Description
This assignment introduces how to search and sort data. In the case of this assignment,
you’ll be searching and sorting a large array of words to count how many times different
words appear in the array. More specifically, we’ll be working with an array containing
all the words that appear in Mary Shelley’s novel Frankenstein. So you’ll be able to
answer questions like: Which of the words “Doctor”, “Frankenstein”, “Monster” or “Igor”
appear more often in the novel?
You will explore two different ways to count words in a piece of text, and learn how to
calculate which one takes longer.
You will implement and time three things:
● A naive counting search: given an array, count how many times a word appears
in it.
● A sorting function: given an array, return a sorted version of that array.
● A binary search: given a sorted array, find where a word should be, and use
that knowledge to count the number of times it appears (faster than before)
The first task is easy, the second two are more challenging.
Running this code
This program takes one command line argument: the words to search for and count.
If you don’t give it any words to search for, it defaults to these words: “doctor”,
“frankenstein”, “the”, “monster”, “igor”, “student”, “college”, “lightning”,
“electricity”, “blood”, and “soul”.
You may want to run it with fewer or different words, eg:
$ java SearchAndSort “frankenstein,doctor,igor,monster”
This will only count how often the words “frankenstein”, “doctor”, “igor”, and
“monster” appear in the text.
Timing things in Java
How long did your code take to run?
In this assignment, we will see some search algorithms run faster than others. To see
how much faster, we will record how long the code takes to run.
Here is an example of timing code:
// Look at the clock when we start
long t0 = (new Date()).getTime();
for (var i = 0; i < 100000; i++) {
// Do something that takes time
}
// Look at the clock when we are finished
long t1 = (new Date()).getTime();
long elapsed = t1 - t0;
System.out.println("My code took " + elapsed + "milliseconds to
run.")
This is already set up for you in main and will output how long your code took to run.
Your job is to implement and call your functions, and make sure that they are working
correctly. If you look in the starter code, you’ll see that we’re running your two different
search and count methods 100 times, so as to get a good average value for how long it
takes. We’re only do the sort once, as sorting the array of 75289 words already takes
awhile.
1. Counting words (naive approach)
● Write and implement a method countWordsInUnsorted
○ parameters: an array of words (String[]) and a query word (String)
which is the word we are looking for
○ return: the number of times that word appears
● Call that method in main
● Checking if you are right: "frankenstein" should appear 26 times.
Implementing the function countWordsInUnsorted
Define a countWordsInUnsorted method that takes an unsorted array of strings and a
query string, and returns an int of how many times that query occurs in the array. Make
it a static method (like main) so that you can call it directly without having to create an
instance of the class SearchAndSort first.
Create a counter variable to record how many times we have seen a word. Create a
loop that iterates over every word in the array. Each time a word matches the query
word, increment the counter.
Note: How can we test if two strings are equal? You have two possibilities in Java:
stringA == stringB and stringA.equals(stringB). These behave differently in
Java! The first tests if they are the same object. The second tests if they have the same
sequences of characters. We need to use the equals method, since we care about
whether two strings have the same characters in them, not that they are the exact same
object.
Calling the function in main
In the main method, within the TODO #1 comment block, you’ll want to replace the 0 in
the line:
int count = 0;
with the call to countWordsInUnsorted. The variable i in the for loop is iterating over
the query words, so you can use this to pass each query word into
countWordsInUnsorted within the loop.
Checking if you are right
Once you’ve completed the two tasks above, you can run your code. Verify that
"frankenstein" appears 26 times. You’ll also see how fast this code takes to execute
(averaged over 100 searches). This amount of time will vary depending on your
computer and what other programs are running on it at the same time.
2. Sorting an array of words with Merge Sort
● Implement mergeSort (Do not use Java's built-in Array.sort for this assignment)
● Call your new method in main
● Checking if you are right: the words (after SORTED in the output) will be in
alphabetical order.
Implementing mergeSort
Implement a mergeSort method that sorts an array of strings. The signature for this
method should be:
public static void mergeSort(String[] arrayToSort, String[]
tempArray, int first, int last)
The first argument is the String[] to sort, the second argument is an empty temporary
array that should be the same size as the array to sort, the third argument is the starting
index for the portion of the array you want to sort, and the fourth argument is the ending
index for the portion of the array you want to sort.
Note that the version of mergeSort we’ll be implementing sorts arrayToSort in place.
This means that you pass an unsorted array in to mergeSort and after the call that
same array is now sorted.
The book provides a good description of mergeSort, on pages 527-533. We’ll also be
talking about how mergeSort works in class.
Note that mergeSort involves comparing to values to see which is larger. With numbers
you can just use < or > to compare them. But for Strings we’ll need another way to
compare. Fortunately, Strings have another method defined on them for comparing
called compareTo. You can read more about this method more here, but long story
short, if you have two Strings s1 and s2, then:
● s1.compareTo(s2) == 0 if s1 and s2 contain the same string.
● s1.compareTo(s2) < 0 if s1 would be alphabetically ordered before s2.
● s1.compareTo(s2) > 0 if s1 would be alphabetically ordered after s2.
Calling mergeSort in main
To call mergeSort in main, you first need to create a temporary string array that is the
same length as allWords. We want to sort the whole array, so the third argument
should be the index of the first word in allWords and the fourth argument should be the
index of the last word in allWords. The four arguments to call mergeSort with are then:
● The array we’re sorting, which is allWords.
● Our new temporary array we’ve created that’s the same length as allWords.
● The index of the first word in allWords (hint: what’s always the index of the very
first element of an array?).
● The index of the last word in allWords (hint: if you know the length of an array,
you can easily compute the index of the last element).
Checking you are right
Did this sort the array of words? The program prints every 500th word. Do they look
sorted?
