path: root/doc/SystemTap_Beginners_Guide/en-US/Array-Operations.xml

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<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
]>
	
<section id="arrayoperators">
	<title>Array Operations in SystemTap</title>
<indexterm>
<primary>array operations</primary>
<secondary>associative arrays</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>associative arrays</secondary>
</indexterm>
	
<para>This section enumerates some of the most commonly used array operations in SystemTap.</para>

<section id="arrayops-assignvalue">
	<title>Assigning an Associated Value</title>

<indexterm>
<primary>array operations</primary>
<secondary>assigning associated values</secondary>
</indexterm>

<indexterm>
<primary>assigning associated values</primary>
<secondary>array operations</secondary>
</indexterm>	

<indexterm>
<primary>values, assignment of</primary>
<secondary>array operations</secondary>
</indexterm>
		<para>Use <command>=</command> to set an associated value to indexed unique pairs, as in:</para>

<screen>
<replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>] = <replaceable>value</replaceable>
</screen>

<para><xref linkend="arraysimplestexample"/> shows a very basic example of how to set an explicit associated value to a unique key. You can also use a handler function as both your <command><replaceable>index_expression</replaceable></command> and <command><replaceable>value</replaceable></command>. For example, you can use arrays to set a timestamp as the associated value to a process name (which you wish to use as your unique key), as in:</para>
<indexterm>
<primary>assigning associated values</primary>
<secondary>array operations</secondary>
<tertiary>associating timestamps to process names</tertiary>
</indexterm>

<indexterm>
<primary>array operations</primary>
<secondary>assigning associated values</secondary>
<tertiary>associating timestamps to process names</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>assigning associated values</secondary>
<tertiary>associating timestamps to process names</tertiary>
</indexterm>

<indexterm>
	<primary>assigning associated values</primary>
	<secondary>associating timestamps to process names</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>associating timestamps to process names</primary>
<secondary>assigning associated values</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
	<primary>timestamps, association thereof to process names</primary>
	<secondary>assigning associated values</secondary>
	<tertiary>array operations</tertiary>
</indexterm>


<example id="arrays-timestampprocessname">
	<title>Associating Timestamps to Process Names</title>
<programlisting>
foo[tid()] = gettimeofday_s()
</programlisting>
</example>

<para>Whenever an event invokes the statement in <xref linkend="arrays-timestampprocessname"/>, SystemTap returns the appropriate <command>tid()</command> value (i.e. the ID of a thread, which is then used as the unique key). At the same time, SystemTap also uses the function <command>gettimeofday_s()</command> to set the corresponding timestamp as the associated value to the unique key defined by the function <command>tid()</command>. This creates an array composed of key pairs containing thread IDs and timestamps.</para>
	
<para>In this same example, if <command>tid()</command> returns a value that is already defined in the array <command>foo</command>, the operator will discard the original associated value to it, and replace it with the current timestamp from <command>gettimeofday_s()</command>.</para>
</section>
<section id="arrayops-readvalues">
	<title>Reading Values From Arrays</title>
<indexterm>
	<primary>reading values from arrays</primary>
	<secondary>array operations</secondary>
</indexterm>

	
<indexterm>
<primary>array operations</primary>
<secondary>reading values from arrays</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>reading values from arrays</secondary>
</indexterm>
	
	<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  
	<command><replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>]</command> 
	statement as an element in a mathematical expression. For example:</para>	
<!--	
	<para>You can also use the <command>=</command> operator to read values from an array. This is accomplished by simply including the <command><replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>]</command> as an element in a mathematical expression. For example:</para>-->
<indexterm>
	<primary>reading values from arrays</primary>
	<secondary>array operations</secondary>
<tertiary>using arrays in simple computations</tertiary>
</indexterm>
	
<indexterm>
<primary>array operations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>using arrays in simple computations</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>using arrays in simple computations</tertiary>
</indexterm>

<indexterm>
<primary>using arrays in simple computations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>algebraic formulas using arrays</primary>
<secondary>reading values from arrays</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>array operations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>computing for timestamp deltas</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>computing for timestamp deltas</tertiary>
</indexterm>

