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<HTML><HEAD><TITLE>Answer to Exercise 55</TITLE></HEAD>
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<TABLE><TR><TD Valign="top"><IMG SRC="../icons/exercise.gif" ALT="??"><TD bgcolor="#FAA89B"><B>ANSWER TO EXERCISE 55</B><TR><TD><TD>
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Firstly, a printing rule to print the state of coins. Coin-objects
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will have a property called <TT>way_up</TT> which is always either 1 or 2:
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[ Face x; if (x.way_up==1) print "Heads"; else print "Tails"; ];
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There are two kinds of coin but we'll implement them with three
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classes: <TT>Coin</TT> and two sub-categories, <TT>GoldCoin</TT> and <TT>SilverCoin</TT>.
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Since the coins only join up into trigrams when present in groups
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of three, we need a routine to detect this:
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[ CoinsTogether cla i x y;
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objectloop (i ofclass cla)
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if (y==0) y=x; else { if (x~=y) return 0; }
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Thus <TT>CoinsTogether(cla)</TT> decides whether all objects of class <TT>cla</TT>
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are in the same place. (<TT>cla</TT> will always be either <TT>GoldCoin</TT> or
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<TT>SilverCoin</TT>.) We must now write the class definitions:
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with name "coin" "coins//p",
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way_up 1, article "the",
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self.way_up = random(2); print (Face) self;
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if (CoinsTogether(self.which_class))
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if (self.which_class == GoldCoin)
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print "gold"; else print "silver";
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" trigram is now ", (Trigram) self.which_class;
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if (inventory_stage==1)
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{ if (self.which_class == GoldCoin)
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print "the gold"; else print "the silver";
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k=CoinsTogether(self.which_class);
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if (k==location k has supporter)
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{ objectloop (i ofclass self.which_class)
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{ 1: print ", "; 2: print " and ";
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3: print " (showing the trigram ",
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(Trigram) self.which_class, ")";
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if (~~(c_style & ENGLISH_BIT)) c_style = c_style + ENGLISH_BIT;
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if (~~(c_style & NOARTICLE_BIT)) c_style = c_style + NOARTICLE_BIT;
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if (c_style & NEWLINE_BIT) c_style = c_style - NEWLINE_BIT;
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if (c_style & INDENT_BIT) c_style = c_style - INDENT_BIT;
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Class GoldCoin class Coin
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with name "gold", which_class GoldCoin,
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list_together [; return CoinLT(); ];
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Class SilverCoin class Coin
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with name "silver", which_class SilverCoin,
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list_together [; return CoinLT(); ];
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(There are two unusual points here. Firstly, the <TT>CoinsLT</TT> routine
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is not simply given as the common <TT>list_together</TT> value in the <TT>coin</TT>
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class since, if it were, all six coins would be grouped together:
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we want two groups of three, so the gold and silver coins have to have
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different <TT>list_together</TT> values. Secondly, if a trigram is together
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and on the floor, it is not good enough to simply append text like
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"showing Tails, Heads, Heads (change)'' at <TT>inventory_stage</TT> 2 since the
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coins may be listed in a funny order: for example, in the order snake,
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robin, bison. In that event, the order the coins are listed in
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doesn't correspond to the order their values are listed in, which is
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misleading. So instead <TT>CoinsLT</TT> takes over entirely at
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<TT>inventory_stage</TT> 1 and prints out the list of three itself, returning
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true to stop the list from being printed out by the library as well.)
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To resume: whenever coins are listed together, they are grouped
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into gold and silver. Whenever trigrams are visible they are to be
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described by either <TT>Trigram(GoldClass)</TT> or <TT>Trigram(SilverClass)</TT>:
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Array gold_trigrams --> "fortune" "change" "river flowing" "chance"
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"immutability" "six stones in a circle"
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"grace" "divine assistance";
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Array silver_trigrams --> "happiness" "sadness" "ambition" "grief"
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"glory" "charm" "sweetness of nature"
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"the countenance of the Hooded Man";
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[ Trigram cla i k state;
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objectloop (i ofclass cla)
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{ print (Face) i; if (k++<2) print ","; print " ";
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state=state*2 + (i.way_up-1);
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if (cla == GoldCoin) i=gold_trigrams; else i=silver_trigrams;
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print "(", (string) i-->state, ")";
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(These interpretations of the coins are quite bogus.) Finally,
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we have to make the six actual coins:
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GoldCoin -> "goat" with name "goat";
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GoldCoin -> "deer" with name "deer";
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GoldCoin -> "chicken" with name "chicken";
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SilverCoin -> "robin" with name "robin";
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SilverCoin -> "snake" with name "snake";
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SilverCoin -> "bison" with name "bison";
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<HR>Back to <A HREF="../section23.html#ex55">the exercise in section 23</A><HR>
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