CSC/ECE 517 Summer 2008/wiki1 6 arraysandhashes

Arrays and Hashes

In this article, we compare arrays and hashes in Ruby and Java. Arrays and hashes are built into Ruby. Arrays, of course, are built into Java too, but hashes are only available through library classes. We demonstrate using logically equivalent code sequences the convenience of programming in both languages. Our Ruby descriptions apply to version 1.8, and for Java, we utilize Java 6 to take advantage of the new constructs that eliminate the drudgery of manually boxing, unboxing, and declaring iterators.

Arrays

While both Java and Ruby provide built-in support for arrays, they differ in the operations that can be performed on them. Arrays in both languages are objects, though this association is implicit in Java arrays, where arrays are container objects whose direct superclass is type Object. In Ruby, arrays are easily immediately identifiable as objects and they are represented by the Array class.

Comparison of Common Operations in Ruby and Java

In this section, we provide a side-by-side examination of common array operations in Ruby and compare and contrast them with Java.

Creating an Array

One way to create an array in Ruby is to use the new class method:

 planets = Array.new(8)

This creates an empty array named planets. This is similar to the Java initialization:

 String[] planets = new String[8];

Arrays can also be initialized to values in both Java and Ruby. For example, we can initialize the arrays of planets by entering the following in Java, noting the use of curly braces in array initialization:

 String[] planets = {"Mercury", "Venus", "Earth", "Mars", "Jupiter", 
        "Saturn", "Uranus", "Neptune" };

Some people consider Pluto a planet, but this is arguable. In Ruby, we can perform a similar operation, instead using square brackets:

 planets = [ :Mercury, :Venus, :Earth, :Mars, :Jupiter,
        :Saturn, :Uranus, :Neptune ]

The similarities between Java and Ruby with respect to array creation end here. Ruby provides additional convenience operators on array creation that Java lacks. Note also that we have introduced the concept of symbols, another concept which Java lacks, and thus we use Strings for the Java implementation.

An array in Ruby can also be created by specifying the default value to each element in the array:

 planets = Array.new(8, :EmptySpace)

Furthermore, since Ruby offers range capabilities, one can initialize an array using ranges:

 digits = Array(1..x)

The above Ruby code creates an array of digits with elements from 1 through x. The equivalent Java code is far more cumbersome and potentially error-prone, as the programmer must micromanage the details of array indexing:

  for (int j = 1; j <= x; j++)
    digits[j - 1] = j;

Accessing Array Elements

Elements in Java and Ruby are accessed in much the same way, though Ruby again offers a few additional methods for accessing elements of arrays. In Java, one could access the element :Venus of the planets array as:

 planets[0];

In Ruby, the operation is much the same:

 planets[0]

In addition, Ruby allows one to index using negative indices, so that:

 planets[-1]

returns :Neptune. Finally, Ruby provides the first and last methods to easily get the first and last elements of an array.

Ruby provides several methods, which, in combination with the range operator, provide an elegant way to obtain sub-arrays of an array. In Ruby, for example, we can generate a sub array of the three elements following :Mercury from the original array:

 some_planets = planets[1..3]

Such tasks can also be accomplished in Java to a certain extent, but require the use of the java.util.Arrays package. The equivalent Java code for the simple operation above, for instance, would be:

 String[] somePlanets = java.util.Arrays.copyOfRange(planets, 1, 4)

Note that the Java method is a half-open interval.

Both languages allow the programmer to iterate over the array. In Ruby, this can be accomplished with the use of blocks. Let us print out all of the planets in uppercase with a simple one-liner, remembering that each planet is actually represented as a symbol:

 planets.each { |p| puts p.to_s.upcase }

In Java, one can use the for-each loop instead, which has similar, but somewhat more verbose syntax:

 for (String p : planets)
   System.out.println(s.toUpperCase());

Finally, let's look at what happens when we access an element that doesn't exist in the array, such as index 100. Java will throw an ArrayIndexOutOfBoundsException. Ruby, on the other hand, returns nil.