<indexterm>
<primary>computing for timestamp deltas</primary>
<secondary>reading values from arrays</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
	<primary>reading values from arrays</primary>
	<secondary>computing for timestamp deltas</secondary>
<tertiary>array operations</tertiary>
</indexterm>
<indexterm>
<primary>timestamp deltas, computing for</primary>
<secondary>reading values from arrays</secondary>
<tertiary>array operations</tertiary>
</indexterm>
<example id="arrayreadingvaluesfrom">
	<title>Using Array Values in Simple Computations</title>
<screen>
delta = gettimeofday_s() - foo[tid()]
</screen>
</example>

<para>
	This example assumes that the array <command>foo</command> was built using the construct in 
	<xref linkend="arrays-timestampprocessname"/> (from <xref linkend="arrayops-assignvalue"/>). This 
	sets a timestamp that will serve as a <emphasis>reference point</emphasis>, to be used in 
	computing for <literal>delta</literal>.
</para>

<para>
	The construct in <xref linkend="arrayreadingvaluesfrom"/> computes a value for the variable 
	<literal>delta</literal> by subtracting the associated value of the key <literal>tid()</literal> 
	from the current <command>gettimeofday_s()</command>. The construct does this by 
	<emphasis>reading</emphasis> the value of <literal>tid()</literal> 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.
</para>	
<!--

<para>
	In <xref linkend="arrayreadingvaluesfrom"/>, the first statement sets a timestamp associated 
	with the returned value of the handler function <command>tid()</command> as a 
	<emphasis>reference point</emphasis>.
	The second statement computes a value for the variable 
	<command>delta</command> by subtracting the associated value the reference point from the 
	current <command>gettimeofday_s()</command>. Note that the first statement writes the value 
	of <command>gettimeofday_s()</command> into the appropriate key of array 
	<literal>foo</literal>, while in the second statement the value of 
	<command>foo[tid()]</command> is <emphasis>read</emphasis> from the array in order to compute 
	for <literal>delta</literal>.
</para>
-->

<indexterm>
	<primary>reading values from arrays</primary>
	<secondary>array operations</secondary>
	<tertiary>empty unique keys</tertiary>
</indexterm>

<indexterm>
<primary>array operations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>empty unique keys</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>reading values from arrays</secondary>
<tertiary>empty unique keys</tertiary>
</indexterm>

<indexterm>
<primary>empty unique keys</primary>
<secondary>reading values from arrays</secondary>
<tertiary>array operations</tertiary>
</indexterm>
<note>
	<title>Note</title>
<para>
	If the <command><replaceable>index_expression</replaceable></command> cannot 
	find the unique key, it returns a value of 0 (for numerical operations, such as 
	<xref linkend="arrayreadingvaluesfrom"/>) or a null/empty string value (for string operations) by 
	default. 
</para>
</note>
</section>
<section id="arrayops-increment">
	<title>Incrementing Associated Values</title>
<indexterm>
<primary>array operations</primary>
<secondary>incrementing associated values</secondary>
</indexterm>

<indexterm>
	<primary>incrementing associated values</primary>
	<secondary>array operations</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>incrementing associated values</secondary>
</indexterm>		
	<para>Use <command>++</command> to increment the associated value of a unique key in an array, as in:</para>

<screen>
<replaceable>array_name</replaceable>[<replaceable>index_expression</replaceable>] ++
</screen>

<para>Again, you can also use a handler function for your <command><replaceable>index_expression</replaceable></command>. For example, if you wanted to tally how many times a specific process performed a read to the virtual file system (using the event <command>vfs.read</command>), you can use the following probe:</para>

<!-- next 3 indexterms for tallying virtual file system reads (VFS reads) -->

<indexterm>
	<primary>incrementing associated values</primary>
	<secondary>array operations</secondary>
	<tertiary>tallying virtual file system reads (VFS reads)</tertiary>
</indexterm>

<indexterm>
<primary>array operations</primary>
<secondary>incrementing associated values</secondary>
<tertiary>tallying virtual file system reads (VFS reads)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>incrementing associated values</secondary>
<tertiary>tallying virtual file system reads (VFS reads)</tertiary>
</indexterm>

