CSE143 Notes for Wednesday, 1/29/14

I began by asking how we could write a program that would count the number of unique words in an input file. I had a copy of the text of Moby Dick that we looked at to think about this. I showed some starter code that constructs a Scanner object tied to a file:

        import java.util.*;
        import java.io.*;
        
        public class WordCount {
            public static void main(String[] args) throws FileNotFoundException {
                Scanner console = new Scanner(System.in);
                System.out.print("What is the name of the text file? ");
                String fileName = console.nextLine();
                Scanner input = new Scanner(new File(fileName));

                while (input.hasNext()) {
                    String next = input.next();
                    // process next
                }
            }
        }

So how do we count the words? Someone suggested that a set would be the perfect structure to solve this problem. It eliminates duplicates, so it will keep track of how many different words there are. So we changed the code to be:

        Set<String> words = new TreeSet<String>();
        while (input.hasNext()) {
            String next = input.next();
            words.add(next);
        }
        System.out.println("Total words = " + words.size());

        What is the name of the text file? moby.txt
        Total words = 32019

        String next = input.next().toLowerCase();

        What is the name of the text file? moby.txt
        Total words = 30368

This program counts the number of unique words, but not the counts of the individual words. To keep track of word counts, we could use many different structures. For example, we could have a List of words and a List of counts where element "i" in one corresponds to element "i" in the other. This approach is often described as "parallel arrays." It's not a very object-oriented approach because we really want to associate the word with its counts rather than have a structure that puts all the words together and another that puts all the counts together. Or we could make a class for a word/count combination and then have a List of that. But Java gives us a better alternative. The collections framework provides a data abstraction known as a map.

The idea behind a map is that it keeps track of key/value pairs. In our case, we want to keep track of word/count pairs (what is the count for each different word). We often store data this way. For example, in the US we often use a person's social security number as a key to get information about them. I would expect that if I talked to the university registrar, they probably have the ability to look up students based on social security number to find their transcript.

In a map, there is only one value for any given key. If you look up a social security number and get three different student transcripts, that would be a problem. With the Java map objects, if you already have an entry in your map for a particular key, then any attempt to put a new key/value pair into the map will overwrite the old mapping.

We looked at an interface in the Java class libraries called Map that is a generic interface. That means that we have to supply type information. It's formal description is Map<K, V>. This is different from the List and Set interfaces in that it has two different types. That's because the map has to know what type of keys you have and what type of values you have. In our case, we have some words (Strings) that we want to associated with some counters (ints). We can't actually use type int because it is a primitive type, but we can use type Integer.

So our map would be of type Map<String, Integer>. In other words, it's a a map that keeps track of String/Integer pairs (this String goes to this Integer). Map is the name of the interface, but it's not an actual implementation. The implementation we will use is TreeMap. So we can construct a map called "count" to keep track of our counts by saying:

        Map<String, Integer> count = new TreeMap<String, Integer>();

I asked what code we need to record a word in our map the first time we see it. Someone suggested using the put method to assign it to a count of 1. So our loop becomes:

        Map<String, Integer> count = new TreeMap<String, Integer>();
        while (input.hasNext()) {
            String next = input.next().toLowerCase();
            count.put(next, 1);
        }

I asked people to think about what to do if a word has been seen before. In that case, we want to increase its count by 1. That means we have to get the old value of the count and add 1 to it:

        count.get(next) + 1

        count.put(next, count.get(next) + 1);

        Map<String, Integer> count = new TreeMap<String, Integer>();
        while (input.hasNext()) {
            String next = input.next().toLowerCase();
            if (!count.containsKey(next)) {
                count.put(next, 1);
            } else {
                count.put(next, count.get(next) + 1);
            }
        }

Then we talked about how to print the results. Clearly we need to iterate over the entries in the map. One way to do this is to request what is known as the "key set". The key set is the set of all keys contained in the map. The Java documentation says that it will be of type Set. We don't have to really worry about this if we use a for-each loop. Remember that a for-each loop iterates over all of the values in a given collection. So we can say:

        for (String word : count.keySet()) {
            // process word
        }

        for (String word : count.keySet()) {
            System.out.println(count.get(word) + "\t" + word);
        }

        What is the name of the text file? moby.txt
        Minimum number of occurrences for printing? 500
        4571    a
        1354    all
        587     an
        6182    and
        563     are
        1701    as
        1289    at
        973     be
        1691    but
        1133    by
        1522    for
        1067    from
        754     had
        741     have
        1686    he
        552     him
        2459    his
        1746    i
        3992    in
        512     into
        1555    is
        1754    it
        562     like
        578     my
        1073    not
        506     now
        6408    of
        933     on
        775     one
        675     or
        882     so
        599     some
        2729    that
        14092   the
        602     their
        506     there
        627     they
        1239    this
        4448    to
        551     upon
        1567    was
        644     were
        500     whale
        552     when
        547     which
        1672    with
        774     you

In the final version that I posted on the calendar, I made one minor change. There is an interface known as SortedMap. When you know that you want to require a map to have its keys sorted, it is more appropriate to use that interface. Every SortedMap is a Map, but not every Map is a SortedMap. So the program changes the declaration to:

        SortedMap<String, Integer> count = new TreeMap<String, Integer>();

The sample program involves keeping track of friendships. You could think of it as keeping track of Facebook friends. One of the first questions that comes up is how do we represent friendships? For example, are friendships bidirectional? If person A is friends with person B, does that mean that person B is friends with person A? For our purposes, we will assume the answer is yes. If we were trying to represent something like "is attracted to", then we'd come to a different conclusion, but for friends, just like on Facebook and other social networking sites, friendship goes both ways.

