lang/C/the.ansi.c.programming.language/notes.accompany.ansi.c/homework/PS6a.html

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   6 <title>Assignment #6 Answers</title>
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   9 <H1>Assignment #6 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 #6 ANSWERS
  22 </B><br>
  23 <br>
  24 <p>Exercise 2.
  25 <I>Modify <TT>io.c</TT> to recognize a new <TT>entrypoint</TT> line
  26 in the data file.
  27 </I><p>Here is the code I added to <TT>io.c</TT>.
  28 Rather than calling <TT>findroom</TT> right away,
  29 and face the possibility of the room not being defined yet,
  30 I chose to just stash the name of the room away
  31 (as a string),
  32 and then try looking it up after the data file had been
  33 completely read.
  34 I added this declaration at the beginning of
  35 <TT>parsedatafile</TT>:
  36 <pre>
  37 char *entryroom = NULL;
  38 </pre>
  39 I added this case to the main <TT>if</TT>/<TT>else</TT> chain
  40 in <TT>parsedatafile</TT>:
  41 <pre>
  42         else if(strcmp(av[0], "entrypoint") == 0)
  43                 {
  44                 struct room *roomp;
  45                 if(ac &lt; 2)
  46                         {
  47                         fprintf(stderr, "missing entry room name\n");
  48                         continue;
  49                         }
  50                 /* don't bother to look up yet; just save name */
  51                 entryroom = chkstrdup(av[1]);
  52                 }
  53 </pre>
  54 I added this code at the end
  55 of <TT>parsedatafile</TT>:
  56 <pre>
  57 if(entryroom != NULL)
  58         {
  59         struct room *roomp = findroom(entryroom);
  60         if(roomp != NULL)
  61                 setentryroom(roomp);
  62         else    fprintf(stderr, "can't find entry room %s\n", entryroom);
  63         }
  64 </pre>
  65 Finally, I added the new <TT>setentryroom</TT> function in <TT>rooms.c</TT>:
  66 <pre>
  67 void
  68 setentryroom(struct room *roomp)
  69 {
  70 entryroom = roomp;
  71 }
  72 </pre>
  73 <p>Exercise 3.
  74 <I>Think about what it would take
  75 to use the <TT>cmdtab</TT> structure,
  76 and the <TT>findcmd</TT> function,
  77 to streamline the code that processes lines in the data file.
  78 </I><p>It would initially seem straightforward to take each case in the
  79 <TT>if</TT>/<TT>else</TT> chain,
  80 break it out to a separate function,
  81 build a table
  82 (an array of <TT>struct cmdtab</TT>)
  83 linking the first words on lines in the data file
  84 to the new functions for parsing those lines,
  85 and finally call <TT>findcmd</TT>
  86 (after reading each line)
  87 to decide which function to call.
  88 <p>The first problem you might face, though,
  89 would be breaking up the old <TT>parsedatafile</TT> function.
  90 It contains a number of local variables
  91 (especially <TT>currentroom</TT> and <TT>currentobject</TT>)
  92 which several of the parsing cases need access to.
  93 Once you broke those parsing cases out into separate functions,
  94 they'd need access to the (formerly local) variables somehow.
  95 You'd probably have to make them global,
  96 although you could restrict them to the source file <TT>io.c</TT>
  97 by declaring them <TT>static</TT>.
  98 <p>The next problem you'd face would be deciding which information
  99 to pass to the broken-out functions.
 100 Most of them use the <TT>av</TT> array to inspect the various
 101 arguments and other words which appeared on the line they're parsing,
 102 but for at least one way I wrote the long description reading code,
 103 it needed to use the original
 104 copy of the complete data file input line
 105 (that is, the <TT>line</TT> array),
 106 the copy that hadn't been
 107 broken apart by <TT>getwords</TT>.
 108 The obvious information to pass to each parsing function is
 109 the <TT>ac</TT> count and the <TT>av</TT> array
 110 (which are, not coincidentally,
 111 quite similar to the <TT>argc</TT> and <TT>argv</TT> with which
 112 <TT>main</TT> is traditionally called).
 113 Would you have to pass along the unbroken <TT>line</TT> to all functions,
 114 for the benefit of just the one or two that needed it?
 115 (Remember, all the functions must accept the same argument list,
 116 because one function call, using the function pointer in the
 117 <TT>func</TT> field of the matching <TT>struct cmdtab</TT>
 118 entry, will be calling all of them.)
 119 <p>Finally, once you decided what information to pass to each
 120 individual line parsing function,
 121 you'd discover (if you've got strict prototype checking turned on
 122 in your compiler, and if you used <TT>cmdtab.c</TT> and
 123 <TT>cmdtab.h</TT> as they appeared in last week's handout),
 124 that the compiler doesn't want you to create an array of
 125 <TT>struct cmdtab</TT> with the <TT>func</TT> fields pointing
 126 at your shiny new functions,
 127 because the <TT>func</TT> field
 128 (again, as it appeared in last week's handout),
 129 is specifically a pointer to a function accepting
 130 a <TT>struct actor *</TT>,
 131 a <TT>struct object *</TT>,
 132 and a <TT>struct sentence *</TT>,
 133 which is almost certainly <em>not</em>
 134 the set of data
 135 you chose to pass to each data file parsing function.
 136 You'd either have to revert the declaration of the <TT>func</TT>
 137 field to
 138 <pre>
 139         int (*func)();
 140 </pre>
 141 (as it was on the disk)
 142 and turn off strict prototype checking,
 143 or write a second whole version of <TT>struct cmdtab</TT> and
 144 a second whole version of <TT>findcmd</TT> that used
 145 <TT>func</TT> fields with a different prototype,
 146 or play some
 147 (fairly ugly)
 148 games with function pointer casts.
 149 <hr>
 150 <hr>
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