<indexterm>
<primary>tallying virtual file system reads (VFS reads)</primary>
<secondary>incrementing associated values</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>VFS reads, tallying of</primary>
<secondary>incrementing associated values</secondary>
<tertiary>array operations</tertiary>
</indexterm>
<example id="simplesimplevfsread">
	<title>vfsreads.stp</title>
<programlisting>
probe vfs.read
{
  reads[execname()] ++
}
</programlisting>
</example>

<para>In <xref linkend="simplesimplevfsread"/>, the first time that the probe returns the process name <command>gnome-terminal</command> (i.e. the first time <command>gnome-terminal</command> performs a VFS read), that process name is set as the unique key <literal>gnome-terminal</literal> with an associated value of 1. The next time that the probe returns the process name <command>gnome-terminal</command>, SystemTap increments the associated value of <literal>gnome-terminal</literal> by 1. SystemTap performs this operation for <emphasis>all</emphasis> process names as the probe returns them.</para>
</section>

<section id="arrayops-foreach">
	<title>Processing Multiple Elements in an Array</title>
<indexterm>
	<primary>multiple elements in an array</primary>
	<secondary>array operations</secondary>
</indexterm>
	
<indexterm>
<primary>array operations</primary>
<secondary>multiple elements in an array</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>multiple elements in an array</secondary>
</indexterm>	

<indexterm>
	<primary>array operations</primary>
	<secondary>processing multiple elements in an array</secondary>
</indexterm>

<indexterm>
	<primary>processing multiple elements in an array</primary>
	<secondary>array operations</secondary>
</indexterm>

<indexterm>
	<primary>operations</primary>
	<secondary>processing multiple elements in an array</secondary>
</indexterm>
<!-- 	<title>Processing Elements in a Tuple as Iterations</title> -->
	<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 <xref linkend="simplesimplevfsread"/>: 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 <xref linkend="simplesimplevfsread"/> useful is to print the key pairs in the array <command>reads</command>, but how?</para>

<!-- next 3 indexterms for foreach -->
<indexterm>
<primary>array operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>foreach</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>foreach</tertiary>
</indexterm>

<indexterm>
	<primary>processing multiple elements in an array</primary>
	<secondary>foreach</secondary>
	<tertiary>array operations</tertiary>
</indexterm>	

<indexterm>
<primary>foreach</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>array operations</tertiary>
</indexterm>	



<!-- next 2 indexterms for iterations, processing elements in an array as -->

<indexterm>
<primary>array operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>iterations, processing elements in an array as</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>iterations, processing elements in an array as</tertiary>
</indexterm>

<indexterm>
<primary>iterations, processing elements in an array as</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>array operations</tertiary>
</indexterm>
	<para>The best way to process all key pairs in an array (as an iteration) is to use the <command>foreach</command> statement. Consider the following example:</para>
<!--	<para>The best way to process all elements in a tuple (treating each element as an iteration) is to use the <command>foreach</command> statement. Consider the following example:</para>-->


<!-- next 2 indexterms for cumulative virtual file system reads, tallying -->

<indexterm>
<primary>array operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>cumulative virtual file system reads, tallying</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>cumulative virtual file system reads, tallying</tertiary>
</indexterm>

<indexterm>
<primary>cumulative virtual file system reads, tallying</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
	<primary>processing multiple elements in an array</primary>
	<secondary>cumulative virtual file system reads, tallying</secondary>
	<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>virtual file system reads (cumulative), tallying</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>array operations</tertiary>
</indexterm>
<example id="simplevfsreadprint">
	<title>cumulative-vfsreads.stp</title>
<programlisting>
global reads
probe vfs.read
{ 
  reads[execname()] ++
}
probe timer.s(3)
{
  foreach (count in reads)
    printf("%s : %d \n", count, reads[count])
}
</programlisting>
</example>

<para>In the second probe of <xref linkend="simplevfsreadprint"/>, the <command>foreach</command> statement uses the variable <command>count</command> to reference each iteration of a unique key in the array <command>reads</command>. The <command>reads[count]</command> array statement in the same probe retrieves the associated value of each unique key.</para> 
	
<para>Given what we know about the first probe in <xref linkend="simplevfsreadprint"/>, 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.</para>
<indexterm>
<primary>array operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>limiting the output of foreach</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>limiting the output of foreach</tertiary>
</indexterm>

