This example is based on the Steady-State 2D Heat Transfer with Conduction tutorial model, which you can find in the Application Libraries at COMSOL Multiphysics > Heat Transfer.

The following code evaluates the temperature at the location defined by a Cut Point 2D feature, demonstrating how to use both an Evaluation Group and a Derived Values feature:

Note: If you use the Record Code or Record Method tools, the generated code will match the default user interface behavior, it writes all evaluated results into COMSOL table objects. It will not include direct assignments into double[][] arrays or double variables as shown above. The 2D array format is needed because each evaluation feature can compute multiple expressions or perform parametric sweeps.

You can evaluate multiple quantities at once by using an Evaluation Group. In this example, both the temperature and the effective thermal conductivity are evaluated at the point by modifying the middle portion of the previous example:

This file also includes a corresponding method that demonstrates the use of Derived Values.

Building on the previous example, the figure below shows the results of a parametric sweep where the rectangle height parameter h1 is varied in four steps: 0.9, 1.0, 1.1, and 1.2 m.

For parametric sweeps, such output can also be generated programmatically using a custom method, as demonstrated in the figure below where the results are written to the Debug Log.

The following code snipped shows how this can be achieved for an Evaluation Group in a method.

Note that the evaluation results are stored in the 2D array pointResult in the same order as shown in the Evaluation Group table in the user interface. The first index corresponds to the sweep parameter index, and the second index corresponds to the evaluated expressions. By default, each row begins with the sweep parameter value, followed by the results of the specified expressions.

Continuing with the Steady-State 2D Heat Transfer with Conduction tutorial model, suppose a parametric sweep is performed over two parameters: the rectangle height h1 and the boundary temperature T0. Assume the Sweep type is set to Specific combinations, with the following values:

Note: When using Specific combinations, the parameter lists must have the same length, in this case 4 simulation runs, with each entry representing one combination.

The figure below shows the Parametric Sweep settings used for this configuration

We can now use a breakpoint in the Method Editor to stop the code execution at the assignment of the pointResult array and display the results using the Data Viewer window:

For information on using breakpoints, see the book Introduction to the Application Builder.

The four-parameter example includes two dummy parameters to demonstrate that the same approach applies when using the Specific combinations option with more than two parameters.

Continuing with the Steady-State 2D Heat Transfer with Conduction tutorial model, assume now that the Sweep type is set to All combinations, with the following values:

This results in 4×3=12 simulation runs, covering all combinations of the two parameters. The figure below shows the Parametric Sweep settings used for this configuration.

Programmatically, this corresponds to a nested for-loop. This loop is indexed, starting from 1. Once an Evaluation Group has been created, the indices can be retrieved using the following call:

The corresponding output for the array sweepIndices in the Debug Log window is:

We can use a breakpoint in the Method Editor to stop the code execution at the assignment of the pointResult array and display the results using the Data Viewer window:

Note that the pointResult array has dimensions [12][4], where:

This structure follows the row-major order of all combinations: for each value of h1, all values of T0 are iterated through.

Expanding on the previous example, let us now introduce a dummy parameter par1, for demonstration purposes.

As before, we can use a breakpoint in the Method Editor to pause execution at the assignment of the pointResult array and inspect the contents using the Data Viewer window.

Note that the pointResult array has dimensions [24][5], where:

The code used for a two-parameter sweep with the All Combinations option can be generalized as follows, by modifying the latter part of the loop:

The process of extracting results using an Evaluation Group remains essentially the same, even when an auxiliary sweep is used.

The auxiliary sweep (inner sweep) is defined in settings window for Step 1: Stationary as shown in the figure below.

In this case, a point dataset is not required, but aside from that, the code remains very similar to the approach demonstrated earlier. One important detail is to ensure that the correct dataset corresponding to the parametric sweep is used. In this example, that is the dataset with the tag dset2.

The following code snippet shows how to use an Evaluation Group to retrieve the average temperature on a boundary within a method.

The feature type used in the Evaluation Group for computing an average along a line is AvLine. Similar feature types exist for other combinations of geometric entities and evaluation operations, as summarized in the table below.

The variable name for the number of DOFs is numberofdofs.

The following code snippet shows how to use an Evaluation Group to retrieve the number of degrees of freedom at each step of a parametric sweep within a method.

Note: As an alternative to using Evaluation Group features such as AvLine and MaxVolume, you can define a nonlocal coupling operator under Component > Definitions > Nonlocal Couplings, and then evaluate it as a global quantity.

This example is part of a collection available for download and is included in the same file as the previous example, Computing the Average Along a Boundary in a Parametric Sweep.

Instead of using a single Parametric Sweep feature with the All combinations or Specified combinations option, you can use multiple nested Parametric Sweep nodes. Accessing results from a nested sweep is very similar to retrieving data from a sweep that uses only one Parametric Sweep node.

Consider the earlier example where the sweep type is set to All combinations, with the following parameter values:

This sweep can also be performed by nesting two Parametric Sweep nodes, as illustrated in the figures below.

This example is based on the Axisymmetric Transient Heat Transfer tutorial model, which you can find in the Application Libraries at COMSOL Multiphysics > Heat Transfer.

This is a transient (time-dependent) simulation and retrieving transient data is very similar to that of a parametric sweep. You will notice that the code below is very similar to the example in Evaluating Quantities For a Parametric Sweep.

If you instead have access to a Plot Group (for example, with the tag pg1), you can use:

Here, the argument to getStepCount represents the parameter index. However, note that getStepCount belongs to a different part of the API that is more tightly linked to the solution data structures. As a result, the ordering of parameters differs from that of an Evaluation Group.

To illustrate this, consider a sweep over the parameters h1, T0, par1, and par2, as shown in the figure below.

The figure below show all the returned values for the inputs 0, 1, and 2, using the Java Shell window.

A list of output times can be very long and there could be reasons to access only the solutions for a subset. Consider the example in the section Retrieving Data from a Transient Simulation. Recall that this example uses a point evaluation at a cut point.

Note that for interpolated times, the time values given as input do not need to match the output times of the Time Dependent study step.

If you want to clear the Debug Log window before running the method, you can call the clearDebugLog() method.

However, keep in mind that the primary purpose of these examples is not to print results to the Debug Log window, but rather to illustrate how data can be retrieved, processed, and used in other methods or incorporated into simulation apps.

As an alternative to using an Evaluation Group, one can instead use a SolutionInfo object. This technique is more general but also more advanced.

The following method updateOutputTimeList is part of an app where a Combo Box form object is used to select the output time used for evaluation and visualization.

The method evaluates a dummy expression, "1", to retrieve the first and last output times. As an alternative, one can use the SolutionInfo method described in the section Accessing a Subset of a List of Output Times.

is used to dynamically update choicelist1, which is used by a Combo Box in the app’s user interface.

The code for the changeOutputTime method is as follows.

This example is based on the Tuning Fork tutorial model, which you can find in the Application Libraries at COMSOL Multiphysics > Structural Mechanics.

This is a combined parametric and eigenfrequency simulation that is also very similar to that of a parametric sweep. You will notice that the code below is very similar to the example in Evaluating Quantities For a Parametric Sweep.

Specifically it creates an Evaluation Group named eg1 and sets its dataset to dset2, which contains results from a parametric eigenfrequency sweep.

It uses dummy expression 1 to trigger evaluation at the selected sweep points. It manually selects which parametric indices and eigenmode indices to evaluate:

Finally, the code logs each parameter and eigenfrequency pair. The eigenfrequencies vary with the parameter L, due to the fact that the resonance frequency is a function of the length of the tuning fork.