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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" type="text/css" href="Common_Content/css/default.css" /><link rel="stylesheet" media="print" href="Common_Content/css/print.css" type="text/css" /><meta name="generator" content="publican 2.8" /><meta name="package" content="Systemtap-SystemTap_Beginners_Guide-2.1-en-US-2.1-2" /><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><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" id="arrayops-foreach"><div class="titlepage"><div><div keep-together.within-column="always"><h3 class="title" id="arrayops-foreach">3.5.4. Processing Multiple Elements in an Array</h3></div></div></div><a id="idm18076768" class="indexterm"></a><a id="idp5226128" class="indexterm"></a><a id="idp8183272" class="indexterm"></a><a id="idp11638744" class="indexterm"></a><a id="idm9751560" class="indexterm"></a><a id="idm9750872" 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">Example 3.16, “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">Example 3.16, “vfsreads.stp”</a> useful is to print the key pairs in the array <code class="command">reads</code>, but how?
</div><a id="idm18079976" class="indexterm"></a><a id="idm440944" class="indexterm"></a><a id="idm4983544" class="indexterm"></a><a id="idp34145344" class="indexterm"></a><a id="idp5424672" class="indexterm"></a><a id="idm9364640" class="indexterm"></a><a id="idm9005904" 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="idm5472304" class="indexterm"></a><a id="idp13114408" class="indexterm"></a><a id="idp1990944" class="indexterm"></a><a id="idp2899744" class="indexterm"></a><a id="idm5523408" class="indexterm"></a><div class="example" id="simplevfsreadprint"><h6>Example 3.17. cumulative-vfsreads.stp</h6><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></div><br class="example-break" /><div class="para">
In the second probe of <a class="xref" href="arrayops-foreach.html#simplevfsreadprint">Example 3.17, “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">Example 3.17, “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="idm4712296" class="indexterm"></a><a id="idm4558528" class="indexterm"></a><a id="idp7920064" class="indexterm"></a><a id="idp10538040" class="indexterm"></a><a id="idp7251848" class="indexterm"></a><a id="idm9537992" class="indexterm"></a><a id="idm1219624" class="indexterm"></a><a id="idp1102448" 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">Example 3.17, “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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