<indexterm>
	<primary>processing multiple elements in an array</primary>
	<secondary>limiting the output of foreach</secondary>
	<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>limiting the output of foreach</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<!-- next 2 indexterms for ordering the output of foreach -->

<indexterm>
<primary>array operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>ordering the output of foreach</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>ordering the output of foreach</tertiary>
</indexterm>

<indexterm>
	<primary>processing multiple elements in an array</primary>
	<secondary>ordering the output of foreach</secondary>
	<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>ordering the output of foreach</primary>
<secondary>processing multiple elements in an array</secondary>
<tertiary>array operations</tertiary>
</indexterm>


<para>Now, remember that the <command>foreach</command> statement in <xref linkend="simplevfsreadprint"/> prints <emphasis>all</emphasis> 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 <command>+</command> (ascending) or <command>-</command> (descending). In addition, you can also limit the number of iterations the script needs to process with the <command>limit <replaceable>value</replaceable></command> option.</para>

<para>For example, consider the following replacement probe:</para>
<screen>
probe timer.s(3)
{
  foreach (count in reads- limit 10)
    printf("%s : %d \n", count, reads[count])
}
</screen>

<para>This <command>foreach</command> statement instructs the script to process the elements in the array <command>reads</command> in descending order (of associated value). The <command>limit 10</command> option instructs the <command>foreach</command> to only process the first ten iterations (i.e. print the first 10, starting with the highest value).</para> 
</section>

<section id="arrayops-deleting">
	<title>Clearing/Deleting Arrays and Array Elements</title>
<indexterm>
<primary>array operations</primary>
<secondary>deleting arrays and array elements</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>deleting arrays and array elements</secondary>
</indexterm>
	
<indexterm>
<primary>array operations</primary>
<secondary>clearing arrays/array elements</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>clearing arrays/array elements</secondary>
</indexterm>

<indexterm>
	<primary>clearing arrays/array elements</primary>
	<secondary>array operations</secondary>
</indexterm>
	<para>Sometimes, you may need to clear the associated values in array elements, or reset an entire array for re-use in another probe. <xref linkend="simplevfsreadprint"/> in <xref linkend="arrayops-foreach"/> allows you to track how the number of VFS reads per process grows over time, but it does not show you the number of VFS reads each process makes per 3-second period.</para>

<!-- next 3 indexterms for delete operator -->
<indexterm>
<primary>array operations</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>delete operator</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>delete operator</tertiary>
</indexterm>

<indexterm>
<primary>clearing arrays/array elements</primary>
<secondary>array operations</secondary>
<tertiary>delete operator</tertiary>
</indexterm>

<indexterm>
<primary>delete operator</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>array operations</tertiary>
</indexterm>	
	<para>To do that, you will need to clear the values accumulated by the array. You can accomplish this using the <command>delete</command> operator to delete elements in an array, or an entire array. Consider the following example:</para>

<indexterm>
<primary>array operations</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>virtual file system reads (non-cumulative), tallying</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>virtual file system reads (non-cumulative), tallying</tertiary>
</indexterm>

<indexterm>
<primary>clearing arrays/array elements</primary>
<secondary>array operations</secondary>
<tertiary>virtual file system reads (non-cumulative), tallying</tertiary>
</indexterm>

<indexterm>
<primary>virtual file system reads (non-cumulative), tallying</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>array operations</tertiary>
</indexterm>	
<example id="simplevfsreadprintnotcumulative">
	<title>noncumulative-vfsreads.stp</title>
<programlisting>
global reads
probe vfs.read
{ 
  reads[execname()] ++
}
probe timer.s(3)
{
  foreach (count in reads)
    printf("%s : %d \n", count, reads[count])
  delete reads	
}
</programlisting>
</example>
<!--	
<example id="simplevfsreadprintnotcumulative">
	<title>vfsreads-per-2secs.stp</title>
<programlisting>
global reads
probe vfs.read
{
  reads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads+) 
    printf("%s : %d \n", count, reads[count])
  delete reads
}
</programlisting>
</example>-->

