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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>3.5.4. Processing Multiple Elements in an Array</title><link rel="stylesheet" href="./Common_Content/css/default.css" type="text/css" /><meta name="generator" content="publican 1.6" /><meta name="package" content="Systemtap-SystemTap_Beginners_Guide-1.0-en-US-2.0-2" /><link rel="home" href="index.html" title="SystemTap Beginners Guide" /><link rel="up" href="arrayoperators.html" title="3.5. Array Operations in SystemTap" /><link rel="prev" href="arrayops-increment.html" title="3.5.3. Incrementing Associated Values" /><link rel="next" href="arrayops-deleting.html" title="3.5.5. Clearing/Deleting Arrays and Array Elements" /></head><body class=""><p id="title"><a class="left" href="http://www.fedoraproject.org"><img src="Common_Content/images/image_left.png" alt="Product Site" /></a><a class="right" href="http://docs.fedoraproject.org"><img src="Common_Content/images/image_right.png" alt="Documentation Site" /></a></p><ul class="docnav"><li class="previous"><a accesskey="p" href="arrayops-increment.html"><strong>Prev</strong></a></li><li class="next"><a accesskey="n" href="arrayops-deleting.html"><strong>Next</strong></a></li></ul><div class="section" title="3.5.4. Processing Multiple Elements in an Array"><div class="titlepage"><div><div><h3 class="title" id="arrayops-foreach">3.5.4. Processing Multiple Elements in an Array</h3></div></div></div><a id="id2879284" class="indexterm"></a><a id="id2874383" class="indexterm"></a><a id="id3051141" class="indexterm"></a><a id="id2901441" class="indexterm"></a><a id="id2944071" class="indexterm"></a><a id="id2944061" class="indexterm"></a><div class="para">
Once you've collected enough information in an array, you will need to retrieve and process all elements in that array to make it useful. Consider <a class="xref" href="arrayops-increment.html#simplesimplevfsread" title="Example 3.14. vfsreads.stp">Example 3.14, “vfsreads.stp”</a>: the script collects information about how many VFS reads each process performs, but does not specify what to do with it. The obvious means for making <a class="xref" href="arrayops-increment.html#simplesimplevfsread" title="Example 3.14. vfsreads.stp">Example 3.14, “vfsreads.stp”</a> useful is to print the key pairs in the array <code class="command">reads</code>, but how?
</div><a id="id4312600" class="indexterm"></a><a id="id2893838" class="indexterm"></a><a id="id4361519" class="indexterm"></a><a id="id3103773" class="indexterm"></a><a id="id3067417" class="indexterm"></a><a id="id2842258" class="indexterm"></a><a id="id2868666" class="indexterm"></a><div class="para">
The best way to process all key pairs in an array (as an iteration) is to use the <code class="command">foreach</code> statement. Consider the following example:
</div><a id="id2960148" class="indexterm"></a><a id="id2749848" class="indexterm"></a><a id="id3016075" class="indexterm"></a><a id="id2714563" class="indexterm"></a><a id="id2874322" class="indexterm"></a><div class="example" id="simplevfsreadprint"><div class="example-contents"><pre class="programlisting">
global reads
probe vfs.read
{
reads[execname()] ++
}
probe timer.s(3)
{
foreach (count in reads)
printf("%s : %d \n", count, reads[count])
}
</pre></div><h6>Example 3.15. cumulative-vfsreads.stp</h6></div><br class="example-break" /><div class="para">
In the second probe of <a class="xref" href="arrayops-foreach.html#simplevfsreadprint" title="Example 3.15. cumulative-vfsreads.stp">Example 3.15, “cumulative-vfsreads.stp”</a>, the <code class="command">foreach</code> statement uses the variable <code class="command">count</code> to reference each iteration of a unique key in the array <code class="command">reads</code>. The <code class="command">reads[count]</code> array statement in the same probe retrieves the associated value of each unique key.
</div><div class="para">
Given what we know about the first probe in <a class="xref" href="arrayops-foreach.html#simplevfsreadprint" title="Example 3.15. cumulative-vfsreads.stp">Example 3.15, “cumulative-vfsreads.stp”</a>, the script prints VFS-read statistics every 3 seconds, displaying names of processes that performed a VFS-read along with a corresponding VFS-read count.
</div><a id="id3065189" class="indexterm"></a><a id="id3001123" class="indexterm"></a><a id="id2787663" class="indexterm"></a><a id="id2986999" class="indexterm"></a><a id="id2938504" class="indexterm"></a><a id="id2901395" class="indexterm"></a><a id="id2722276" class="indexterm"></a><a id="id2989119" class="indexterm"></a><div class="para">
Now, remember that the <code class="command">foreach</code> statement in <a class="xref" href="arrayops-foreach.html#simplevfsreadprint" title="Example 3.15. cumulative-vfsreads.stp">Example 3.15, “cumulative-vfsreads.stp”</a> prints <span class="emphasis"><em>all</em></span> iterations of process names in the array, and in no particular order. You can instruct the script to process the iterations in a particular order by using <code class="command">+</code> (ascending) or <code class="command">-</code> (descending). In addition, you can also limit the number of iterations the script needs to process with the <code class="command">limit <em class="replaceable"><code>value</code></em></code> option.
</div><div class="para">
For example, consider the following replacement probe:
</div><pre class="screen">
probe timer.s(3)
{
foreach (count in reads- limit 10)
printf("%s : %d \n", count, reads[count])
}
</pre><div class="para">
This <code class="command">foreach</code> statement instructs the script to process the elements in the array <code class="command">reads</code> in descending order (of associated value). The <code class="command">limit 10</code> option instructs the <code class="command">foreach</code> to only process the first ten iterations (i.e. print the first 10, starting with the highest value).
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