#!F-adobe-helvetica-medium-r-normal--18*
#!N 
#!CNavyBlue #!N  #!Rall265 Conditional Execution: 
Route and Switch #!N #!EC #!N #!N Two of Data Explorer's 
mechanisms to control execution flow through a visual program are the 
Switch and Route modules. Switch allows you to switch between one 
or more inputs to drive a single output; Route is the 
inverse of Switch, having a single input that can be routed 
to zero, one, or more than one output. Switch is typically 
used to choose between different paths in a visualization program; for 
example, to pass an imported data field through either the Glyph 
module or through Isosurface, depending on user choice or characteristics of 
the data field itself. Route is typically used to turn off 
portions of the visualization program; for example, to turn off WriteImage 
or Export, or to prevent rendering to an image window unless 
the user chooses to create an image. Switch can be thought 
of as turning off portions of the visual program logically  #!F-adobe-times-medium-i-normal--18*   
above #!EF Switch; Route can be thought of as turning off 
portions of the visual program logically  #!F-adobe-times-medium-i-normal--18*   below #!EF Route. Note 
that while Route turns off modules that receive its unused outputs, 
the Collect module is an exception: it runs even if some 
of its inputs have been turned off by Route. #!N #!N 
 #!Lxmodx238,dxall265 f Figure 38  #!EL  shows an example of a Switch module controlling whether an 
Isosurface or a MapToPlane is passed to Image. In a simple 
data-flow execution model, this Switch module will be executed when its 
inputs are available (i.e., the results of the Isosurface and MapToPlane 
modules, and the value of the selector). On execution, the Switch 
module chooses whether to pass the Isosurface or MapToPlane result to 
the output based on the  #!F-adobe-times-bold-r-normal--18*   selection #!EF input to Switch. 
In the case of a pure data-flow model both the Isosurface 
and MapToPlane modules execute before the decision as to which will 
actually be used is known. Since both operations can be computationally 
expensive, the execution of both of them is very inefficient. #!N 
#!N Again, this problem is handled in Data Explorer within the 
simple data-flow execution module by an examination of the graph prior 
to execution. If the  #!F-adobe-times-bold-r-normal--18*   selection #!EF value comes from an 
external source (e.g., an interactor) and is known a priori, the 
selection may be performed by a simple transformation of the graph: 
excising the Switch module altogether, and substituting arcs from the selected 
source (either Isosurface or MapToPlane) to each of the modules that, 
in the original network, received the result of the Switch module. 
This leaves the unselected module dangling. It and any of its 
ancestors that are therefore made unnecessary will not be executed. #!N 
#!N A different procedure is used if the controlling value is 
not static (e.g., if it is determined elsewhere in the network), 
as shown in  #!Lxmodx339,dxall266 f Figure 39  #!EL  . Suppose either an isosurface or a 
set of vector glyphs is selected depending on whether the data 
are scalar or vector. The determination of the type of the 
data is made using the Inquire module (i.e. at run time). 
In this case, the selection value for the Switch module cannot 
be determined before the execution of the graph. Instead, the graph 
must be evaluated in stages: 1) determine the selection value, 2) 
determine the necessary input to the Switch module and 3) evaluate 
the remainder of the graph. Since dynamic inputs may themselves be 
descended from other non-static inputs (e.g., computed in the network), this 
process may have to be performed repeatedly. #!Cbrown #!N  #!F-adobe-times-medium-r-normal--18*    #!Rxmodx339 
#!N Graphics omitted from Online Documentation. Please see the manual. #!N 
#!N Figure 39. Example 3 #!EF #!N #!EC #!N #!N #!N 
 #!F-adobe-times-medium-i-normal--18*   Next Topic #!EF #!N #!N  #!Literseq,dxall267 h Iteration using the Sequencer  #!EL  #!N  #!F-adobe-times-medium-i-normal--18*   #!N