<para>In <xref linkend="simplevfsreadprintnotcumulative"/>, the second probe prints the number of VFS reads each process made <emphasis>within the probed 3-second period only</emphasis>. The <command>delete reads</command> statement clears the <command>reads</command> array within the probe.</para>

<note>
	<title>Note</title>
	
<!-- next 2 indexterms for multiple array operations within the same probe -->

<indexterm>
<primary>array operations</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>multiple array operations within the same probe</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>multiple array operations within the same probe</tertiary>
</indexterm>

<indexterm>
<primary>clearing arrays/array elements</primary>
<secondary>array operations</secondary>
<tertiary>multiple array operations within the same probe</tertiary>
</indexterm>

<indexterm>
<primary>multiple array operations within the same probe</primary>
<secondary>clearing arrays/array elements</secondary>
<tertiary>array operations</tertiary>
</indexterm>	
	<para>You can have multiple array operations within the same probe. Using the examples from <xref linkend="arrayops-foreach"/> and <xref linkend="arrayops-deleting"/> , you can track the number of VFS reads each process makes per 3-second period <emphasis>and</emphasis> tally the cumulative VFS reads of those same processes. Consider the following example:</para>
	
<screen>
global reads, totalreads

probe vfs.read
{
  reads[execname()] ++
  totalreads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads-) 
    printf("%s : %d \n", count, reads[count])
  delete reads
}

probe end
{
  printf("TOTALS\n")
  foreach (total in totalreads-)
    printf("%s : %d \n", total, totalreads[total])
}
</screen>

<para>In this example, the arrays <command>reads</command> and <command>totalreads</command> track the same information, and are printed out in a similar fashion. The only difference here is that <command>reads</command> is cleared every 3-second period, whereas <command>totalreads</command> keeps growing.</para> 
</note>
</section>	
<section id="arrayops-conditionals">
	<title>Using Arrays in Conditional Statements</title>
<indexterm>
<primary>array operations</primary>
<secondary>conditional statements, using arrays in</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>conditional statements, using arrays in</secondary>
</indexterm>

<indexterm>
<primary>conditional statements, using arrays in</primary>
<secondary>array operations</secondary>
</indexterm>
	
<indexterm>
<primary>if/else statements, using arrays in</primary>
<secondary>array operations</secondary>
</indexterm>

<para>You can also use associative arrays in <command>if</command> statements. This is useful if you want to execute a subroutine once a value in the array matches a certain condition. Consider the following example:</para>

<example id="simplevfsreadprintif">
	<title>vfsreads-print-if-1kb.stp</title>
<programlisting>
global reads
probe vfs.read
{
  reads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads-)
    if (reads[count] >= 1024)
      printf("%s : %dkB \n", count, reads[count]/1024)
    else
      printf("%s : %dB \n", count, reads[count])
}
</programlisting>
</example>	

<para>Every three seconds, <xref linkend="simplevfsreadprintif"/> prints out a list of all processes, along with how many times each process performed a VFS read. If the associated value of a process name is equal or greater than 1024, the <command>if</command> statement in the script converts and prints it out in <command>kB</command>.</para>  
<!--	<title>vfsreads-stop-on-stapio.stp</title>
<programlisting>
global reads
probe kernel.function("vfs_read")
{
  reads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads+) 
    printf("%s : %d \n", count, reads[count])
  if(reads["stapio"] >= 20) {
    exit()
  }
}
</programlisting>
</example>

<para>The <command>if(reads["stapio"] >= 20)</command> instructs the script to execute the subroutine <command>exit()</command> once the value associated with the unique key <command>stapio</command> (in the array <command>reads</command>) is greater than or equal to 20.</para>
-->
<formalpara>
	<title>Testing for Membership</title>
	
<!-- next 3 indexterms for testing for array membership -->
<indexterm>
<primary>array operations</primary>
<secondary>conditional statements, using arrays in</secondary>
<tertiary>testing for array membership</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>conditional statements, using arrays in</secondary>
<tertiary>testing for array membership</tertiary>
</indexterm>

<indexterm>
<primary>conditional statements, using arrays in</primary>
<secondary>array operations</secondary>
<tertiary>testing for array membership</tertiary>
</indexterm>

