CSC/ECE 517 Summer 2008/wiki1 6 c9)

Arrays and Hashes

Arrays and Hashes are indexed collection of objects, that are accessible using a key. In arrays, the 'key' is always an integer. The 'key' is called as an 'index' in array. In hashes, the 'key' can be an object of any type. Arrays provide efficient access to array elements, while hashes provide more flexibility.

Arrays

Array in Java and Ruby

In Java - An array is an indexed collection of items of same data type.We are not allowed to declare an array consist of multiple data type(like double and integer in same array).

In Ruby - Arrays are the indexed collection of objects in Ruby, which is accessible using keys.The key is always an integer in a an array and the array can grow as needed.Ruby support to store different type of data to store in one array.An array can can have integer, floating point number or a string.

Declaration of array

In Java - integer [] employee; double employee[];

The square bracket indicates the array declaration.

It can also be declared in one line as :-

integer employee = new integer [5];

Now the Employee array has allocated the memory to store the five integers.Unless we have initialised the array at the time of declaration we need to specify the size of an array before using it.

The indexing of an array starts from 0 to size-1.It means if the array is of size 5 then first element is stored at employee[0] and the last one is at employee[4] .In an array the elements can be initialised one by one upto the previously declared size of an array.

for ex:-

        String employee = new String [5]
       for(int i = 0; i <5 ; i++)
       {
         employee[i] = i;
       } 

If elements are initialised at the time of the declaration then ther is no need of specifying a size to an array.The size of an array is same as the number of initialised elements. for ex:-

String [] weekday = {Monday,Tuesday,wednesday,Thursday,friday,Saturday,Sunday};

In this example the size of an array is 7.

in Ruby -

Ruby does not require to declare the size of an array.

x = ['cat' , 5 ,6 ,'fly']
y = ['apple' , 'orange', 'pineapple']

An array can be declared with the new keyword also. for ex:

num = array.new

num << 3

num << 4

Now that array num has two element 3 and 4.

The indexing of an array starts from 0 and goes up to the size-1. Ruby supports the negative indexing in array.Negative index counts backward and x[-1] is same as referencing the last element of x.

Adding elements in the existing array

Java does not allow to add more elements than the size of an existing array.If user is interested to add more items,the size of the array needs to be increased. Ruby is very flexible with adding the number of elements in the array at run time of any data type since ruby is not bound to declare the size of an array before using it.For ex:-

 num = [1 ,2 ,4, 3, 5]
 num + ["apple", "cycle"]
 num =>[1, 2, 4 , 3,"5", "apple", "cycle"]

Another way to add the element is :-

 num.push "berry"

It adds the berry at last in the array.

 num =>[1, 2, 4 , 3,5,"Berry"]

Adding an element at a particular index is

 num[index number] = item
or num.insert(2,7)

 num =>[1, 2,7,4,3,5]

The size of the array can be increased multiple times with the same element in the same order.

 num = [1 ,2 , 3, 4, 5]
 num * 2 =>[1, 2, 3, 4, 5, 1, 2, 3, 4, 5]
 num * 3 =>[1, 2, 3, 4, 5, 1, 2, 3, 4, 5,1, 2, 3 ,4, 5]

Passing an array as a argument in a method:-

In Java an array can be passed to a method as an argument like a variable is passed to a method with some additional rules. For ex: -

       integer company (integer []employee)
       {
        return 0;        
       }
       company(employee);

When an array is passed to a method only the reference of the array is passed the copy of the array is not created in the method.

Ruby also provide the functionality of passing an array as a argument in methods. The argument name in the declaration of the method starts with the "*" symbol indicates that the argument is an array.

def hello(*args)
puts #{args.join(",")}
end
hello('jon' ,'Mary')   #=> jon,Mary

Deletion of an element from an array:-

In Java Deletion requires some kind of search to locate to particular item in the array.There are two ways to remove an element from an array. The first approach is to reset the array element to null but this can cause holes in the array.The second approach is to store the real occurence at the begining of the array and null refernces can be kept at the end of the array. The second approach comes up with two possible solutions.If the jth element is removed then the element starting from the j+1th position to the null will be shifted one position lower.And the second possibility is to replace the removed null refernce with the last refernece of an array.This technique can be used only if array is not arranged in aparticular order otherwise the first possibility is suitable to use.

In ruby deleting an element from an array is very easy as compared to Java.The adjustment of elements are done automatically.If an element is removed from an jth position then the elements from the position j+1 moves one position down. For ex:-

num = [1, 2, 3, 4, 5]
num.delete(3)              #=> [1 ,2 ,3, 5]
num .delete_at(2)        #=> [1, 2,4 ,5]

Some useful functionality with array:-

num = [1,2,3,4,5]

In Java to find the length of an array we can use an inbuilt function num.length => 5 and to sort the array ,reverse the array etc we have to write the algorthim depending on the requirment.

