Name: _________________________________________

CSC 107       Fall 2003

Lab 2: Functions and Grammars

In an earlier in-class exercise, you executed JavaScript code that generated random sequences of characters, some of which turned out to be words. In this lab, you will take the next step of generating random sentences based on grammar rules. To accomplish this, you will write simple functions that call library functions and each other, and so build complexity out of simplicity.

Random Parts of Speech

Chapter 5 introduced the random.js library, which contains several useful functions for generating random values. For example, the RandomOneOf function randomly selects an item from a list of options. Consider the following JavaScript function:

function noun() // Assumes: nothing // Returns: a random noun { return RandomOneOf(["man", "woman", "ball"]); }

This function uses RandomOneOf to select at random from a list of three nouns. The call noun() will thus return one of the three nouns each time it is executed.

EXERCISE 1:    Create a Web page named grammar.html and enter the noun function definition in the HEAD of that page. In the BODY of the page, call the noun function and display the returned value.

Load this page 10 times and list the displayed values below:

What is displayed each time the following JavaScript statement is executed? Explain.

document.write(noun() + " " + noun());

EXERCISE 2:    Similar to the noun function above, define additional functions called verb and article that return a random verb (e.g., "hit", "liked", "smelled") and article (e.g., "the", "a", "some"), respectively. Feel free to add to the vocabulary of these functions.

Important: Each function should return its randomly generated word, NOT write it!

Make the appropriate calls to test these functions in your grammar.html page.

Grammar Rules

If you recall from your grade school days, sentences can be parsed according to grammar rules -- syntactic rules that specify how the different parts of speech can fit together. For example, a simple sentence can be constructed with an article, followed by a noun, followed by a verb, such as "The ball smelled." or "Some woman hit." This can be written as a grammar rule in the following notation:

sentence <--- article + noun + verb

EXERCISE 3:    Define a function called sentence that returns a sentence of the above form. Your function should have no inputs (similar to noun, article, and verb), and should work by calling each of the previously defined functions and concatenating the resulting words with spaces in between. Note that the format of a sentence is not random, it will always consist of an adjective followed by a noun followed by a verb. The random appearance of sentences is due to the fact that the individual words are randomly generated by the other functions.

Important: Your function should return the sentence, NOT write it!

Once you have defined your function, modify the body of your grammar.html page to call this function and display the return value. Load the page 10 times and list the displayed sentences below:

EXERCISE 4:    Now consider the following grammar rules that define a more complex sentence structure. sentence <--- nounPhrase + verbPhrase nounPhrase <--- article + adjective + noun + prepPhrase verbPhrase <--- verb + nounPhrase prepPhrase <--- preposition + article + noun

Update your page to generate sentences based on these grammar rules. In particular, you must define new functions called adjective, preposition, nounPhrase, verbPhrase, and prepPhrase that return the appropriate type of word or phrase, and also modify sentence to call these functions appropriately.

For example, the call sentence() might produce "the big man by a tree hit a green ball above some woman" or "some nice woman beside the ball liked a big man by the house". Once you have defined/modified the necessary functions, load the page 10 times and list the displayed sentences below:

Optional Parts of Speech

The grammar rules in the previous exercise leave no options when constructing a sentence. Every sentence generated using these rules will have the exact same structure, even the same number of words (13). In practice, sentences and phrases can be defined with optional parts. For example, the adjective and prepositional phrase in a noun phrase are not necessary, since "the man", "the nice man", "the man by the tree", and "the nice man by the tree" are equally valid. Similarly, a verb phrase does not always require a noun phrase after the verb. For example, consider the following generalized grammar rules:

sentence <--- nounPhrase + verbPhrase nounPhrase <--- article + [adjective] + noun + [prepPhrase] verbPhrase <--- verb + [nounPhrase] prepPhrase <--- preposition + article + noun Here, the notation [PART_OF_SPEECH] is used to denote an optional part of speech. Note: The brackets used in grammar rules should not be confused with JavaScript brackets, which are used to represent lists in JavaScript code.

EXERCISE 5:    From looking at these grammar rules, what is the fewest number of words possible in a sentence? Similarly, what is the greatest number of words possible in a sentence. Justify your answers.

In order to generate sentences based on these more flexible grammar rules, you must modify the nounPhrase and verbPhrase functions so that they sometimes include the optional parts of speech and sometimes they don't. The RandomOneOf function will also prove useful here. In particular, whenever you want an optional part of speech, you can call the RandomOneOf function with two options to select from: the part of speech, and the empty string. For example, the following will either return an adjective (as a result of the call adjective() or the empty string.

RandomOneOf([ adjective(), "" ])

EXERCISE 6:    Modify the nounPhrase and verbPhrase functions to match the new grammar rules. In particular, you should include calls to RandomOneOf so that the specified parts of speech are optional. Using the new versions of these functions, load the page 20 times and list the displayed sentences below:

EXERCISE 7:    Referring to your 20 sentences from the previous exercise, answer the following questions:

Hand in a printout of grammar.html, attached to these sheets.