<indexterm>
<primary>testing for array membership</primary>
<secondary>conditional statements, using arrays in</secondary>
<tertiary>array operations</tertiary>
</indexterm>	

<indexterm>
<primary>membership (in array), testing for</primary>
<secondary>conditional statements, using arrays in</secondary>
<tertiary>array operations</tertiary>
</indexterm>


<para>You can also test whether a specific unique key is a member of an array. Further, membership in an array can be used in <command>if</command> statements, as in:</para>
</formalpara>

<screen>
if([<replaceable>index_expression</replaceable>] in <replaceable>array_name</replaceable>) <replaceable>statement</replaceable>
</screen>

<para>To illustrate this, consider the following example:</para>

<example id="simplesimplevfsreadprintifmember">
	<title>vfsreads-stop-on-stapio2.stp</title>
<programlisting>
global reads

probe vfs.read
{
  reads[execname()] ++
}

probe timer.s(3)
{
  printf("=======\n")
  foreach (count in reads+) 
    printf("%s : %d \n", count, reads[count])
  if(["stapio"] in reads) {
    printf("stapio read detected, exiting\n")
    exit()
  }
}
</programlisting>		
</example>

<para>The <command>if(["stapio"] in reads)</command> statement instructs the script to print <computeroutput>stapio read detected, exiting</computeroutput> once the unique key <command>stapio</command> is added to the array <command>reads</command>.</para>
</section>

<section id="arrayops-aggregates">
	<title>Computing for Statistical Aggregates</title>
<indexterm>
	<primary>statistical aggregates</primary>
	<secondary>array operations</secondary>
</indexterm>	
<indexterm>
	<primary>aggregates (statistical)</primary>
	<secondary>array operations</secondary>
</indexterm>

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
</indexterm>
<para>Statistical aggregates are used to collect statistics on numerical values where it is important to accumulate new data quickly and in large volume (i.e. storing only aggregated stream statistics). Statistical aggregates can be used in global variables or as elements in an array.</para>
<!-- next 3 indexterms for adding values to statistical aggregatest -->
<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>adding values to statistical aggregates</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>adding values to statistical aggregates</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>adding values to statistical aggregates</tertiary>
</indexterm>

<indexterm>
<primary>adding values to statistical aggregates</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<para>To add value to a statistical aggregate, use the operator <command>&lt;&lt;&lt; <replaceable>value</replaceable></command>.</para>

<remark>need more examples of supported rvalues, e.g. length, count, and what each one does.</remark>

<example id="simpleaggregates">
	<title>stat-aggregates.stp</title>
<programlisting>
global reads	
probe vfs.read
{
  reads[execname()] &lt;&lt;&lt; count
}
</programlisting>
</example>

<!-- next 2 indexterms for count operator -->

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>count (operator)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>count (operator)</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>count (operator)</tertiary>
</indexterm>

<indexterm>
<primary>count operator</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array (operator)</tertiary>
</indexterm>

<para>In <xref linkend="simpleaggregates"/>, the operator <command>&lt;&lt;&lt; count</command> <emphasis>stores</emphasis> the amount returned by <literal>count</literal> to to the associated value of the corresponding <command>execname()</command> in the <literal>reads</literal> array. Remember, these values are <emphasis>stored</emphasis>; they are not added to the associated values of each unique key, nor are they used to replace the current associated values. In a manner of speaking, think of it as having each unique key (<command>execname()</command>) having multiple associated values, accumulating with each probe handler run.</para>

<note>
	<title>Note</title>	
	<para>In the context of <xref linkend="simpleaggregates"/>, <literal>count</literal> returns the amount of data written by the returned <command>execname()</command> to the virtual file system.</para>
</note>


<!-- next 2 indexterms for extracting data collected by statistical aggregates -->

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>extracting data collected by statistical aggregates</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>extracting data collected by statistical aggregates</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>extracting data collected by statistical aggregates</tertiary>
</indexterm>

<indexterm>
<primary>extracting data collected by statistical aggregates</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<indexterm>
<primary>integer extractors</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>


