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authorTobias Klauser <tklauser@distanz.ch>2008-01-27 11:37:44 +0100
committerTobias Klauser <tklauser@xenon.tklauser.home>2008-01-27 11:37:44 +0100
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+<title>Link Lists</title>
+<body bgcolor="#ffffcc">
+<hr>
+<center> <h1>Link Lists</h1> </center>
+<hr>
+<p>
+<ul>
+<li><a href="#long">Long explanation</a>
+<li><a href="#short">Short explanation</a>
+</ul>
+
+<a name=long>
+Question, how would you write a program to meet the following requirements?
+
+<ol>
+<li>Read an unknown number of records from a file into memory. A typical
+record would look like:
+<pre>
+ Ref Title Cost
+ --- ----- ----
+ 1234 Oracle_Guide 22.95
+</pre>
+<li>Perform a numeric sort based on the first field.
+<li>Delete duplicate records based on the first field.
+</ol>
+
+One method is to define an
+<a href="../CONCEPT/arrays.html#chard">array</a> of records as below:
+
+<pre>
+ main()
+ {
+ char array[50][80]; /* 50 records, 80 bytes long */
+ }
+</pre>
+
+The data can then be read into the array and then actions performed
+on the data.
+This is OK but has some major problems.
+<ol>
+<li>The arrary will hold all the data in character format. It would be
+nice to hold the integer and decimal numbers in a more appropriate form.
+<li>If you have more than 50 records the program has to be altered
+and recompiled.
+</ol>
+The first problem could be fixed by defining an
+<a href="../SYNTAX/struct.html#array">array of structures</a> BUT both problems
+can be solved with linked lists.<p>
+
+<a name=short>
+
+<img src=../../GRAPHICS/linklist.gif align=right>
+<p>
+The concept of a linked list is fairly simple. It is a group of structures
+linked together by pointers, here is a picture:<p>
+<br clear=right>
+<p>
+<p>
+The "Name Age Pointer" block could be defined as below:
+<pre>
+ struct record {char name[20]; int age; struct record *next_rec;};
+</pre>
+
+The important bit is "struct record *next_rec" this is the pointer to the next
+structure (record). It will either point to the next record which
+will require memory reserved
+with the <a href="../FUNCTIONS/malloc.html">malloc</a> function or
+<a href="../SYNTAX/null.html">NULL</a> if its the last record.
+<p>
+
+<h2>Examples</h2>
+
+<img src=../../GRAPHICS/computer.gif align=left>
+<a href="../EXAMPLES/linklst1.c"> Here is the first example.</a>
+This program is more complicated than I would
+like, but its the simplest I can come up with.
+<br clear=left>
+<p>
+<img src=../../GRAPHICS/computer.gif align=center>
+<a href="../EXAMPLES/linklst2.c">
+This is the same program</a> but without the heavy commenting.
+<br clear=left>
+<p>
+<img src=../../GRAPHICS/computer.gif align=center>
+<a href="../EXAMPLES/linklst3.c">
+This is still the same program</a> but it is slightly simplified with the use
+of <a href="../SYNTAX/typedef.html">typedef</a>.
+<br clear=left>
+
+<hr>
+<p>
+<center>
+<table border=2 width=80% bgcolor=ivory>
+<tr align=center>
+<td width=25%>
+<a href="../cref.html" target="_top">Top</a>
+</td><td width=25%>
+<a href="../master_index.html" target="_top">Master Index</a>
+</td><td width=25%>
+<a href="../SYNTAX/keywords.html" target="_top">C Keywords</a>
+</td><td width=25%>
+<a href="../FUNCTIONS/funcref.htm" target="_top">Functions</a>
+</td>
+</tr>
+</table>
+</center>
+<p>
+
+
+
+<hr>
+<address>Martin Leslie
+<script language="JavaScript">
+<!-- //
+document.write(document.lastModified);
+// -->
+</script>
+</address>
+