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<TITLE>User's Guide - Preserving Explicit State</TITLE>
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<H2><A NAME="HDRSTATES" ></A>4.6 Preserving Explicit State</H2>
<A NAME="IDX602"></A><A NAME="IDX603"></A>
<A NAME="IDX604"></A>
<A NAME="IDX605"></A>
<A NAME="IDX606"></A>
<A NAME="IDX607"></A>
<A NAME="IDX608"></A>
<P><B><A NAME="FIGXMODX6" HREF="../usrguide.htm#FT_FIGXMODX6">Figure 22. Example
5</A></B><BR>
<B><BR><CENTER><IMG SRC="../images/xmodf6.gif" ALT="Figure xmodf6 not
displayed."></CENTER><BR></B><BR>
<P>
Some visualization applications require the retention
of state from one execution to the next, which as
discussed earlier, cannot be supported within the
context of pure data flow. Consider, for example, the
creation of a plot of data values at a given point while
sequencing through a time series.
The state of the plot from the prior execution is retrieved.
It is updated by appending the new time-step information, and the
result is then preserved by resaving the state of the plot
for the next execution.
Data Explorer provides two sets of tools for preserving state depending on
whether the state needs to be preserved over one execution of the
network or over multiple executions of the network. The tools for
preserving state are GetLocal, SetLocal, GetGlobal, and SetGlobal. The
Set tools enable you to save an object (in Data Explorer's cache) for
access in a subsequent execution or iteration. The Get tools enable you
to retrieve the object saved by the Set tools.
<P>
You pair a GetLocal and SetLocal in a visual program by creating an
arc from GetLocal's <TT><STRONG>link</STRONG></TT> output parameter to
SetLocal's <TT><STRONG>link</STRONG></TT> input parameter.
In a visual program a GetLocal typically appears logically above a
SetLocal.
When GetLocal runs, it checks if an object has been saved in the cache.
If no object was saved (as would be the case if SetLocal has
not yet run) or the <TT><STRONG>reset</STRONG></TT>
parameter to GetLocal is set,
GetLocal outputs an initial value that you can set
using the
<TT><STRONG>initial</STRONG></TT> parameter.
Otherwise, GetLocal retrieves the saved
object from the cache and outputs it.
When SetLocal runs, it saves its input object in the cache and then
indicates that its paired GetLocal
should simply be scheduled
during the next iteration of a loop
or the next time an execution is called for.
(Note that if GetLocal is inside a macro, it will be executed only if the
macro needs to be executed; that is, if the macro's inputs have changed
or there is a side effect module in the macro.)
<P>
GetGlobal and SetGlobal are paired in the same way as GetLocal and SetLocal.
They also save and retrieve items from the cache. The main
difference is that GetGlobal and SetGlobal will preserve state over
more than one execution of a program. (However, recall that a complete
loop takes place within a <I>single</I> execution.)
<P>
Using GetGlobal and SetGlobal is comparable to using
a static variable in C-language programming. GetLocal and SetLocal are
good for saving state inside of a looping construct. Once the loop is
terminated, the state is reset for the next execution of the loop. To
save state in a program that uses a Sequencer module, you should
use GetGlobal and SetGlobal, since each iteration of the Sequencer
is a separate execution of the program as described in <A
HREF="usrgu030.htm#HDRLOOP">4.5 , "Iteration using Looping"</A>.
<P>
<P>
Illustrated in <A HREF="#FIGXMODX6">Figure 22</A>
is a simple macro that sums the numbers from 1 to N, where N is an input
parameter.
The <TT><STRONG>start</STRONG></TT> parameter to ForEachN has been set to 1.
GetLocal and SetLocal are used to accumulate the sum.
Sum is a trivial macro consisting of a Compute where the
expression is "a+b."
On the first iteration of the loop,
GetLocal will output its <TT><STRONG>initial</STRONG></TT> value,
which has been set to 0.
On subsequent iterations GetLocal will output the accumulated
value SetLocal saved during the previous iteration. When the loop
terminates the final accumulated value is the output of the macro.