Checking Equality

Ruby provides the == method to compare arrays for equality. Given the following code:

 planets_a = [:Mercury, :Venus, :Earth]
 planets_b = [:Mercury, :Venus, :Earth]
 planets_c = [:Earth, :Neptune, :Pluto]

It is easy to see that:

 planets_a == planets_b => true
 planets_a == planet_c => false

While Java provides both == and equals(), these operators and methods derive from java.lang.Object and do not provide the intended result. Given the following code:

 planetsA = {"Mercury", "Venus", "Earth"};
 planetsB = {"Mercury", "Venus", "Earth"};
 planetsC = {"Earth", "Neptune", "Pluto"};

The expressions:

 planetsA == planetsB;
 planetsA.equals(planetsB);

both return false. Instead, to get the desired behavior one must again utilize the package java.util.Arrays, in particular, the java.util.Arrays.equals() method:

 java.util.Arrays.equals(planetsA, planetsB)

This is just one example of where Ruby follows the principle of least surprise, such that the language behaves in such a way as to minimize confusion for experienced users.

Inserting, Removing, and Changing Elements

Most static languages, including Java, require you to increase the size of an array in order to add elements to it. In Java, one must create a new array of larger size and copy the elements into the newly created array before adding the new element. To compensate for these deficiencies, Java provides classes like ArrayList, but traditional arrays are still subject to this limitation.

Moreover, while Java provides both System.arraycopy, and convenience methods in java.util.Arrays, the resulting Java code is far from transparent. Ruby, on the other hand, makes such operations as insertion and addition trivial out of the box to implement, largely due to its dynamic handling of arrays.

Let us first change some elements in an array, without affecting its size, by changing :Venus, :Earth, and :Mars with :Larry, :Curly, and :Moe. In Java:

 planets[1] = "Larry";
 planets[2] = "Curly";
 planets[3] = "Moe";

All of these can be changed simultaneously in Ruby with a single statement, again, with the help of the range operator:

 planets[1..3] = [:Larry :Curly :Moe]

Let us now assume that :Pluto has been defined as a proper planet. It should now be added to the end of the planets array. In Java, one must use the library call java.util.Arrays.copyOf, or roll their own loop. In either case, the algorithm requires the creation of a new array. We choose to implement the former approach:

 planets = java.util.Arrays.copyOf(planets, planets.length + 1);
 planets[planets.length - 1] = "Pluto";

The same operation in Ruby is incredibly straight-forward:

 planets << :Pluto

Finally, let us remove :Earth from the planets array (possibly, it has been destroyed as a result of global warming) in Java. The implementation again is tedious and cumbersome as one must pay attention to the array indexing of each method argument. In fact, a significant amount of time was spent establishing the array boundaries:

 String[] new_planets = new String[planets.length-1];
 System.arraycopy(planets, 0, new_planets, 0, 2);
 System.arraycopy(planets, 2 + 1, new_planets, 2, 
     new_planets.length - 2);

In Ruby, one can simply say:

 planets.delete :Earth

Or, with absolute indices:

 planets.delete_at(2)

Array Summary

To be fair, in Java 6, many of the vanilla array deficiencies have been addressed with additional classes and APIs, in particular, the ArrayList, which provides methods such as clear, clone, and add that mimic operations found in Ruby. Most experienced Java programmers use ArrayList when additional array flexibility is desired. Despite these classes, the number of pre-defined methods on arrays available in Java lack sorely when compared with Ruby. For example, Java offers no built-in equivalent to the Ruby method uniq. Finally, in methods that are nearly equivalent between Java and Ruby, the Ruby syntax tends to be less verbose and more declarative.

Hashes

Ruby provides a hash table through the Hash class, and offers built-in syntax to access hash table functions. Java provides hash tables through the java.util.Hashtable class, which extends java.util.Dictionary. Both Java hash tables and Ruby hash tables offer similar functionality at the basic level, but Ruby provides a host of additional methods that make munging hash tables easier.

Comparison of Common Operations in Ruby and Java

In this section, we provide a side-by-side examination of common hash operations in Ruby and compare and contrast them with Java.

Creating a Hash Table

In Java, we can initialize an empty hash table called planets as follows:

 import java.util.Hashtable;
 Hashtable<String, Integer> planets = 
      new Hashtable<String, Integer>();

A similar operation can be performed in Ruby with:

 planets = Hash.new

If we so choose, Ruby allows us to initialize the planets array through the use of curly braces in a single statement, eliminating the Hash.new call entirely:

 planets = {
   :Mercury => 4900,
   :Venus => 12100,
   :Earth => 12800,
   :Mars => 6800,
   :Jupiter => 143000,
   :Saturn => 125000,
   :Uranus => 51100,
   :Neptune =>49500
 }

