Speller is a CS50 problem set that I have solved.
Theoretically, on input of size n, an algorithm with a running time of n is “asymptotically equivalent,” in terms of O, to an algorithm with a running time of 2n. Indeed, when describing the running time of an algorithm, we typically focus on the dominant (i.e., most impactful) term (i.e., n in this case, since n could be much larger than 2). In the real world, though, the fact of the matter is that 2n feels twice as slow as n.
The challenge ahead of you is to implement the fastest spell checker you can! By “fastest,” though, we’re talking actual “wall-clock,” not asymptotic, time.
In speller.c, we’ve put together a program that’s designed to spell-check a file after loading a dictionary of words from disk into memory. That dictionary, meanwhile, is implemented in a file called dictionary.c. (It could just be implemented in speller.c, but as programs get more complex, it’s often convenient to break them into multiple files.) The prototypes for the functions therein, meanwhile, are defined not in dictionary.c itself but in dictionary.h instead. That way, both speller.c and dictionary.c can #include the file. Unfortunately, we didn’t quite get around to implementing the loading part. Or the checking part. Both (and a bit more) we leave to you! But first, a tour.
Open up dictionary.h, and you’ll see some new syntax, including a few lines that mention DICTIONARY_H. No need to worry about those, but, if curious, those lines just ensure that, even though dictionary.c and speller.c (which you’ll see in a moment) #include this file, clang will only compile it once.
Next notice how we #include a file called stdbool.h. That’s the file in which bool itself is defined. You’ve not needed it before, since the CS50 Library used to #include that for you.
Also notice our use of #define, a “preprocessor directive” that defines a “constant” called LENGTH that has a value of 45. It’s a constant in the sense that you can’t (accidentally) change it in your own code. In fact, clang will replace any mentions of LENGTH in your own code with, literally, 45. In other words, it’s not a variable, just a find-and-replace trick.
Finally, notice the prototypes for five functions: check, hash, load, size, and unload. Notice how three of those take a pointer as an argument, per the *:
bool check(const char *word); unsigned int hash(const char *word); bool load(const char *dictionary); Recall that char * is what we used to call string. So those three prototypes are essentially just:
bool check(const string word); unsigned int hash(const string word); bool load(const string dictionary); And const, meanwhile, just says that those strings, when passed in as arguments, must remain constant; you won’t be able to change them, accidentally or otherwise!
Now open up dictionary.c. Notice how, atop the file, we’ve defined a struct called node that represents a node in a hash table. And we’ve declared a global pointer array, table, which will (soon) represent the hash table you will use to keep track of words in the dictionary. The array contains N node pointers, and we’ve set N equal to 26 for now, to match with the default hash function as described below. You will likely want to increase this depending on your own implementation of hash.
Next, notice that we’ve implemented load, check, size, and unload, but only barely, just enough for the code to compile. Notice too that we’ve implemented hash with a sample algorithm based on the first letter of the word. Your job, ultimately, is to re-implement those functions as cleverly as possible so that this spell checker works as advertised. And fast!
Okay, next open up speller.c and spend some time looking over the code and comments therein. You won’t need to change anything in this file, and you don’t need to understand its entirety, but do try to get a sense of its functionality nonetheless. Notice how, by way of a function called getrusage, we’ll be “benchmarking” (i.e., timing the execution of) your implementations of check, load, size, and unload. Also notice how we go about passing check, word by word, the contents of some file to be spell-checked. Ultimately, we report each misspelling in that file along with a bunch of statistics.
Notice, incidentally, that we have defined the usage of speller to be
Usage: speller [dictionary] text where dictionary is assumed to be a file containing a list of lowercase words, one per line, and text is a file to be spell-checked. As the brackets suggest, provision of dictionary is optional; if this argument is omitted, speller will use dictionaries/large by default. In other words, running
./speller text will be equivalent to running
./speller dictionaries/large text where text is the file you wish to spell-check. Suffice it to say, the former is easier to type! (Of course, speller will not be able to load any dictionaries until you implement load in dictionary.c! Until then, you’ll see Could not load.)
Within the default dictionary, mind you, are 143,091 words, all of which must be loaded into memory! In fact, take a peek at that file to get a sense of its structure and size. Notice that every word in that file appears in lowercase (even, for simplicity, proper nouns and acronyms). From top to bottom, the file is sorted lexicographically, with only one word per line (each of which ends with \n). No word is longer than 45 characters, and no word appears more than once. During development, you may find it helpful to provide speller with a dictionary of your own that contains far fewer words, lest you struggle to debug an otherwise enormous structure in memory. In dictionaries/small is one such dictionary. To use it, execute
./speller dictionaries/small text where text is the file you wish to spell-check. Don’t move on until you’re sure you understand how speller itself works!
