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<html><head><title>XLISP: An Object-oriented Lisp</title></head>
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<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
<hr>
<h1>File I/O</h1>
<hr>
<ol>
<li><nobr><a href="#1">File I/O Examples</a> - from the XLISP 2.0 manual, by David Betz</nobr></li>
<ul>
<li><nobr><a href="#1-1">Input from a File</a></nobr></li>
<li><nobr><a href="#1-2">Output to a File</a></nobr></li>
<li><nobr><a href="#1-3">A Slightly More Complicated File Example</a></nobr></li>
<li><nobr><a href="#1-4">Closing Files via 'unwind-protect'</a></nobr></li>
</ul>
<li><nobr><a href="#2">Files and Directories</a></nobr></li>
<ul>
<li><nobr><a href="#2-1">setdir</a></nobr></li>
<li><nobr><a href="#2-2">listdir</a></nobr></li>
</ul>
<li><nobr><a href="#3">Testing Existence</a></nobr></li>
<ul>
<li><nobr><a href="#directory-exists-p">directory-exists-p</a></nobr></li>
<li><nobr><a href="#file-exists-p">file-exists-p</a></nobr></li>
<li><nobr><a href="#file-or-directory-exists-p">file-or-directory-exists-p</a></nobr></li>
</ul>
<li><nobr><a href="#4">Text File I/O</a></nobr></li>
<ul>
<li><nobr><a href="#">Reading from a Text File</a></nobr></li>
<li><nobr><a href="#">Writing to a Text File</a></nobr></li>
<li><nobr><a href="#">Appending to a Text File</a></nobr></li>
</ul>
<li><nobr><a href="#5">Binary File I/O</a></nobr></li>
</ol>
<a name="1"></a>
<hr>
<h2>1 File I/O Examples</h2>
<a name="1-1"></a>
<hr>
<h2>1.1 Input from a File</h2>
<hr>
<p>The basics of file i/o with XLISP:</p>
<ul>
<li><p>To open a file for input, use the
<a href="../reference/open.htm">open</a> function with the keyword argument
:direction set to :input.</p></li>
<li><p>To open a file for
output, use the <a href="../reference/open.htm">open</a> function with the
keyword argument :direction set to :output.</p></li>
<li><p>To close a file, use the <a href="../reference/close.htm">close</a>
function.</p></li>
</ul>
<p>The <a href="../reference/open.htm">open</a> function takes a single
required argument which is the name of the file to be opened. This name can
be in the form of a string or a symbol. <nobr>The
<a href="../reference/open.htm">open</a></nobr> function returns an object
of type '<nobr>file-stream</nobr>' if it succeeds in opening the specified
file. <nobr>It returns</nobr> the value
<a href="../reference/nil.htm">NIL</a> if it fails.</p>
<p>In order to manipulate the file, it is necessary to save the value
returned by the <a href="../reference/open.htm">open</a> function.
This is usually done by assigning it to a variable with the
<a href="../reference/setq.htm">setq</a> special form or by binding it using
<a href="../reference/let.htm">let</a> or
<a href="../reference/let-star.htm">let*</a>. Here is an example:</a>
<pre class="example">
(setq file-stream (open "init.lsp" :direction :input))
</pre>
<p>Evaluating this expression will result in the file 'init.lsp' being
opened. The file object that will be returned by the
<a href="../reference/open.htm">open</a> function will be
assigned to the variable '<nobr>file-stream</nobr>'.</p>
<p>It is now possible to use the file for input. <nobr>To read</nobr> an
expression from the file, just supply the value of the
'<nobr>file-stream</nobr>' variable as the optional 'stream' argument to the
<a href="../reference/read.htm">read</a> function:</p>
<pre class="example">
(read file-stream)
</pre>
<p>Evaluating this expression will result in reading the first expression
from the file 'init.lsp'. The expression will be returned as the
result of the <a href="../reference/read.htm">read</a> function.
More expressions can be read from the file using further calls to the
<a href="../reference/read.htm">read</a> function. When there
are no more expressions to read, the
<a href="../reference/read.htm">read</a> function will return
<a href="../reference/nil.htm">NIL</a> [or whatever value was
supplied as the second argument to
<a href="../reference/read.htm">read</a>].</p>
<p>Once you are done reading from the file, you should close it. To close
the file, use the <a href="../reference/close.htm">close</a>
function:</p>
<pre class="example">
(close file-stream)
</pre>
<p>Evaluating this expression will cause the file to be closed.</p>
<p> <a href="#top">Back to Top</a></p>
<a name="1-2"></a>
<hr>
<h2>1.2 Output to a File</h2>
<hr>
<p>Writing to a file is pretty much the same as reading from one. You need
to open the file first. This time you should use the
<a href="../reference/open.htm">open</a> function to indicate
that you will do output to the file:</p>
<pre class="example">
(setq file-stream (open "test.dat" :direction :output))
</pre>
<p>Evaluating this expression will open the file 'test.dat' for output.