Ruby symbols are an excellent choice for a hash key. In Java, the hash table can only be populated with individualized calls to the put method, and Strings must be used:

 planets.put("Mercury", 4900);
 planets.put("Venus", 12100);
 planets.put("Earth", 12800);
 planets.put("Mars", 6800);
 planets.put("Jupiter", 143000);
 planets.put("Saturn", 125000);
 planets.put("Uranus", 51100);
 planets.put("Neptune", 49500);

The hash tables above associate a planet with its diameter in kilometers. If later we decide that :Pluto should be a proper planet, it can be added to our hash table like this:

 planets[:Pluto] = 2300

In Java, we again use the put method:

 planets.put("Pluto", 2300);

Accessing Table Entries

In the previous examples we created a hash table named planets in Java and in Ruby. The hash table contains the names of planets which can be accessed with their corresponding key maps. In this section we are going to explore the differences in accessing the data in each of these hash tables.

To retrieve the entries from the planets table in Java we can do the following:

 planets.get("Saturn");
 planets.get("Jupiter");

The same task can be accomplished in Ruby as follows:

 planets[:Saturn]
 planets[:Jupiter]

Both sets of code would retrieve 125000 and 143000.

Ruby will return the default value of nil if a key is not found, but this default value can be overridden during creation of the hash table. Java always returns null when a key is not found. A common operation is have a key that is not found map to zero. Here are two different ways to accomplish this in Ruby:

 planets = Hash.new(0)
 planets = Hash.new { |h,k| h[k] = 0 }

The first method will return a default value of 0 for entries not in the table. The second method will return a default value of 0, and also add the missing entry to the hash table.

Much like arrays, hash tables can also be iterated over in both Java and Ruby. In Ruby, we can use each:

 planets.each {|k,v| puts "#{k}: #{v}" }

In Java, we once again use for-each to accomplish the same goal, but the resulting code is not nearly as succinct:

 for (String k: planets.keySet())
   System.out.println(k + ": " + planets.get(k));

Verifying, Removing, and Replacing Entries

To prevent errors, it is common to verify that an entry or a key is in a table before trying to do an operation on it. The following Java code will check the hash table planets to see whether or not they are empty, check if the key :Saturn exists, and check if the value 125000 exists in the hash table.

 planets.isEmpty();
 planets.containskey("Saturn");
 planets.containsValue(125000);

Ruby has similar functions:

 planets.empty?
 planets.has_key?(:Saturn)
 planets.has_value?(125000)

Finding the size of a hash table is also very similar in Java and Ruby. Finding the size of hash table in Java:

 planets.size();

Compared with Ruby:

 planets.size

The following Java code will remove the :Jupiter and its corresponding entry from the hash table:

 planets.remove ("Jupiter");

Ruby can accomplish the same task with:

 planets.delete(:Jupiter);

To replace an entry in a hash table in Java, we can simply use the put method again. The method returns the previous value or null if there is no previous value (yes, this ruins our planet/diameter value to key pairing, but it illustrates replacement operations):

 planets.put("Pluto", 1000);

In Ruby this operation would look like:

 planets[:Pluto] = 1000

Converting a Hash Table Into an Array

As a final operation, let us take the planets hash table and turn its keys into an array, so that it resembles the original planets array at the beginning of this article. In Ruby, the operation is trivial:

 planets_array = planets.keys

In Java, we must first convert the hash table to a keySet and then use it's toArray method to return a planets_array as a String:

 String[] planetsArray = planets.keySet().toArray(new String[0]);

Since toArray normally returns an Object array, some effort is needed in toying with the types so that the compiler is satisfied.

Of course, while this array will have the same contents as the original array, the order of the array elements may be different depending on the internal hash structure.

Hash Table Summary

The difference in creating and using hash tables in Java as opposed to Ruby is mostly a matter of cosmetic syntax. Unlike arrays, hash tables in both Ruby and Java are dynamic data structures, so many of the methods are nearly identical in functionality. As with arrays, the Ruby code is a little more readable and the same task in Java can often be accomplished with less code in Ruby.

External Links

• Java Platform, Standard Edition 6 API Specification, for see java.util.Arrays and java.util.ArrayList; for hash tables see java.util.Hashtable.
• The Java Language Specification, Chapter 10: Arrays.
• The Java Tutorials, see the section on Arrays.
• RDoc Documentation, on the Array class and Hash class.
• Learning Ruby by Michael Fitzgerald, O'Reilly. Chapters 6 (Arrays) and 7 (Hashes). [Offline Book].
• To Ruby From Java
• Source code for Ruby Array class and Hash class. Sun JDK source code available once license agreements are accepted.

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