Ruby comes up with lots of easy built in methods that makes array very easy to use. For ex:-

num = [1 ,2 , 3, 4, 5,"apple"]

checks if array is empty : - num.empty # =>true (if empty) false otherwise

size of an array :- num.size # =>6 (the size of an array)

type of the element:- num[0].size # =>fixnum

                                        num[5].class   	 #=>string

first element of the array:- num.first # =>1

last element of the array :- num.last #=>apple

Adding an element :- num.push "orange" # =>[1 ,2 , 3, 4,5,"apple","orange"]

removing the last element num.pop # =>[1 ,2 , 3, 4, 5,"apple"])

accesing array element num.at[0] # =>1

removes all elements from array num.clear # =>[]

reverse the array num.reverse #=> ["apple", 5, 4, 3, 2, 1]

return the index of an element num.rindex("b") #=> nil

num.rindex("2") #=> 2

sorting an array (x = [2, 4 3]) x.sort #=>[2, 3, 4]

Two dimensional array:-

In two dimensional array an array is arranged in the form of table with rows and coloumns.The array can be declared as :-

integer employee = new integer[4][5]; where number of rows are 4 and number of coloums are 5.

The array can be initialise as :-

       for(int i = 0; i<4 ; i++)
       {
             for(int j = 0; j<5 ; j++)
             {
                 employee[i][j] = 1;
                     }
                }

Ruby does not support two dimensional array but still the same effect can be generated through nested array:-

num = [ [1, 2], [3, 4] ] num [0][0] -> 1 num [0][1] -> 2

Hashes

Hashing in Ruby and Java:-

A collection of key value pair is called 'hash'. It is sometimes called as 'associative array' or 'dictionaries' or a 'paired group of elements in simpler terms'. It is similar to array except hashes don't use numeric indices, the word keys are used instead. The objects that the keys refer to are called values.Keys and values occur in pairs, so there must be an even number of arguments.

In Ruby, variables must be initialized, the content of a variable that doesn't exist return the nil.

Language Support

In Ruby -

Hashes are built-in objects in Ruby.

In Java - Java does not provide direct support for implementing hashes. We need to import Java's API named 'Java.util.*' in order to implement hashes.

Decalaration and initialisation of hash:-

If we want to map the fruits with their respective colors, we can do that by defining hashes. The following paragraphs will show the difference in initialization using Java and Ruby.

In Java -

Map food_color = new HashMap();
    food_color.put( "Apple", "red" );
    food_color.put( "Banana", "yellow" );
    food_color.put( "Lemon", "yellow" );
    food_color.put( "Carrot", "orange" );

In Ruby -

food_color = [  
   {"Fruit" => "Apple", "Color" => "red"},  
   {"Fruit" => "Banana", "Color" => "yellow"},      
   {"Fruit" => "Lemon", "Color" => "yellow"},
   {"Fruit" => "Carrot", "Color" => "orange"}    
   ]  

Another way of declaration is:-

food_color = {”Apple” => "red", “Banana” =>"yellow", “Lemon” =>"yellow", “Carrot” =>"orange"}

or

food_color = {”apple” , "red", “Banana”, "yellow", “Lemon” , "yellow", “Carrot”, "orange"}

or

food_color = {}    # create an empty hash
food_color["Apples"]="red"
food_color["Banana"]="yellow"
food_color["Lemon"]="yellow"
food_color["Carrot"]="orange"   

Adding an element to Hash

In Java - To add an element to hash in Java we need to use '.puts' method.

Map map = new HashMap(); 
Object key = null; Object value = null;
map.put( key, value );
food_color.put( "Strawberry", "red" );  

In Ruby - In Ruby, we can add element to hash just like array except the difference that the index is an object and does'nt have to be integer.

food_color["Strawberry"]="red"
 

To print the whole hash

In Ruby -

  food_color.each {|key, value| puts "#{key} => #{value}"}  

In Java -

for ( Iterator it = food_color.keySet().iterator(); it.hasNext(); ) {
   System.out.println( it.next() );

}

To acces single element:-

fruits["apple"] => sweet

To sort an hash:-

fruits.sort.each {|key, value| puts "#{key} => #{value}"}  

=> apple=>sweet =>orange=>sweet and sour =>strawberry=>delicious

Hash allows the functionality of adding the numbers in the exixting key value.

num["x"] = 2 num["x"] +1 = 1 => 3

If the key-value does not exist and value is added it returns nil.For ex:- num["y"] += 3 =>return nil