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<div class="ChapSects"><a href="chap10_mj.html#X7EADF566874177D8">10 Examples of usage</a>
<div class="ContSect"> <a href="chap10_mj.html#X7EC25CA68794860A">10.1 Writing and reading a file</a>

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<div class="ContSect"> <a href="chap10_mj.html#X7AECE665813D6E4D">10.2 Using filtering programs to read and write files</a>

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<div class="ContSect"> <a href="chap10_mj.html#X7A8A180D7B335151">10.3 Using filters when reading or writing files sequentially</a>

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<div class="ContSect"> <a href="chap10_mj.html#X7BD2BF7F87573C69">10.4 Accessing a web page</a>

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<div class="ContSect"> <a href="chap10_mj.html#X81BD8400832EE20B">10.5 (Un-)Pickling</a>

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<h3>10 Examples of usage</h3>

For larger examples see the <code class="file">example</code> directory of the package. You find there a small server using the TCP/IP protocol and a corresponding client and another small server using the UDP protocol and a corresponding client.

Further, there is an example for the usage of <code class="code">File</code> objects, that read from or write to strings.

Another example there shows starting up a child process and piping a few megabytes through it using <code class="func">IO_Popen2</code> (<a href="chap4_mj.html#X7E05CF6481CBC0A1">4.4-4</a>).

In the following, we present a few explicit, interactive short examples for the usage of the functions in this package. Note that you have to load the IO package with the command <code class="code">LoadPackage("IO");</code> before trying these examples.

<a id="X7EC25CA68794860A" name="X7EC25CA68794860A"></a>

<h4>10.1 Writing and reading a file</h4>

The following sequence of commands opens a file with name <code class="file">guck</code> and writes some things to it:

<div class="example"><pre>
gap> f := IO_File("guck","w");
<file fd=3 wbufsize=65536 wdata=0>
gap> IO_Write(f,"Hello world\n");
12
gap> IO_WriteLine(f,"Hello world2!");
14
gap> IO_Write(f,12345);
5
gap> IO_Flush(f);
true
gap> IO_Close(f);
true
</pre></div>

There is nothing special about this, the numbers are numbers of bytes written. Note that only after the <code class="func">IO_Flush</code> (<a href="chap4_mj.html#X7873A49F7CC7ECFB">4.2-10</a>) command the data is actually written to disk. Before that, it resides in the write buffer of the file. Note further, that the <code class="func">IO_Flush</code> (<a href="chap4_mj.html#X7873A49F7CC7ECFB">4.2-10</a>) call here would not have been necessary, since the <code class="func">IO_Close</code> (<a href="chap4_mj.html#X820DA9C97C2E80BA">4.2-16</a>) call flushes the buffer anyway.

The file can again be read with the following sequence of commands:

<div class="example"><pre>
gap> f := IO_File("guck","r");
<file fd=3 rbufsize=65536 rpos=1 rdata=0>
gap> IO_Read(f,10);
"Hello worl"
gap> IO_ReadLine(f);
"d\n"
gap> IO_ReadLine(f);
"Hello world2!\n"
gap> IO_ReadLine(f);
"12345"
gap> IO_ReadLine(f);
""
gap> IO_Close(f);
true
</pre></div>

Note here that reading line-wise can only be done efficiently by using buffered I/O. You can mix calls to <code class="func">IO_Read</code> (<a href="chap4_mj.html#X84F3D862836BF17C">4.2-6</a>) and to <code class="func">IO_ReadLine</code> (<a href="chap4_mj.html#X8738708E7B9DF8DB">4.2-3</a>). The end of file is indicated by an empty string returned by one of the read functions.

<a id="X7AECE665813D6E4D" name="X7AECE665813D6E4D"></a>

<h4>10.2 Using filtering programs to read and write files</h4>

If you want to write a big amount of data to file you might want to compress it on the fly without using much disk space. This can be achieved with the following command:

<div class="example"><pre>
gap> s := "";; for i in [1..10000] do Append(s,String(i)); od;;
gap> Length(s);
38894
gap> IO_FileFilterString("guck.gz",[["gzip",["-9c"]]],s);
true
gap> sgz := StringFile("guck.gz");;
gap> Length(sgz);
18541
gap> ss := IO_StringFilterFile([["gzip",["-dc"]]],"guck.gz");;
gap> s=ss;
true
</pre></div>

This sequence of commands needs that the program <code class="file">gzip</code> is installed on your system.