This macro is roughly equivalent to the
following C-language statements:
<PRE>
b = 0;
for (a=1; a<=10; a++)
b = b+a;
</PRE>
<P>
If the macro were run again, on the first iteration of the loop
GetLocal would again output its <TT><STRONG>initial</STRONG></TT> value.
(Note that the macro
will only run again if the input to the macro changes or the output of the
macro has been removed from cache.)
<P>
If you replaced the GetLocal and SetLocal in <A HREF="#FIGXMODX6">Figure 22</A>
with
GetGlobal and SetGlobal it would be equivalent to the following
C-language statements:
<PRE>
int a;
static int b = 0;
for (a=1; a<=10; a++)
b = b+a;
</PRE>
While when SetLocal is used, the sum is reset each time the macro is
run, if SetGlobal is used, the sum of a previous execution is added to the
sum of the current execution. For example, let macro_local be the macro
shown in <A HREF="#FIGXMODX6">Figure 22</A> and macro_global be the same macro
but with
SetGlobal and GetGlobal substituted for SetLocal and GetLocal. If the
input to both macros is 10 then both macros will output 55 (the sum of numbers
1 to 10) the first time they are run. If an execution takes place
without the input to the macros changing then neither macro will run again
and the value 55 will be used as the output again. If you change the input
to 3 then macro_local will output 6 and macro_global will output 61 (55+6).
<P>
Illustrated in <A HREF="#FIGXMODX7">Figure 23</A> is a macro that returns the
accumulated volumes of the members of a group and the number of
members in the group.
ForEachMember is used to iterate through the group.
Measure is used to determine the volume of a member and the GetLocal
and SetLocal pair on the left side of the macro is used to accumulate
the volumes.
For illustrative purposes, a loop containing GetLocal, SetLocal, and Increment
is used to count the number of members in the group.
(Inquire also provides this function, as does the
<TT><STRONG>index</STRONG></TT> output of
ForEachMember.)
Increment is a trivial macro consisting of a
Compute where the expression is set to "a+1."
The <TT><STRONG>initial</STRONG></TT> values to both GetLocal tools are 0.
<P><B><A NAME="FIGXMODX7" HREF="../usrguide.htm#FT_FIGXMODX7">Figure 23. Example
6</A></B><BR>
<B><BR><CENTER><IMG SRC="../images/xmodf7.gif" ALT="Figure xmodf7 not
displayed."></CENTER><BR></B><BR>
<P>
Illustrated in <A HREF="#FIGXMODX13">Figure 24</A> is a visual program that
saves the
current camera settings for use in the next execution of the
program. The initial value of GetGlobal is NULL. The Inquire
module checks to see that the output of GetGlobal is
a valid camera object. If it's not a camera
object, then Route is used to ensure that the Display module is
not scheduled to run.
When a new camera is chosen (for example by rotating the object in the Image
window) the Display window will show the image
using the previous execution's camera settings.
<P><B><A NAME="FIGXMODX13" HREF="../usrguide.htm#FT_FIGXMODX13">Figure 24. Example
7</A></B><BR>
<B><BR><CENTER><IMG SRC="../images/xmodf13.gif" ALT="Figure xmodf13 not
displayed."></CENTER><BR></B><BR>
<P>
As mentioned previously, in a true data-flow implementation, all
modules are pure functions (i.e. their outputs are fully defined
by their inputs). Hence, processes are stateless with no side
effects. A macro in Data Explorer is considered to be a function,
with its outputs being fully defined by its inputs. This is no
longer true when a GetGlobal module is added to a macro. GetLocal
maintains state information only within one execution of the macro.
GetGlobal maintains state information between executions, and therefore
the outputs of a macro containing GetGlobal
are no longer entirely defined by the inputs. The outputs
from macros with state (containing a GetGlobal module) are
guaranteed to stay in the cache until the inputs for that macro
change. At that point, the results of the previous execution
are discarded to make room for the new
results. This is equivalent to setting the cache attribute of the macro to
<TT><STRONG>cache last</STRONG></TT> for each of the outputs. These cache
settings
cannot be overwritten by the user. This guarantees coherency when
executing macros with state.
<P>
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