.

Arrays

• Java has a built-in (but not primitive) type called the array that uses a different syntax than other objects.
• Like vectors, arrays provide storage for a fixed number of things.
  • The things may be objects.
  • The things may be primitive types.
• To declare an array of things, you write
  thing[] name;
  • For example, to declare an array of integers called numbers, you would write
    int[] numbers;
• You create new arrays with the new command, followed by the type, followed by the number of things you want. If I wanted numthings numbers in the previous array, I could write
  numbers = new int[numthings];
• I could also combine the two
  int[] numbers = new int[numthings];
• You can create an initialize the array to a collection of values by surrounding the values with curly braces.
  • For example,
    int[] fibs = { 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89 }
• Java uses zero-based arrays. The initial thing in an array is element 0. If there are n things in an array, then the last thing is number n-1.
• You can refer to a particular element of the array as
  name[number]
  • For example, I could get the tenth element of numbers with
    numbers[10]
• You can determine the length of an array with name.length

How Do Vectors and Arrays Differ?

• Java provides both built-in arrays and a java.util.Vector class.
• How are they similar?
  • They both provide collections of elements that you can index by number.
  • Both work like objects. You create reference variables and then make the variables refer to them.
• How are they different? In a number of ways:
  • Arrays are built in to Java; Vectors are part of a library.
  • Arrays use a special syntax; Vectors use a normal syntax.
  • Arrays are fixed size; Vectors can grow and shrink.
  • You can store primitive types in arrays; you cannot store primitive types in Vectors.
  • You can have gaps in arrays; you cannot have gaps in Vectors.
  • You can initialize arrays when defining them; it is much more difficult to initialize Vectors.