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Date: Mon, 11 Nov 1996 17:01:26 GMTServer: NCSA/1.5Content-type: text/htmlLast-modified: Mon, 07 Oct 1996 00:18:37 GMTContent-length: 9909<html><head><title> Notes on the Hangman Programming Assignment </title></head><body><h2> Notes on the Hangman Programming Assignment </h2>I have been getting a lot of questions about the Hangman program. Somepeople have had minor problems getting the string class to work, orinterpreting a particular error message. However, others claim to becompletely lost, and cannot figure out how to approach the problem ofwriting a program to play Hangman. Maybe I didn't prepare you properly forthis assignment. I don't think it is a terribly hard assignment, but I mayhave rushed through the material on the string class.<p>So I decided to write a detailed document describing how to do theassignment. Much of the code that you will need to use in your program isgiven to you right here; all you have to do is assemble it into a coherentstructure and fill in the details.<p><b>Note:</b> The code in this document does not address a small problem:treating both upper- and lower-case letters as the same. Instead, it treatsthe two versions of each letter as totally different letters. This is notcorrect behavior for the assignment, though. You are responsible formodifying the program to handle upper- and lower-case letters. If the userguesses the letter 'e' (either upper- or lower-case), your program should"reveal" all letters 'e' in the secret phrase (both upper- and lower-case).However, I recommend that you do this only <i>after</i> you have the rest ofthe program working.<h3> Basic Implementation </h3>One way to implement the Hangman game is to keep two strings: the secretphrase, and the "partially-revealed" secret phrase. The secret phrase isinitialized at the beginning of the program and doesn't change. Thepartially-revealed secret phrase is the same as the secret phrase, but allof the letters are "blanked out". When the user guesses a letter in thesecret phrase, the corresponding letter in the partially-revealed secretphrase is "unblanked". For example, say the secret phrase is <tt>HowdyDoody!</tt>. Then the partially-revealed secret phrase will contain thestring <tt>_____ _____!</tt> - each of the letters has been replaced with theunderscore character.<p>Now, if the user guesses the letter "o", then the partially-revealed secretphrase will be modified; for each "o" in the secret phrase <tt>HowdyDoody!</tt>, the corresponding character in the partially-revealed secretphrase is changed from the underscore character to the letter "o".<h3> Pseudo-Code </h3><ul><li> get random secret phrase<li> initialize the "partially revealed phrase"<li> initialize number of chances<li> display partially revealed secret phrase<li> initialize guessed letters to empty string</ul><ul><li> loop until out of chances or secret phrase has been guessed <ul> <li> ask user for guess <li> if not a letter <ul> <li> error "Not a letter" </ul> <li> else if already guessed <ul> <li> error "Already guessed" </ul> <li> else <ul> <li> add guess to string of guessed letters <li> if guess is in secret phrase <ul> <li> message "Good guess" <li> reveal matching letters in partially revealed phrase </ul> <li> else <ul> <li> message "Bad guess" <li> subtract one from number of chances </ul> </ul> <li> display partially revealed secret phrase </ul><li> if secret phrase guessed <ul> <li> message "Congratulations" </ul><li> else <ul> <li> message "Sorry, secret phrase was: " secret_phrase </ul></ul><h3> Get the Random Secret Phrase </h3>For this program, you "get" the random secret phrase by calling the function<tt>random_secret_phrase</tt>. This function has the following prototype:<p><pre> string random_secret_phrase ();</pre><p> This prototype tells us that the <tt>random_secret_phrase</tt> functionhas no inputs, and returns (outputs) a string. So we call this function bysimply putting an empty pair of parentheses after its name:<pre> random_secret_phrase ();</pre>However, if we just wrote this line in our program, it would call thefunction, but throw away its return value! This is definitely not what wewant. In general, we must use the return value of a function in some way. For <tt>random_secret_phrase</tt>, its return value is a string which israndomly selected from a list. The appropriate thing to do with this stringis to store it in a variable, so we can refer to that value whenever we needto.<pre> string secret_phrase = random_secret_phrase ();</pre>This creates a variable <tt>secret_phrase</tt> of type <tt>string</tt>, andinitializes it with the value returned by the <tt>random_secret_phrase</tt>function call.