Frequently Asked Questions (General)

What is meant by an immutable data type? An object is immutable if its data-type value (state) never changes once it is constructed. A data type is immutable if every object of that type is immutable. For example, to make the Autocomplete data type immutable, you must make a defensive copy of the terms[] array in the constructor (because the client might change one of the entries in the original terms[] array after constructing the Autocomplete object).

Can my methods have side effects that are not described in the API? No. For example, the Autocomplete data type should not change one of the entries in the terms[] array (or it could have a serious side effect on the client). Similarly, the firstIndexOf() and lastIndexOf() should not change the entries in their argument arrays. We usually allow methods to have benign side effects, such as changing the state of a random number generator.

Frequently Asked Questions (Term)

What is the meaning of the modifier static in the following function declarations?

public static Comparator<Term> byReverseWeightOrder()
It means that there is one (static) method for the entire class, as opposed to one (instance) method per object. Since there is a single total order (by weight), the static modifier is appropriate.

Can a nested class have a constructor and instance variables? Yes, absolutely. Recall your nested iterator class in RandomizedQueue.

What is lexicographic order? Lexicographic order (or alphabetical order) is a total order in which strings of characters are ordered based on the position of the characters in the underlying alphabet (such as the Roman alphabet, ASCII, or Unicode). To determine which of two strings comes first, you compare the first characters of the two strings. If they differ, the string whose first character appears earlier in the alphabet comes first. If the first characters are the same, you compare the second characters, and so forth. If you reach a position where one of the two strings has no more characters, the shorter string comes first. For example, "cat" is less than "dog" and "dog" is less than "dogcatcher".

How can I compare two strings lexicographically? Lexicographic order is the natural order for Java strings, so you can use the compareTo() method in String to do so. Alternatively, you can use the charAt() method in the String class to access the corresponding characters of the two strings and then compare their Unicode values.

What exactly does it mean to compare two string by their first r characters? You should compare the two strings lexicographically, as in the natural order for Java strings, but you should never examine beyond the first r characters of either string. For example, if r = 3, then "cat" is less than "dog"; "dog" is equal to "dogcatcher"; and "do" is less than "dogcatcher". Note that in the last example, "do" has fewer than 3 characters, so we compare the whole string "do" to the first 3 characters of "dogcatcher".

What exactly is meant by the requirement that the the string comparison functions must take time proportional to the number of characters needed to resolve the comparison? As an example, if you are comparing the two strings "ABCAAAAAAAAAAAAA" and "ABDEEEEEEEEEEEEE", you should compare only the first three corresponding pairs of characters because, while the first two characters are the same, the third characters are different.

What are the running times of the charAt() and length() methods in the String library? Constant time.

What is the running time of the length() method in the String library? Constant time.

What is the running time of the compareTo() method in the String library? Comparing two strings takes time proportional to the number of characters needed to resolve the comparison.

What is the running time of the substring() method in the String library? Extracting a substring of length r takes \(\Theta(r)\) time. Consequently, most uses of the ​substring()​ method in the string comparison functions will not meet the performance requirements.

Frequently Asked Questions (BinarySearchDeluxe)

What is the meaning of the type parameter Key in the following function declaration?

public static <Key> int firstIndexOf(Key[] a, Key key, Comparator<Key> comparator)
This is an example of a generic method, which introduces its own type parameter Key. The type parameter serves as a placeholder for the argument types that are passed to the generic method. In this case, it enforces the constraint that the type of the elements in the array a[] must match the type of the search key key (so that the compiler would allow searching for an Apple object in an Apple[] array but reject searching for an Orange object in an Apple[] array). This bit of arcane Java syntax enables the program to compile without warnings.

When I compile I get the compiler warning "unchecked call to compare(T, T) as a member of the raw type java.util.Comparator." Is that OK? No, your program should compile without any errors or warnings.

What’s a good way to get a Comparator object to use for testing? You can create an array of Term objects and use Term.byReverseWeightOrder(). But, it's probably easier (and less error-prone) to create an array of String objects and use String.CASE_INSENSITIVE_ORDER.

String[] a = { "A", "A", "C", "G", "G", "T" };
Comparator comparator = String.CASE_INSENSITIVE_ORDER;
int index = BinarySearchDeluxe.firstIndexOf(a, "G", comparator);

Will I meet the performance requirement if I use binary search to find any matching key and then do a linear scan left to find the first such key? No. That approach will make \(\Theta(n)\) compares in the worst case (e.g., if all keys are equal to the search key).

Can I use Arrays.binarySearch()? Arrays.binarySearch() returns the index of any matching key—not necessarily the first or last such key. So, it will not be of much help.

My binary search makes more \(1 + \lceil \log_2 n \rceil\) compares. Is this a big problem? If it makes \(\Theta(n)\) compares in the worst case, that is a serious performance bug. If it makes \(\Theta(\log n)\) compares in the worst case (but more than the requisite number), that is just a minor deduction.

