Loop Parameters (Concurrency)

Synopsis

This Operator iterates over its subprocess for all the defined parameter combinations. The parameter combinations can be set by the wizard provided in parameters.

Description

The Loop Parameters Operator is a nested Operator. It executes the subprocess for all combinations of selected values of the parameters. This can be very useful for plotting or logging purposes and sometimes for simply configuring the parameters for the inner Operators as a sort of meta step. Any results of the subprocess are delivered through the output ports. This Operator can be run in parallel.

The entire configuration of this Operator is done through the edit parameter settings parameter. Complete description of this parameter can be found in the parameters section.

The inner performance port can be used to log the performance of the inner subprocess. When it is connected, a log is created automatically to capture the number of the run, the parameter settings and the main criterion or all criteria of the delivered performance vector, depending on the parameter log all criteria. Please note that if no results are delivered at the end of a process, the log tables still can be seen in the Results View even if it is not automatically shown.

Please note that this Operator has two modes: synchronized and non-synchronized. They depend on the setting of the synchronize parameter. In the latter, all parameter combinations are generated and the subprocess is executed for each combination. In the synchronized mode, no combinations are created but the parameter values are treated as a list of combinations. For the iteration over a single parameter there is no difference between both modes. Please note that the number of parameter possibilities must be the same for all parameters in the synchronized mode. As an Example, having two boolean parameters A and B (both with true(t)/false(f) as possible parameter settings) will produce four combinations in non-synchronized mode (t/t, f/t, t/f, f/f) and two combinations in synchronized mode (t/t, f/f).

If the synchronize parameter is not set to true, selecting a large number of parameters and/or large number of steps (or possible values of parameters) results in a huge number of combinations. For example, if you select 3 parameters and 25 steps for each parameter then the total number of combinations would be above 17576 (i.e. 26 x 26 x 26). The subprocess is executed for all possible combinations. Running a subprocess for such a huge number of iterations will take a lot of time. So always carefully limit the parameters and their steps.

Differentiation

Optimize Parameters (Grid)

The Optimize Parameters (Grid) Operator executes the subprocess for all combinations of selected values of the parameters and then delivers the optimal parameter values. The Loop Parameters Operator, in contrast to the optimization Operators, simply iterates through all parameter combinations. This might be especially useful for plotting purposes.

Tutorial Processes

Iterating through the parameters of the SVM Operator

The 'Weighting' data set is loaded using the Retrieve Operator. The Loop Parameters Operator is applied on it. Have a look at the Edit Parameter Settings parameter of the Loop Parameters Operator. You can see in the Selected Parameters window that the C and gamma parameters of the SVM Operator are selected. Click on the SVM.C parameter in the Selected Parameters window, you will see that the range of the C parameter is set from 0.001 to 100000. 11 values are selected (in 10 steps) logarithmically. Now, click on the SVM.gamma parameter in the Selected Parameters window, you will see that the range of the gamma parameter is set from 0.001 to 1.5. 11 values are selected (in 10 steps) logarithmically. There are 11 possible values of two parameters, thus there are 121 ( i.e. 11 x 11) combinations. The subprocess will be executed for all combinations of these values because the synchronize parameter is set to false, thus it will iterate 121 times. In every iteration, the value of the C and/or gamma parameters of the SVM(LibSVM) Operator is changed. The value of the C parameter is 0.001 in the first iteration. The value is increased logarithmically until it reaches 100000 in the last iteration. Similarly, the value of the gamma parameter is 0.001 in the first iteration. The value is increased logarithmically until it reaches 1.5 in the last iteration.

Have a look at the subprocess of the Loop Parameters Operator. First the data is split into two equal partitions using the Split Data Operator. The SVM (LibSVM) Operator is applied on one partition. The resultant classification model is applied using an Apply Model Operator on the other partition. The statistical performance of the SVM model on the testing partition is measured using the Performance (Classification) Operator. At the end the Loop Parameters Operator automatically logs the parameter settings and performance.

The log contains the following four things: The iteration numbers of the Loop Parameters Operator are counted. This is stored in a column named 'Iteration'. The classification error of the performance of the testing partition is logged in a column named 'classification error'. The value of the C parameter of the SVM (LibSVM) Operator is stored in a column named 'SVM.C'. The value of the gamma parameter of the SVM (LibSVM) Operator is stored in a column named 'SVM.gamma'.

Run the process and turn to the Results View. Now have a look at the values logged by the Loop Parameters Operator.

Parameters

edit_parameter_settings The parameters are selected through the edit parameter settings menu. You can select the parameters and their possible values through this menu. This menu has an Operators window which lists all the operators in the subprocess of this Operator. When you click on any Operator in the Operators window, all parameters of that Operator are listed in the Parameters window. You can select any parameter through the arrow keys of the menu. The selected parameters are listed in the Selected Parameters window. Only those parameters should be selected for which you want to iterate the subprocess. This Operator iterates through parameter values in the specified range. The range of every selected parameter should be specified. When you click on any selected parameter (parameter in Selected Parameters window), the Grid/Range and Value List option is enabled. These options allow you to specify the range of values of the selected parameters. The Min and Max fields are for specifying the lower and upper bounds of the range respectively. As all values within this range cannot be checked, the steps field allows you to specify the number of values to be checked from the specified range. Finally the scale option allows you to select the pattern of these values. You can also specify the values in form of a list. Range: menu
error_handling This parameter allows you to select the method for handling errors occurring during the execution of the inner process. It has the following options:
- fail_on_error: In case an error occurs, the execution of the process will fail with an error message.
- ignore_error: In case an error occurs, the error will be ignored and the execution of the process will continue with the next iteration.
Range: selection
log_performance This parameter will only be visible if the inner performance port is connected. If it is connected, the main criterion of the performance vector will be automatically logged with the parameter set if this parameter is set to true. Range: boolean
log_all_criteria This parameter allows for more logging. If set to true, all performance criteria will be logged. Range: boolean
synchronize This Operator has two modes: synchronized and non-synchronized. They depend on the setting of this parameter. If it is set to false, all parameter combinations are generated and the inner Operators are applied for each combination. If it is set to true, no combinations are created but the parameter values are treated as a list of combinations. For the iteration over a single parameter there is no difference between both modes. Please note that the number of parameter possibilities must be the same for all parameters in the synchronized mode. Range: boolean
enable_parallel_execution This parameter enables the parallel execution of the subprocess. Please disable the parallel execution if you run into memory problems. Range: boolean

Input

input (IOObject)
This Operator can have multiple inputs. When one input is connected, another input port becomes available which is ready to accept another input (if any). The order of inputs remains the same. The Object supplied at the first input port of this Operator is available at the first input port of the nested chain (inside the subprocess). Do not forget to connect all inputs in correct order. Make sure that you have connected the right number of ports at the subprocess level.

Output

output (Collection)
Any results of the subprocess are delivered through the output ports. This Operator can have multiple outputs. When one output port is connected, another output port becomes available which is ready to deliver another output (if any). The order of outputs remains the same. The Object delivered at the first output port of the subprocess is delivered at the first outputport of the Operator. Don't forget to connect all outputs in correct order. Make sure that you have connected the right number of ports.

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