Odds are, you didn’t spend enough time looking over speller.c. Go back one square and walk yourself through it again!
So that you can test your implementation of speller, we’ve also provided you with a whole bunch of texts, among them the script from La La Land, the text of the Affordable Care Act, three million bytes from Tolstoy, some excerpts from The Federalist Papers and Shakespeare, and more. So that you know what to expect, open and skim each of those files, all of which are in a directory called texts within your pset5 directory.
Now, as you should know from having read over speller.c carefully, the output of speller, if executed with, say,
./speller texts/lalaland.txt will eventually resemble the below.
Below’s some of the output you’ll see. For information’s sake, we’ve excerpted some examples of “misspellings.” And lest we spoil the fun, we’ve omitted our own statistics for now.
MISSPELLED WORDS
[...] AHHHHHHHHHHHHHHHHHHHHHHHHHHHT [...] Shangri [...] fianc [...] Sebastian's [...]
WORDS MISSPELLED: WORDS IN DICTIONARY: WORDS IN TEXT: TIME IN load: TIME IN check: TIME IN size: TIME IN unload: TIME IN TOTAL: TIME IN load represents the number of seconds that speller spends executing your implementation of load. TIME IN check represents the number of seconds that speller spends, in total, executing your implementation of check. TIME IN size represents the number of seconds that speller spends executing your implementation of size. TIME IN unload represents the number of seconds that speller spends executing your implementation of unload. TIME IN TOTAL is the sum of those four measurements.
Note that these times may vary somewhat across executions of speller, depending on what else your codespace is doing, even if you don’t change your code.
Incidentally, to be clear, by “misspelled” we simply mean that some word is not in the dictionary provided.
And, lastly, recall that make automates compilation of your code so that you don’t have to execute clang manually along with a whole bunch of switches. However, as your programs grow in size, make won’t be able to infer from context anymore how to compile your code; you’ll need to start telling make how to compile your program, particularly when they involve multiple source (i.e., .c) files, as in the case of this problem. And so we’ll utilize a Makefile, a configuration file that tells make exactly what to do. Open up Makefile, and you should see four lines:
The first line tells make to execute the subsequent lines whenever you yourself execute make speller (or just make). The second line tells make how to compile speller.c into machine code (i.e., speller.o). The third line tells make how to compile dictionary.c into machine code (i.e., dictionary.o). The fourth line tells make to link speller.o and dictionary.o in a file called speller. Be sure to compile speller by executing make speller (or just make). Executing make dictionary won’t work!
How to check whether your program is outting the right misspelled words? Well, you’re welcome to consult the “answer keys” that are inside of the keys directory that’s inside of your speller directory. For instance, inside of keys/lalaland.txt are all of the words that your program should think are misspelled.
You could therefore run your program on some text in one window, as with the below.
./speller texts/lalaland.txt And you could then run the staff’s solution on the same text in another window, as with the below.
./speller50 texts/lalaland.txt And you could then compare the windows visually side by side. That could get tedious quickly, though. So you might instead want to “redirect” your program’s output to a file, as with the below.
./speller texts/lalaland.txt > student.txt ./speller50 texts/lalaland.txt > staff.txt You can then compare both files side by side in the same window with a program like diff, as with the below.
diff -y student.txt staff.txt Alternatively, to save time, you could just compare your program’s output (assuming you redirected it to, e.g., student.txt) against one of the answer keys without running the staff’s solution, as with the below.
diff -y student.txt keys/lalaland.txt If your program’s output matches the staff’s, diff will output two columns that should be identical except for, perhaps, the running times at the bottom. If the columns differ, though, you’ll see a > or | where they differ. For instance, if you see
MISSPELLED WORDS MISSPELLED WORDS
TECHNO TECHNO L L > Thelonious Prius Prius > MIA L L that means your program (whose output is on the left) does not think that Thelonious or MIA is misspelled, even though the staff’s output (on the right) does, as is implied by the absence of, say, Thelonious in the lefthand column and the presence of Thelonious in the righthand column.
Finally, be sure to test with both the default large and small dictionaries. Be careful not to assume that if your solution runs successfully with the large dictionary it will also run successfully with the small one. Here’s how to try the small dictionary:
./speller dictionaries/small texts/cat.txt