<a id="X7A8A180D7B335151" name="X7A8A180D7B335151"></a>

<h4>10.3 Using filters when reading or writing files sequentially</h4>

If you want to process bigger amounts of data you might not want to store all of it in a single GAP string. In that case you might want to access a file on disk sequentially through a filter:

<div class="example"><pre>
gap> f := IO_FilteredFile([["gzip",["-9c"]]],"guck.gz","w");
<file fd=5 wbufsize=65536 wdata=0>
gap> IO_Write(f,"Hello world!\n");
13
gap> IO_Write(f,Elements(SymmetricGroup(5)),"\n");
1359
gap> IO_Close(f);
true
gap> f := IO_FilteredFile([["gzip",["-dc"]]],"guck.gz","r");
<file fd=4 rbufsize=65536 rpos=1 rdata=0>
gap> IO_ReadLine(f);
"Hello world!\n"
gap> s := IO_ReadLine(f);; Length(s);
1359
gap> IO_Read(f,10);
""
gap> IO_Close(f);
true
</pre></div>

<a id="X7BD2BF7F87573C69" name="X7BD2BF7F87573C69"></a>

<h4>10.4 Accessing a web page</h4>

The IO package has an HTTP client implementation. Using this you can access web pages and other web downloads from within GAP. Here is an example:

<div class="example"><pre>
gap> r := SingleHTTPRequest("www.math.rwth-aachen.de",80,"GET",
> "/~Max.Neunhoeffer/index.html",rec(),false,false);;
gap> RecNames(r);
[ "protoversion", "statuscode", "status", "header", "body", "closed" ]
gap> r.status;
"OK"
gap> r.statuscode;
200
gap> r.header;
rec( date := "Thu, 07 Dec 2006 22:08:22 GMT",
 server := "Apache/2.0.55 (Ubuntu)",
 last-modified := "Thu, 16 Nov 2006 00:21:44 GMT",
 etag := "\"2179cf-11a5-3c77f600\"", accept-ranges := "bytes",
 content-length := "4517", content-type := "text/html; charset=ISO-8859-1" )
gap> Length(r.body);
4517
</pre></div>

Of course, the time stamps and exact sizes of the answer may differ when you do this.

<a id="X81BD8400832EE20B" name="X81BD8400832EE20B"></a>

<h4>10.5 (Un-)Pickling</h4>

Assume you have some GAP objects you want to archive to disk grouped together. Then you might do the following:

<div class="example"><pre>
gap> r := rec( a := 1, b := "Max", c := [1,2,3] );
rec( a := 1, b := "Max", c := [ 1, 2, 3 ] )
gap> r.c[4] := r;
rec( a := 1, b := "Max", c := [ 1, 2, 3, ~ ] )
gap> f := IO_File("guck","w");
<file fd=3 wbufsize=65536 wdata=0>
gap> IO_Pickle(f,r);
IO_OK
gap> IO_Pickle(f,[(1,2,3,4),(3,4)]);
IO_OK
gap> IO_Close(f);
true
</pre></div>

Then, to read it in again, just do:

<div class="example"><pre>
gap> f := IO_File("guck");
<file fd=3 rbufsize=65536 rpos=1 rdata=0>
gap> IO_Unpickle(f);
rec( a := 1, b := "Max", c := [ 1, 2, 3, ~ ] )
gap> IO_Unpickle(f);
[ (1,2,3,4), (3,4) ]
gap> IO_Unpickle(f);
IO_Nothing
gap> IO_Close(f);
true
</pre></div>

Note that this works for a certain amount of builtin objects. If you want to archive your own objects or more sophisticated objects you have to use extend the functionality as explained in Section <a href="chap5_mj.html#X7B1C9A9C7D3C0312">5.3</a>. However, it works for lists and records and they may be arbitrarily self-referential.

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