<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>3.5.2. Reading Values From Arrays</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="arrayoperators.html" title="3.5. Array Operations in SystemTap" /><link rel="next" href="arrayops-increment.html" title="3.5.3. Incrementing Associated Values" /></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="arrayoperators.html"><strong>Prev</strong></a></li><li class="next"><a accesskey="n" href="arrayops-increment.html"><strong>Next</strong></a></li></ul><div class="section" id="arrayops-readvalues"><div class="titlepage"><div><div keep-together.within-column="always"><h3 class="title" id="arrayops-readvalues">3.5.2. Reading Values From Arrays</h3></div></div></div><a id="idp1974176" class="indexterm"></a><a id="idp31678408" class="indexterm"></a><a id="idm8619992" class="indexterm"></a><div class="para"> You can also read values from an array the same way you would read the value of a variable. To do so, include the <code class="command"><em class="replaceable"><code>array_name</code></em>[<em class="replaceable"><code>index_expression</code></em>]</code> statement as an element in a mathematical expression. For example: </div><a id="idm4897592" class="indexterm"></a><a id="idp37250840" class="indexterm"></a><a id="idm17006536" class="indexterm"></a><a id="idm3201568" class="indexterm"></a><a id="idp7578200" class="indexterm"></a><a id="idm789592" class="indexterm"></a><a id="idp1979704" class="indexterm"></a><a id="idm3140968" class="indexterm"></a><a id="idp12159656" class="indexterm"></a><a id="idp1126368" class="indexterm"></a><div class="example" id="arrayreadingvaluesfrom"><h6>Example 3.15. Using Array Values in Simple Computations</h6><div class="example-contents"><pre class="screen">delta = gettimeofday_s() - foo[tid()]</pre></div></div><br class="example-break" /><div class="para"> This example assumes that the array <code class="command">foo</code> was built using the construct in <a class="xref" href="arrayoperators.html#arrays-timestampprocessname">Example 3.14, “Associating Timestamps to Process Names”</a> (from <a class="xref" href="arrayoperators.html#arrayops-assignvalue">Section 3.5.1, “Assigning an Associated Value”</a>). This sets a timestamp that will serve as a <span class="emphasis"><em>reference point</em></span>, to be used in computing for <code class="literal">delta</code>. </div><div class="para"> The construct in <a class="xref" href="arrayops-readvalues.html#arrayreadingvaluesfrom">Example 3.15, “Using Array Values in Simple Computations”</a> computes a value for the variable <code class="literal">delta</code> by subtracting the associated value of the key <code class="literal">tid()</code> from the current <code class="command">gettimeofday_s()</code>. The construct does this by <span class="emphasis"><em>reading</em></span> the value of <code class="literal">tid()</code> from the array. This particular construct is useful for determining the time between two events, such as the start and completion of a read operation. </div><a id="idm1841560" class="indexterm"></a><a id="idm2923088" class="indexterm"></a><a id="idp2011416" class="indexterm"></a><a id="idp39103712" class="indexterm"></a><div class="note"><div class="admonition_header"><h2>Note</h2></div><div class="admonition"><div class="para"> If the <code class="command"><em class="replaceable"><code>index_expression</code></em></code> cannot find the unique key, it returns a value of 0 (for numerical operations, such as <a class="xref" href="arrayops-readvalues.html#arrayreadingvaluesfrom">Example 3.15, “Using Array Values in Simple Computations”</a>) or a null/empty string value (for string operations) by default. </div></div></div></div><ul class="docnav"><li class="previous"><a accesskey="p" href="arrayoperators.html"><strong>Prev</strong>3.5. 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