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<title editable="nocomment">Chap11a</title>

         
<h1>Chap11a: assertz</h1>

         
<p>Prolog has two database manipulation commands:

         
assertz and retract. Let’s see how these are used.

         
Suppose we start with an empty database. So if we

         
give the command:</p>

         

         
<table style="width: 100%">

         
   <tr>

         
       <td class="input" style="width: 100%"

         
           contenteditable="true" id="text0" spellcheck="false"

         
           >?- listing.

         
</td>

         
       <td rowspan="2" valign="top">

         
           <button class="run" id="launch0"

         
           >⏵</button>

         
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       <td class="result" id="demo0"></td>

         
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<p>then Prolog will simply respond true; the listing (of course)

         
is empty.</p><p>Suppose we now give this command:<p>

         

         
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   <tr>

         
       <td class="input" style="width: 100%"

         
           contenteditable="true" id="text1" spellcheck="false"

         
           >?- assertz(happy(mia)).

         
</td>

         
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           <button class="run" id="launch1"

         
           >⏵</button>

         
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       <td class="result" id="demo1"></td>

         
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<p>This succeeds ( assertz/1 commands always succeed). But what is

         
important is not that it succeeds, but the side-effect it

         
has on the database. For if we now give the command</p>

         

         
<table style="width: 100%">

         
   <tr>

         
       <td class="input" style="width: 100%"

         
           contenteditable="true" id="text2" spellcheck="false"

         
           >?- listing.

         
</td>

         
       <td rowspan="2" valign="top">

         
           <button class="run" id="launch2"

         
           >⏵</button>

         
       </td>

         
   </tr>

         
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       <td class="result" id="demo2"></td>

         
   </tr>

         
</table>

         

         
<p>We get the fact. That is, the database is no longer empty:

         
it now contains the fact we asserted.</p>

         
<p>Suppose we then made four more assert commands:</p>

         

         
<table style="width: 100%">

         
   <tr>

         
       <td class="input" style="width: 100%"

         
           contenteditable="true" id="text3" spellcheck="false"

         
           >?- assertz(happy(vincent)).

         
?- assertz(happy(marcellus)).

         
?- assertz(happy(butch)).

         
?- assertz(happy(vincent)).

         
</td>

         
       <td rowspan="2" valign="top">

         
           <button class="run" id="launch3"

         
           >⏵</button>

         
       </td>

         
   </tr>

         
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       <td class="result" id="demo3"></td>

         
   </tr>

         
</table>

         

         
<p>and then ask for a listing:</p>

         

         
<table style="width: 100%">

         
   <tr>

         
       <td class="input" style="width: 100%"

         
           contenteditable="true" id="text4" spellcheck="false"

         
           >?- listing.

         
</td>

         
       <td rowspan="2" valign="top">

         
           <button class="run" id="launch4"

         
           >⏵</button>

         
       </td>

         
   </tr>

         
   <tr>

         
       <td class="result" id="demo4"></td>

         
   </tr>

         
</table>

         

         
<p>All the facts we asserted are now in the knowledge base.

         
Note that happy(vincent) is in the knowledge base twice.

         
As we asserted it twice, this seems sensible.</p>

         
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