Hashes

A Hash is a collection of unique keys and their corresponding values in pairs. 
	   In Smojo, the word 
# ( -- # )
 creates an empty hash on the stack:
	
		# constant my-hash
		my-hash .
		{} ok
	

	Here 
constant
 binds the hash created by 
#
 to 
my-hash
. Therefore, we can get the hash directly by 
my-hash
.
	

	The word 
#! ( k v # -- )
 writes a key-value pair into an existing hash. 
	It requires three arguments: key (k), value (v) and the target hash (#):
	
		"year" 2017 my-hash #!
		my-hash . 
		{year=2017} ok
	

	Both the key and value can be anything -- text and integer ( as in the example above ) or XTs. Anything. 
	The word 
#@ ( k # -- v )
 returns a matching value given its key:
	
		"year" my-hash #@ .
		2017 ok
	

	Note that if the key is not on the hash, the result is a special object called 
NULL ( -- null )
, which can be tested
	using the word 
NULL? ( * -- f )
. For example: 
	
		"bingo" my-hash #@ null? . 
		true ok
		"year" my-hash #@ null? .
		false ok 
	

	You may test for the presence of a key-value pair on a hash by using the word 
#CONTAINS? ( k # -- f )
.
	This word puts 
TRUE
 on the stack if a key exists on the hash and 
FALSE
 otherwise:
	
		"year" my-hash #contains? . 
		true ok
	

	The word 
#DROP ( k # -- )
 removes a key-value pair from the hash; 
	It requires two arguments: key and the target hash table:
	
		"year" my-hash #drop
		my-hash . 
		{} ok
	

	

	
In summary, hashes are a simple data structure in which you can store and retrieve data temporarily (i.e., as long as your program is running.). Here are some other useful words for a hash:
		


	  	

	    	Word
	    	Action
	  	

	  	
	  	

        
#@ ( k # -- v )

        Retrieves a value (v) from a hash given the corresponding key (k).
        


        

        
#! ( k v # -- )

        Stores a key-value pair (k v) into a hash.
        


        

        
#CONTAINS? ( k # -- f )

        Returns TRUE if the key (k) is on the hash and FALSE otherwise.
        


		

	    	
#KEYS ( # -- seq )

	    	Takes a hash and returns the sequence of all keys. Note that the keys are returned in no particular order.
	  	


		

	    	
#VALUES ( # -- seq )

	    	Takes a hash and returns the sequence of all values. Note that the values are returned in no particular order.
  		


		

	    	
#SORTED ( -- # )

	    	This puts a special "sorted" hash on the stack. 
		The 
#KEYS
 of a sorted hash will be sorted in "alphabetical" (or more precisely, lexicographical) order.		
  		


		

	    	
># ( #t #s -- )

	    	Copies the contents of the source hash (#s) into the target hash (#t).
  		


		

	    	
#EMPTY? ( # -- f )

	    	Returns 
TRUE
 if the hash is empty, 
FALSE
 otherwise.		
  		


		

	    	
#SIZE ( # -- n )

	    	Returns the number of key-value pairs in the hash.
  		



		
	
		
		
	
		
		
	
Quiz

	
Question 1

	
Is is possible to have 
NULL
 as key or associated value in a hash? 
		Prove your answer using a program.

	
	
	


	

	
Question 2

	
What happens if a key-value pair is overwritten by the same key but with a different value?
		Prove your answer using a program.

		
	
	


	

	
Question 3

	
Create a hash called "me" and write your name, age, and gender into the hash. Then try the following words on your hash:
	
		#SIZE       \ returns the number of key-value pairs in the hash.
		#EMPTY?     \ returns true if the hash is empty.
		#CONTAINS?  \ returns true if the has contains the given key
	

	Continue with your hash "me", remove all the key-value pairs from hash "me" and then try 
#EMPTY?
 to see if it is empty now.

	

	
	


	

	
Question 4*

	
The words 
#KEYS
 and 
#VALUES
 return the keys and the values in a hash as a sequence respectively. Write a word 
#CLEAR
 which removes all the key-value pairs in a hash. Hint: which word can remove key-value pairs from a hash and which word returns the required input for that word?

		
	
	


	

	
Question 5*

	
Can a hash be used as a value in a hash? When might this be useful? Hint: how would you store the details of each student (name, height, gender) in a class to be retrieved by that student's ID?


	
	


	
 
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