Introduction

For example, to turn stone into money, one possible ladder is (replace 't' by 'h', replace 'o' by 'i', etc.):

All of these words can be found in the Knuth file and in a dictionary. Your program will find the shortest word ladder between two words you enter when running the program. There may be more than one shortest ladder, your program must find one, but not all such ladders.

Input/Output

The data files are linked here too:

• knuth.dat
• words5.dat

You are to write a program named ladderq.cc that uses a file of 5-letter words to find the shortest ladder from one word to another using a process outlined below. You must develop a class to do this, the class Ladder has been started for you, but you will need to add more member functions (both public and private).

Your program should:

• Read and store the words from a file (specified by the user).
• Prompt the user to enter two five-letter words (and continue to prompt), for each pair of words, output a shortest ladder from the first word to the second. If one of the words is not of length 5, your program should halt.

The file knuth.dat has extraneous information in it. Ignore lines that begin with *, and only process the first 5 characters on other lines. Knuth asks that the file not be altered, hence these restrictions. Code to read this file is included as the member function LoadWords, already written for you to use.

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Coding/Algorithm

The vector is diagrammed as:

A ladder is found by putting the starting word (or rather a pointer to the Wnode that the word is in) on the queue, then putting all words 1 letter away from this word on the queue, then putting all words 2 letters away on the queue, then all words 3 letters away, etc. As each word is taken off the queue, if the last (target) word is found the process can stop (there may be other words on the queue, but they'll be ignored). This process is guaranteed to find the shortest word ladder.

A Word w isn't actually stored on the queue, a pointer to a struct containing w is stored. The other field of the struct is a pointer to the word that is one letter away from w and that caused w to be put on the queue (the word's predecessor). For example, if w is bears, then pointers to structs containing dears, fears, gears, beard (and so on) are enqueued with each struct pointing to bears since this word preceeded the others and caused them to be enqueued. The first word doesn't have a predecessor. It's field cannot be 0/NULL since this is used for another purpose. An easy fix is to make the pointer self-referential, it points to the struct itself (and this will need to be checked when printing ladders).

This is diagrammed below

More Details

The first word (entered by the user) is looked up in the list of words, and a pointer to the struct containing the word is enqueued. For extra credit your program should be able to handle a first word even if the word is NOT in the list of words (all other words in the ladder, except perhaps for the last, mut be in the list of words).

Put a pointer to the struct containing the first word onto the queue (it's a queue of Wnode pointers). Then repeat the dequeue/enqueue process below.

• Dequeue an element (it's a pointer). Find each word one letter apart from the dequeued word. If one of these is the target word, you're done (or if one of the words is one apart from the target word you're done, you can stop early).

• Otherwise, if each of the words one apart isn't the last word, enqueue the the word if it hasn't been queued up before (you can use the prev pointer fields in a Wnode to determine if a word has been enqueued before --- initially all prev fields should be set to 0/NULL, this helps determine if a word has been enqueued before). This means each word is enqueued at most once.

When the target word is derived, you'll need to print out the ladder from the first word to the target word. The prev pointer in the Wnode stores information that will allow the ladder to be recreated, you may need to use recursion or a vector since the ladder will be backwards (but should be printed properly). You could also search backwards, so to find a ladder from smart to brain search from brain to smart.

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Ladder Member Functions

• Clear(), sets all prev fields to 0/NULL.
• FindLadder(), tries to find a ladder between two words that are parameters to the function. FindLadder can return a boolean (and set some state in the Ladder class) or it can return a pointer. It makes more sense to return a boolean and store the beginning of the ladder (if one was found) in a private variable that can be accessed by the function Print. Pass the strings to FindLadder as const reference parameters.
• Print prints the word ladder. Private data may be needed to store the last node of the ladder (the prev field of the last node can be used to access all other ladder nodes, the first node in the ladder has a self-referential pointer).

You'll probably find it useful to write a function IsOneApart that is used to determine if two strings are one letter apart. To do this, count the letters that are equal. If this is one less than the total number of letters in the words, the words are one apart. This function does NOT need to be a member function, it has two strings as parameters (const reference) and returns true if the strings are one letter apart. You can just define this function in ladder.cc and use it there.

You'll probably want debugging code/member functions to verify what's going on. If you build helping/debugging member functions into your class you'll save time in the long run since the member functions can be used to help debug code.

You may want to write a separate function to find a word in the vector of Wnode * read and constructed using the function Ladder::LoadWords. The function can be useful in debugging and developing the program.

You may find it useful to write a function that gives back all the words that are one letter apart from a given word. This list of words could be stored in a vector or the List class (random access isn't needed). It's not at all necessary to do this, and there is no grade improvement, but it may be helpful.

Using Templates

(Note taht QueueAr.cc includes Queue.h.) If you want several kinds of queue, just put another definition in the template.cc file. Once the template.cc file is compiled to template.o, you only need to relink, not recompile, every time you make a change in ladder.cc. This will make your recompiles much faster. If you want to use the List class, #include "list.cc" in template.cc and then instantiate the kind of List you want, e.g., template class List<string>.

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Submission

To submit your assignment, type:

Extra Credit

The idea is that for each word, all words one-letter away are determined (and stored somehow) when the words are loaded. When looking for candidate words to enqueue, only words that are one-letter away (these are already known) are checked for previous use. This saves searching through the entire list of words and checking whether each is one letter away.

For other extra credit, replace the queue with a stack and discuss (in your README file) the differences in behavior between the stack and queue versions of the program.

Submitting Extra Credit