<h3> Initialize the Partially-Revealed Secret Phrase </h3>The partially-revealed secret phrase should be the same as the secretphrase, but with all letters changed to underscores. Non-letters, such asthe space character or punctuation should be left as is. To decide if acharacter is a letter, us the <tt>isalpha</tt> function. This function isdeclared in the header file <tt>ctype.h</tt>. It has the followingprototype:<pre> bool isalpha (char ch);</pre>This prototype tells us that the <tt>isalpha</tt> function takes one input,a character, and returns a boolean value (true or false). If the givencharacter is a letter, it returns true.<pre> string partial_phrase = secret_phrase; for (int i = 0; i < partial_phrase.length(); i++) { if ( isalpha (partial_phrase [i]) ) partial_phrase [i] = '_'; }</pre>This code loops over each of the characters in the partially-revealed secretphrase; each character that is a letter is changed to an underscore. Sincewe know the number of characters in the partially-revealed secret phrase is<tt>partial_phrase.length()</tt>, we can use a simple counting loop. We usethe bracket operator to access each character; the index variable <tt>i</tt>tells us which character we are working on.<h3> Checking Whether a Letter has Already Been Guessed </h3>To decide if the user has guessed a letter previously, you need to keeptrack of which letters they have already guessed. This can be done with astring that holds all previously-guessed letters. At the beginning of thegame, this will be the empty string. Each time the user guesses a newletter, add it to the end of the string.<p>To decide whether a letter has already been guessed, use the function<tt>char_in_string</tt>. This function has the following prototype:<pre> bool char_in_string (char ch, string str);</pre>This tells us that the function has two inputs (one character and onestring), and returns a boolean (i.e., true or false) value. If it returnstrue, then one of the characters of the given string is the character<tt>ch</tt>.<p>So, if you call this function, and supply as inputs the guess and the stringof previously-guessed letters, it will tell you whether or not the guess hasbeen made before. The function call will look like:<pre> char_in_string (guess, guessed_letters)</pre>But, as usual, we need to do something with the return value of thisfunction! Since <tt>char_in_string</tt> returns a boolean value, we can usethe return value as the condition of an if statement:<pre> if ( char_in_string (guess, guessed_letters) )</pre><h3> Add Guess to String of Guessed Letters </h3><pre> guessed_letters += guess;</pre>Remember that the <tt>+=</tt> is just a shorthand for the following:<pre> guessed_letters = guessed_letters + guess;</pre>The character <tt>guess</tt> is automatically converted to a string, and the<tt>+</tt> operator concatenates the two strings into a single string.<h3> Checking If the Guess is in the Secret Phrase </h3>This step is similar to checking whether the guess had been made previously. Use the <tt>char_in_string</tt> function, but instead of searching thestring of previously-guessed characters, search the secret phrase itself.<pre> if ( char_in_string (guess, secret_phrase) )</pre><h3> Reveal the Matching Letters in the Secret Phrase </h3>When the user guesses a letter which is in the secret phrase, you must"unblank" the correct letters in the partially-revealed secret phrase bychanging them to the guessed letter. Do this by looping over each characterin the secret phrase; if that character is the same as the guess, then"reveal" the corresponding character in the partially-revealed secret phraseby changing it from an underscore to the corresponding character in thesecret-phrase.<pre> for (int i = 0; i < secret_phrase.length(); i++) { if (secret_phrase [i] == guess) partial_phrase [i] = secret_phrase [i]; }</pre><h3> Checking whether the Game is Over </h3>The game ends when either the player has run out of chances or hascompletely guessed the secret phrase. The first condition can be checkedwith the boolean expression<pre> num_chances == 0</pre>The second condition can be checked by comparing the secret phrase to thepartially-revealed secret phrase. If they are the same, then the player haswon the game. This can be checked with the boolean expression<pre> secret_phrase == partial_phrase</pre>The game ends when either of these two boolean expressions is true:<pre> num_chances == 0 || secret_phrase == partial_phrase</pre>However, the game must continue as long as neither of these conditions istrue. Thus the condition in our loop would be:<pre> while (num_chances > 0 && secret_phrase != partial_phrase) {</pre><hr><i> <a href="mailto:mbirk@cs.wisc.edu">mbirk@cs.wisc.edu</a> </i></body></html>⌨️ 快捷键说明
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