<para>To extract data collected by statistical aggregates, use the syntax format <command>@<replaceable>extractor</replaceable>(<replaceable>variable/array index expression</replaceable>)</command>. <command><replaceable>extractor</replaceable></command> can be any of the following integer extractors:</para>
	
<variablelist>
	
<varlistentry>
	<term>count</term>
<listitem>
<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@count (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@count (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>@count (integer extractor)</tertiary>
</indexterm>

<indexterm>
	<primary>@count (integer extractor)</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>	
<para>
	Returns the number of all values stored into the variable/array index expression. Given the sample probe in <xref linkend="simpleaggregates"/>, the expression <command>@count(writes[execname()])</command> will return <emphasis>how many values are stored</emphasis> in each unique key in array <literal>writes</literal>. 
</para>
</listitem>
</varlistentry>	
	
<varlistentry>
	<term>sum</term>
<listitem>

<!-- next 2 indexterms for sum (integer extractor) -->

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@sum (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@sum (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>@sum (integer extractor)</tertiary>
</indexterm>

<indexterm>
	<primary>@sum (integer extractor)</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<para>
	Returns the sum of all values stored into the variable/array index expression. Again, given sample probe in <xref linkend="simpleaggregates"/>, the expression <command>@sum(writes[execname()])</command> will return <emphasis>the total of all values stored</emphasis> in each unique key in array <literal>writes</literal>.  
</para>
	</listitem>
</varlistentry>

<varlistentry>
	<term>min</term>
<listitem>

<!-- next 2 indexterms for min (integer extractor) -->

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@min (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@min (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>@min (integer extractor)</tertiary>
</indexterm>

<indexterm>
	<primary>@min (integer extractor)</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>

<para>
	Returns the smallest among all the values stored in the variable/array index expression.
</para>
</listitem>
</varlistentry>

<varlistentry>
	<term>max</term>
<listitem>

<!-- next 2 indexterms for max (integer extractor) -->

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@max (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@max (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>@max (integer extractor)</tertiary>
</indexterm>

<indexterm>
	<primary>@max (integer extractor)</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>
	
<para>
	Returns the largest among all the values stored in the variable/array index expression.
</para>
</listitem>
</varlistentry>

<varlistentry>
	<term>avg</term>
<listitem>

<!-- next 2 indexterms for avg (integer extractor) -->

<indexterm>
<primary>array operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@avg (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>operations</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>@avg (integer extractor)</tertiary>
</indexterm>

<indexterm>
<primary>computing for statistical aggregates</primary>
<secondary>array operations</secondary>
<tertiary>@avg (integer extractor)</tertiary>
</indexterm>

<indexterm>
	<primary>@avg (integer extractor)</primary>
<secondary>computing for statistical aggregates</secondary>
<tertiary>array operations</tertiary>
</indexterm>


<para>
	Returns the average of all values stored in the variable/array index expression.
</para>
</listitem>
</varlistentry>



<!--
<varlistentry>
	<term></term>
<listitem>
<para>
</para>
</listitem>
</varlistentry>
-->

</variablelist>

<para>
	When using statistical aggregates, you can also build array constructs that use multiple index 
	expressions (to a maximum of 5). This is helpful in capturing additional contextual information 
	during a probe. For example:
</para>

<example id="multiplearrayindices">
	<title>Multiple Array Indexes</title>
<programlisting>
global reads
probe vfs.read
{
  reads[execname(),pid()] &lt;&lt;&lt; 1
}
probe timer.s(3)
{
  foreach([var1,var2] in reads)
    printf("%s (%d) : %d \n", var1, var2, @count(reads[var1,var2]))
}
</programlisting>
</example>

<para>
	In <xref linkend="multiplearrayindices"/>, the first probe tracks how many times each process 
	performs a VFS read. What makes this different from earlier examples is that this array associates
	a performed read to both a process name <emphasis>and</emphasis> its corresponding process ID.
</para>

<para>
	The second probe in <xref linkend="multiplearrayindices"/> demonstrates how to process and print
	the information collected by the array <literal>reads</literal>. Note how the 
	<command>foreach</command> statement uses the same number of variables (i.e. 
	<literal>var1</literal> and <literal>var2</literal>) contained in the first instance of the array 
	<literal>reads</literal> from the first probe.
</para>


</section>

</section>