3. Counting words (with Binary Search) and timing it
● Implement public static int binarySearch(String[] sortedWords,
String query, int startIndex, int endIndex)
● Implement public static int getSmallestIndex(String[] words,
String query, int startIndex, int endIndex)
● Implement public static int getLargestIndex(String[] words, String
query, int startIndex, int endIndex)
● Call getSmallestIndex and getLargestIndex in main to get the smallest and
largest indices at which the word you’re looking for appears in the sorted array.
Use these two values to compute how many times that word appears.
● Checking if you are right: you will get the same values as in the first search
section, but much faster.
Implementing binarySearch
The arguments to binarySearch are:
● The sorted array of words to search in.
● The word to search for.
● The index in the array at which to start searching.
● The index in the array at which to stop searching.
There’s a nice description of how to implement the binary search algorithm for integers
on pages 168-172 of the book
Binary search returns the array index where it found the word. If the word only appears
once in the array, then this index will be where it occurs. But if the word appears
multiple times in the sorted array (so all the instances of the word will be next to each
other in the array), then this index will be to one of the words in the middle of the group.
The binary search algorithm doesn’t guarantee that this will be the first element in the
group or the last element in the group. So we need to implement some other methods to
do this.
Implementing getSmallestIndex
The method getSmallestIndex will be a recursive method that uses the
binarySearch method to find the smallest index for which a word is found in the array.
The outline for this method is:
● Use binarySearch to find an index to the word. If the index binarySearch
returns is -1, then the word wasn’t found and getSmallestIndex should just
return -1. This is the base case.
● If binarySearch did find the word, then recursively call getSmallestIndex on
the portion of the array before where the word was found. This is from index 0 up
to (but not including) the index where the word was found. If this returns -1 then
we know we already had the smallest index, otherwise the recursive call to
getSmallestIndex found the smallest index and we should return that. This is
the recursive case.
Implementing getLargestIndex
The method getLargestIndex will be a recursive method that uses the binarySearch
method to find the largest index for which a word is found in the array. The outline for
this method is very similar to the outline above for getSmallestIndex, except that the
recursive call should search the portion of the array starting after where binarySearch
has found the word.
Using getSmallestIndex and getLargestIndex in main to count words
Since the array that you’re working with has been sorted by this point, all the same
words appear next to each other (e.g. all 26 appearances of the word “frankenstein”
are next to each other in the array). So if you’ve found the smallest and largest index for
a word, then you can just subtract the two indices to count the words! But it’s not quite
as simple as just subtracting. To figure out what it should be, consider the case where a
word appears only once (so the first and last index are the same) and the case where
the word doesn’t appear at all.
Checking if you are right
Check that you’re getting the same answers as the naive approach that iterates through
the whole array. Also, look to see how much faster this approach is to the naive
approach!
Example arguments and output
$ java SearchAndSort “student,college,frankenstein”
SEARCH AND SORT
Searching and counting the words student,college,frankenstein
NAIVE SEARCH:
student:3
college:3
frankenstein:26
49 ms for 300 searches, 0.163333 ms per search
SORTING:
2393 ms to sort 75289 words
SORTED (every 500 word): a a above after all am and and and and and and another are as astonished
authority be been beneath bosom but buy called chair closed concussion cooking covered darkness deemed
despair dire doing earnest endure even exert failure feeble fiend fondly for friend from gloom guilt had
hardly have he hellish hide his hope hurried i i i i i if in in in inspiring is it journey labours
lesson little lullaby many me me miles monotonous mould my my my my nearly no not obtain of of of of of
on one or own paused place preceded prosperity rays remained restrained s saw seems she shrieks snowy
sometimes spoke strong support teeth that that the the the the the the the the their there thirst those
timid to to to to transcendent undertaking urged views was was waves were when which who will with woman
wound you your
BINARY SEARCH:
student:3
college:3
frankenstein:26
2 ms for 300 searches, 0.006667 ms per search
Of course the actual timing for running the searches and sort will vary on your computer,
depending on how fast your computer is and how many other processes are running on it.
Turning the code in
● Create a directory with the following name:
you replace
student ID is 1234567, then the directory name is 1234567_assignment1.
● Put a copy of your edited SearchAndSort.java file in the directory.
● Compress the folder using zip. Zip is a compression utility available on mac, linux
and windows that can compress a directory into a single file. This should result in
a file named
your real ID of course).
● Upload the zip file through the page for Assignment 1 in canvas
(https://canvas.ucsc.edu/courses/12730/assignments/38334).
Helpful tips:
Visualizations
● Visual animations of different sorting types:
https://visualgo.net/bn/sorting?slide=1
● Merge sort as a dance: https://www.youtube.com/watch?v=XaqR3G_NVoo
● An explanation of Binary Search:
Code things
● Using Java’s printf:
https://docs.oracle.com/javase/tutorial/java/data/numberformat.html
● Did you use myString == “foo”, or myString.equals(“foo”) In Java, these don’t
mean the same thing!
(https://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java)
Use lots of test output to help make sure that your Binary Search and Merge sort are
operating correctly at every level of recursion. For example, here is a debug statement
that I made for myself to test the Binary Search.
System.out.println(query + ” pivot:” + sortedWords[pivot] + ” ” + startIndex + “-”
+ endIndex + ” ” + sortedWords[startIndex] + “-” + sortedWords[endIndex-1]);
SEARCH: soul soul pivot:me 0-100 a-you soul pivot:task 50-100 me-you soul pivot:of
50-75 me-such soul pivot:play 62-75 of-such soul pivot:sister 68-75 play-such soul
pivot:success 71-75 sister-such soul pivot:streets 71-73 sister-streets soul not
found
Be sure to comment out extra debug statements when you turn in your work!
