lang/C/the.ansi.c.programming.language/notes.accompany.ansi.c/homework/PS3a.html

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   6 <title>Assignment #3 Answers</title>
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   9 <H1>Assignment #3 Answers</H1>
  10
  11
  12
  13
  14
  15 <B>Intermediate C Programming
  16 <br>
  17 <br>
  18 UW Experimental College
  19 </B><br>
  20 <br>
  21 <B>Assignment #3 ANSWERS
  22 </B><br>
  23 <br>
  24 <p>Exercise 2.
  25 <I>Add some other commands, such as ``break'' or
  26 ``cut'' or ``read''.
  27 </I><p>Here is the code I added
  28 (to <TT>commands.c</TT>, of course)
  29 to implement all three commands.
  30 They are all straightforward but quite boring,
  31 because they don't really <em>do</em> anything yet.
  32 (Soon we'll be able to make them more interesting.)
  33 <pre>
  34 else if(strcmp(verb, "break") == 0)
  35         {
  36         if(objp == NULL)
  37                 {
  38                 printf("You must tell me what to break.\n");
  39                 return FALSE;
  40                 }
  41         if(cmd-&gt;preposition == NULL || strcmp(cmd-&gt;preposition, "with") != 0 ||
  42                         cmd-&gt;xobject == NULL)
  43                 {
  44                 printf("You must tell me what to break with.\n");
  45                 return FALSE;
  46                 }
  47         if(!contains(player-&gt;contents, cmd-&gt;xobject))
  48                 {
  49                 printf("You have no %s.\n", cmd-&gt;xobject-&gt;name);
  50                 return FALSE;
  51                 }
  52         printf("Oh, dear.  Now the %s is broken.\n", objp-&gt;name);
  53         }
  54
  55 else if(strcmp(verb, "cut") == 0)
  56         {
  57         if(objp == NULL)
  58                 {
  59                 printf("You must tell me what to cut.\n");
  60                 return FALSE;
  61                 }
  62         if(cmd-&gt;preposition == NULL || strcmp(cmd-&gt;preposition, "with") != 0 ||
  63                         cmd-&gt;xobject == NULL)
  64                 {
  65                 printf("You must tell me what to cut with.\n");
  66                 return FALSE;
  67                 }
  68         if(!contains(player-&gt;contents, cmd-&gt;xobject))
  69                 {
  70                 printf("You have no %s.\n", cmd-&gt;xobject-&gt;name);
  71                 return FALSE;
  72                 }
  73         printf("The %s is now cut in two.\n", objp-&gt;name);
  74         }
  75
  76 else if(strcmp(verb, "read") == 0)
  77         {
  78         if(objp == NULL)
  79                 {
  80                 printf("You must tell me what to read.\n");
  81                 return FALSE;
  82                 }
  83
  84         printf("There isn't much to read on the %s.\n", objp-&gt;name);
  85
  86         }
  87 </pre>
  88 <p>Exercise 3.
  89 <I>Write a function <TT>plural</TT>
  90 so that the game can print slightly more interesting messages.
  91 </I><p>Here is my first <TT>plural</TT> function:
  92 <pre>
  93 #include &lt;string.h&gt;
  94 #include "game.h"
  95
  96 #define MAXRETBUF 20
  97
  98 char *
  99 plural(char *word)
 100 {
 101 static char retbuf[MAXRETBUF];
 102 strcpy(retbuf, word);
 103 strcat(retbuf, "s");
 104 return retbuf;
 105 }
 106 </pre>
 107 Notice that the <TT>retbuf</TT> array is declared <TT>static</TT>,
 108 so that it will persist after <TT>plural</TT> returns,
 109 so that the pointer that <TT>plural</TT> returns will remain valid.
 110 <p>Here is the change to
 111 (the newer version of)
 112 <TT>parser.c</TT>:
 113 <pre>
 114 if(ac &lt; 2)
 115         cmd-&gt;object = NULL;
 116 else if((cmd-&gt;object = findobject(actor, av[1])) == NULL)
 117         {
 118         printf("I don't see any %s.\n", plural(av[1]));
 119         return FALSE;
 120         }
 121 </pre>
 122 (The only change is to the <TT>printf</TT> call.
 123 You could also make a similar change to the code just below
 124 which checks for the presence of the object of the preposition,
 125 if any.)
 126 <p>To keep my compiler happy,
 127 I also added the prototype
 128 <pre>
 129         extern char *plural(char *);
 130 </pre>
 131 to <TT>game.h</TT>.
 132 <p>Finally, here is an improved version of the <TT>plural</TT> function.
 133 If the last character of the word being pluralized is s,
 134 it tacks on ``es''.
 135 (Unfortunately, if the word is <em>already</em> plural,
 136 this means that it ends up changing
 137 ``marbles'' to ``marbleses,''
 138 and sounding like Gollum.)
 139 Since this version has to compute
 140 the length of the original word anyway,
 141 it's easy to make sure that it won't overflow the return buffer.
 142 (In this case,
 143 since <TT>plural</TT>'s function is relatively unimportant,
 144 I chose to have it return the original word,
 145 unpluralized,
 146 rather than printing error messages or aborting or anything.)
 147 <pre>
 148 #include &lt;string.h&gt;
 149 #include "game.h"
 150
 151 #define MAXRETBUF 20
 152
 153 char *
 154 plural(char *word)
 155 {
 156 static char retbuf[MAXRETBUF];
 157 int len = strlen(word);
 158 if(len + 3 &gt;= MAXRETBUF)
 159         return word;
 160 strcpy(retbuf, word);
 161 if(word[len-1] != 's')
 162         strcat(retbuf, "s");
 163 else    {
 164         /* It ends in s. */
 165         /* maybe we should add "es" */
 166         strcat(retbuf, "es");
 167         /* (or maybe it'a already plural and we should add nothing...) */
 168         }
 169 return retbuf;
 170 }
 171 </pre>
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