<nobr>If the</nobr> file already exists, its current contents will be
discarded. <nobr>If it</nobr> doesn't already exist, it will be created.
<nobr>In any</nobr> case, a '<nobr>file-stream</nobr>' object will be
returned by the <a href="../reference/open.htm">open</a> function. This file
object will be assigned to the '<nobr>file-stream</nobr>' variable.</p>
<p>It is now possible to write to this file by supplying the value of the
'<nobr>file-stream</nobr>' variable as the optional 'stream' parameter in
the <a href="../reference/print.htm">print</a> function.</p>
<pre class="example">
(print "Hello there" file-stream)
</pre>
<p>Evaluating this expression will result in the string <nobr>"Hello
there"</nobr> being written to the <nobr>file
"test.dat"</nobr>. More data can be written to the file using the
same technique.</p>
<p>Once you are done writing to the file, you should
<nobr><a href="../reference/close.htm">close</a> it</nobr>. Closing an
output file is just like closing an input file:</p>
<pre class="example">
(close file-stream)
</pre>
<p>Evaluating this expression will
<a href="../reference/close.htm">close</a> the output file and
make it permanent.</p>
<p> <a href="#top">Back to Top</a></p>
<a name="1-3"></a>
<hr>
<h2>1.3 A Slightly More Complicated File Example</h2>
<hr>
<p>This example shows how to
<a href="../reference/open.htm">open</a> a file,
<a href="../reference/read.htm">read</a> each Lisp expression
from the file and <a href="../reference/print.htm">print</a> it.
It demonstrates the use of files and the use of the optional 'stream'
argument to the <a href="../reference/read.htm">read</a>
function.</p>
<pre class="example">
(do* ((file-stream (open "test.dat" :direction :input))
(expression (read file-stream) (read file-stream)))
((null expression) nil)
(print expression))
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="1-4"></a>
<hr>
<h2>1.4 Closing Files via 'unwind-protect'</h2>
<hr>
<p>To make sure that the file gets closed in the end, the
<nobr>file i/o</nobr> functions can be wrapped by an <nobr><a
href="../reference/unwind-protect.htm">unwind-protect</a> form:</nobr></p>
<pre class="example">
(let ((file-stream (open "test.dat" :direction :input)))
(unwind-protect
<font color="#008844">;; protect-form</font>
(do ((expression (read file-stream) (read file-stream)))
((null expression) nil)
(print expression))
<font color="#008844">;; clean-up form</font>
(when file-stream (close file-stream))))
</pre>
<p>This pattern can be found in many <nobr>file i/o</nobr> functions:</p>
<pre class="example">
(let ((<font color="#AA0000">file-stream</font> (open <font color="#0000CC">filename</font> :direction <font color="#0000CC">direction</font>)))
(unwind-protect
(progn
<font color="#008844">;; do something with the file-stream</font>
)
(when <font color="#AA0000">file-stream</font> (close <font color="#AA0000">file-stream</font>))))
</pre>
<p><nobr> <a href="#top">Back to top</a></nobr></p>
<a name="3"></a>
<hr>
<h2>3 Testing Existence</h2>
<hr>
<p><div class="box">
<p>Note that these function are meant to prevent accidents during basic
<nobr>file i/o</nobr>, they may not be able to test if a file or directory
physically exists or is a link to a different place.</p>
</div></p>
<a name="directory-exists-p"></a>
<p>The Nyquist <a href="../reference/listdir.htm">listdir</a> function can
be used to test if a directory exists:</p>
<pre class="example">
(defun <font color="#0000CC">directory-exists-p</font> (string)
(and (listdir string) t))
</pre>
<a name="file-exists-p"></a>
<p>Testing if a file exists is a bit more tricky because on Unix [including
Linux and <nobr>Mac OS X]</nobr> a directory is a special kind of file, so
the XLISP <a href="../reference/open.htm">open</a> function also can open
directories. That's why we first must make sure that the filename string is
not the name of an existing directory:</p>
<pre class="example">
(defun <font color="#0000CC">file-exists-p</font> (string)
(or (stringp string) (error <font color="#880000">"not a string"</font> string))
(unless (listdir string)
(let ((file-stream (open string)))
(when file-stream
(close file-stream)
string))))
</pre>
<a name="file-or-directory-exists-p"></a>
<p>Before creating a new file it's always a good idea to check
if a file or directory with the same name already exists:</p>
<pre class="example">
(defun <font color="#0000CC">file-or-directory-exists-p</font> (string)
(or (stringp string) (error <font color="#880000">"not a string"</font> string))
(when (or (listdir string)
(let ((file-stream (open string)))
(when file-stream
(close file-stream)
t)))
string))
</pre>
<p><nobr> <a href="#top">Back to Top</a></nobr></p>
<hr>
<a href="../start.htm">Nyquist / XLISP 2.0</a> -
<a href="../manual/contents.htm">Contents</a> |
<a href="tutorials.htm">Tutorials</a> |
<a href="../examples/examples.htm">Examples</a> |
<a href="../reference/reference-index.htm">Reference</a>
</body></html>
|