I said that a good way to visualize friendships is to draw a graph in which each person is represented with a node (an oval) and each friendship is represented by an edge connecting two nodes (a line drawn between two ovals). I am using a program called Graphviz, which is an open-source graph viewer.. For example, here is a sample friendship graph:

This information is stored in a file with lines that list pairs of friendships, as in:

        graph {
            Ashley -- Christopher
            Ashley -- Emily
            Ashley -- Joshua
            Bart -- Lisa
            Bart -- Matthew
            Christopher -- Andrew
            Emily -- Joshua
            Jacob -- Christopher
            Jessica -- Ashley
            JorEl -- Zod
            KalEl -- JorEl
            Kyle -- Lex
            Kyle -- Zod
            Lisa -- Marge
            Matthew -- Lisa
            Michael -- Christopher
            Michael -- Joshua
            Michael -- Jessica
            Samantha -- Matthew
            Samantha -- Tyler
            Sarah -- Andrew
            Sarah -- Christopher
            Sarah -- Emily
            Tyler -- Kyle
            Stuart -- Jacob
        }

For example, here is a sample execution using our data file for finding the connection between Stuart and Ashley:

        Welcome to the cse143 friend finder.
        starting name? Stuart
        target name? Ashley
        
        Starting with Stuart
            1 away: [Jacob]
            2 away: [Christopher]
            3 away: [Andrew, Ashley, Michael, Sarah]
        found at a distance of 3

Here is a sample execution where the connection is not found, asking for a connection between Stuart and Bart:

        Welcome to the cse143 friend finder.
        starting name? Stuart
        target name? Bart 
        
        Starting with Stuart
            1 away: [Jacob]
            2 away: [Christopher]
            3 away: [Andrew, Ashley, Michael, Sarah]
            4 away: [Emily, Jessica, Joshua]
            5 away: []
        not found

We looked at one more example that involved a fairly long chain:

        Welcome to the cse143 friend finder.
        starting name? Bart  
        target name? JorEl
        
        Starting with Bart
            1 away: [Lisa, Matthew]
            2 away: [Marge, Samantha]
            3 away: [Tyler]
            4 away: [Kyle]
            5 away: [Lex, Zod]
            6 away: [JorEl]
        found at a distance of 6

If we want a structure that keeps track of these kind of friendships, then we want to use names as keys into the structure. We ask the structure, "Who are the friends of Samantha?" or "Who are the friends of Ashley?". So a name, a String, will be used as the key. But what should it return? If we map a String to a String, then we can store only one friendship. We want to be able to return more than one friendship. Someone suggested that we want to use a set. That is exactly right.

The idea is that we want to have a map that converts a String into a Set of String values. Given the name of a person, we can get a Set with the names of that person's friends. For our sample file:

        "Andrew"        => maps to => [Christopher, Sarah]
        "Andrew"	=> maps to => [Christopher, Sarah]
        "Ashley"	=> maps to => [Christopher, Emily, Jessica, Joshua]
        "Bart"	        => maps to => [Lisa, Matthew]
        "Christopher"	=> maps to => [Andrew, Ashley, Jacob, Michael, Sarah]
        "Emily"	        => maps to => [Ashley, Joshua, Sarah]
        "Jacob"	        => maps to => [Christopher, Stuart]
        "Jessica"	=> maps to => [Ashley, Michael]
        "JorEl"	        => maps to => [KalEl, Zod]
        "Joshua"	=> maps to => [Ashley, Emily, Michael]
        "KalEl"	        => maps to => [JorEl]
        "Kyle"	        => maps to => [Lex, Tyler, Zod]
        "Lex"	        => maps to => [Kyle]
        "Lisa"	        => maps to => [Bart, Marge, Matthew]
        "Marge"	        => maps to => [Lisa]
        "Matthew"	=> maps to => [Bart, Lisa, Samantha]
        "Michael"	=> maps to => [Christopher, Jessica, Joshua]
        "Samantha"	=> maps to => [Matthew, Tyler]
        "Sarah"	        => maps to => [Andrew, Christopher, Emily]
        "Stuart"	=> maps to => [Jacob]
        "Tyler"	        => maps to => [Kyle, Samantha]
        "Zod"	        => maps to => [JorEl, Kyle]

        Map<String, Set<String>>

        Map<String, Set<String>> friends = new TreeMap<String, Set<String>>();

To fill up this structure, we need to process the input file. Remember that the input file has lines that have two names separated by a "--", as in:

    Ashley -- Christopher

        while (input.hasNextLine()) {
            String line = input.nextLine();
            if (line.contains("--")) {
                Scanner lineData = new Scanner(line);
                String name1 = lineData.next();
                lineData.next();  // this skips the "--" token
                String name2 = lineData.next();
                // process name1 and name2
            }
        }

I mentioned that this is a good place to introduce an extra method because we're going to do the same thing twice. So we replaced the comment above with the following two lines of code:

        addTo(friends, name1, name2);
        addTo(friends, name2, name1);

        public static void addTo(Map<String, Set<String>> friends, String name1, 
                                 String name2) {
            ...
        }

        get the set for name1
        add name2 to that set

        Set<String> names = friends.get(name1);
        names.add(name2);

Although we can write the code in this way as two lines of code, most programmers would write this as one line of code. There is no need to introduce the local variable called names. So we can instead write this as:

        friends.get(name1).add(name2);

The very first time we see a name, we want to put it into the map. When we do that, we want to associate it with a brand new set that can be used to store the names of that person's friends:

        friends.put(name1, new TreeSet<String>());

        if (!friends.containsKey(name1)) {
            friends.put(name1, new TreeSet<String>());
        }
        friends.get(name1).add(name2);

This completes the task of constructing the friends map. I included a version of this part of the code on the calendar. We will complete this program in the next lecture.