My firstIndexOf() is correct but lastIndexOf() has a bug. Any suggestions? Your code for the two methods should be symmetric since you can view lastIndexOf() as finding the first matching index in the reverse of the array (which is in reverse-sorted order). That is, replace less with greater, lo with hi, and index 0 with n-1.

Frequently Asked Questions (Autocomplete)

Should I re-implement mergesort or quicksort? No. Don’t re-invent the wheel. Call either Arrays.sort() or one of the sorting algorithms in algs4.jar.

Can I assume that the weights are all distinct? No. However, if there are two terms with equal weights, the allMatches() method can return them in arbitrary order.

Can I assume that the Autocomplete constructor receives the terms in descending order by weight? No. While many of the sample input files are in descending order by weight, do not make any assumptions about the order.

What should allMatches() return if there are no matching terms? It should still return an array containing all of the matching terms—an array of length 0.

Should Autocomplete be case sensitive? For instance, if I enter the prefix "prince" and one of my terms has a query "Princeton",  should "Princeton" be included among the matches? No. On this assignment, queries are case sensitive. In a real application, you might prefer a case-insensitive (or diacritic-insensitive) order.

Can a prefix or query string be the empty string? Yes and yes. For example, the empty-string prefix "" should return all queries, in descending order by weight. It might be harder to think of an application for an empty-string query, but the API does not exclude this possibility.

What do I need to do to handle Unicode characters and UTF-8 encoding? You shouldn’t need to do anything special. The Java String data type handles Unicode naturally; The supplied test files are encoded using UTF-8. When reading input or writing output, StdIn, In, and StdOut assume UTF-8 encoding.

Why do I get a java.util.NoSuchElementException from readLong() when I run the sample client? Did you cut-and-paste the file instead of downloading it? Your text editor may have corrupted the file by changing the encoding from UTF-8 to something else.

How can I view the Unicode characters that my program prints out?

Note that even if you can’t view Unicode properly, it’s likely that your program is still handling Unicode properly. You can use the program to test whether your system is configured to display Unicode.

Sample Input Files

Input files (for use with Autocomplete). Below are some input files for testing. Each input file consists of an integer n followed by n pairs of terms and integer weights, one pair per line, with the term and weight separated by a tab. The terms can be arbitrary strings consisting of Unicode characters, including spaces (but neither tabs nor newlines).

file number term weight source
pu-buildings.txt 166 Princeton buildings age (in Y10K)
pokemon.txt 729 Pokemon popularity Allen Qin
fortune1000.txt 1,000 company revenue Fortune 1000
nasdaq.txt 2,658 NASDAQ market cap
metal-albums.txt 3,000 metal albums votes Metal Storm
pu-courses.txt 6,771 course enrollment Princeton Registrar
wiktionary.txt 10,000 word frequency Wiktionary
redditors.txt 10,000 reddittor subscribers
baby-names.txt 31,109 first name frequency U.S. Social Security
trademarks.txt 92,254 company U.S. trademark number USPTO
cities.txt 93,827 city population
mandarin.txt 94,339 Mandarin word frequency Invoke IT
bing.txt 250,000 word Bing search frequency Microsoft Web N-gram
2grams.txt 277,718 2-gram frequency Google Web Trillion Word Corpus
3grams.txt 1,020,009 3-gram frequency Corpus of Contemporary American English
4grams.txt 1,034,308 4-gram frequency Corpus of Contemporary American English
5grams.txt 1,044,269 5-gram frequency Corpus of Contemporary American English
words-3333333.txt 333,333 word frequency Google Web Trillion Word Corpus
artists.txt 43,849 music artist familiarity Million Song Dataset
songs.txt 922,230 song hotttnesss Million Song Dataset
alexa.txt 1,000,000 web site frequency Alexa
imdb-votes.txt 82,455 movie number of votes Internet Movie Database
movies.txt 229,447 movies revenue (USD) Internet Movie Database
actors.txt 2,875,183 actors revenue (USD) Internet Movie Database

Possible Progress Steps

These are purely suggestions for how you might make progress. You do not have to follow these steps.


Suppose that I want to find only the top-k matching terms (instead of all matching terms). Are there better algorithms? Yes. The problem of finding the top term matching a given prefix can be formulated as a range maximum query problem or equivalently as a lowest common ancestor problem. Here are some lecture notes by Erik Demaine on the equivalence.

Any way to find the range of words that start with a given prefix in time proportional to the length of the prefix (in the worst case)? Yes, use a suffix tree (or a suffix array with the lcp array), where the terms are separated by a word-separation symbol and concatenated together. To use space proportional to the number of terms consider a suffix array on words. Or consider a compact DAWG.