Calculator List

../../../../_images/calculator1.svg

Create columns by evaluating python calculations.

Documentation

The calculations are written as Python code and can consist of simple arithmetic calculations, Python function calls, or calls to functions defined in plugins. The result of each calculation is written to a column in the output table.

Configuration gui

You declare each calculation by typing a name in the text line labeled Signal name and entering the calculation in the textfield labeled Calculation. You can use any of the signals in the list Signal names in your calculation.

To use a signal from an incoming table type simply drag-and-drop the signal name from the list of available signals to the calculation field. To use a signal from the incoming generic data use arg in a way that fits the data format as can be seen in the examples below.

To add a function, drag-and-drop it from the Avaliable functions tree structure. Note that any signal that you reference in the calculation must exist in all incoming data structures.

To add a new calculation, press the New button and the Calculation field as well as Signal name will be cleared. If you want to edit a calculation simply click on the calculation in the List of calculations. The signal name will then appear under Signal name and the calculation will appear in the Calculation field. The calculation will be updated in the Calculation field, List of calculations and preview simultaneously. To remove a calculation, mark a calculation in List of calculations and press the Remove button.

If something goes wrong when you define the calculations you will get an error or warning message in the preview window and at the top of the window. The message at the top of the window has a menu which can be used to choose how often it should be shown. Access it by right-clicking on a message that is shown and choose an appropriate setting, for example, show always or hide always.

Some commonly used operators and functions can be found under the function tree structure and can be added to a calculation by double-clicking or dragging the function name to the calculation area. If you want more information about a function, hover its name and its documentation will appear as a tooltip.

Calculations

A calculation in the calculator must be a python expression which evaluates to either a numpy array, a list, or a scalar value (str/int/float etc.).

Things that can be used in expressions include literal values, operators, function calls, list comprehensions, lambda function definitions, and conditional expressions. Things that can not be used in expressions include if statements, for/while loops, function definitions with the def keyword, class definitions, and import statements.

The data on the input port is available under the name arg and any calculated columns are accessable from the Table named res.

Working with Table columns

Note

This node does not support the ${COLUMN_NAME} notation of the older Calculator Table nodes. Use arg['COLUMN_NAME'] instead.

If the input is a Table you can get a column from it with arg['COLUMN_NAME']. The column will be returned as a numpy array.

To get the number of rows in the Table use arg.number_of_rows(). To get a list of the column names, use arg.column_names(). For more information see the Table API reference.

Element wise calculations

Simple arithmetics and many functions can work directly on numpy arrays, but for cases where this doesn’t work you can use either list comprehensions or np.vectorize.

One such case which comes up often is when working with arrays of strings. Say for example that you have a Table with a column (paths) of file paths and want to get the path to the containing directory for each file. This is easy to do with a list comprehension:

[os.path.dirname(p) for p in arg['paths']]

or with np.vectorize:

np.vectorize(os.path.dirname, otypes=[str])(arg['paths'])

The otypes argument declares that the output should be of string type and is needed to allow the calculation to be performed even if arg['paths'] is empty (i.e. if the input table has no rows).

Generic input

The Calculator nodes can perform calculations on any given input. Any type can by used as input and it is accessed by the keyword arg. The API of the incoming data type can be used in the calculator.

Some examples:

- Table         - Columns are accessed as ``arg['signal1']``
- List of Table - Columns are accessed as ``arg[0]['signal1']``
- ADAF          - Time series are accessed as ``arg.sys['system0']['raster0']['signal0'].y``
- Tuple         - Elements are accessed as ``arg[0]``

Some useful numpy functions

Numpy is available under the name np in all calculations. A few numpy functions that are very useful in the calculator node are np.where, np.flat_nonzero, and np.vectorize. See their respective documentations for more information.

Avoiding errors if Table column is missing

If a calculation uses a column from the incoming Table (e.g. arg['COLUMN_NAME']) and that column doesn’t exist in the input Table the calculator node will fail with an error. The simplest way to fix this would be to change the error handling option to “Skip calculation” which would simply ignore the calculation if there is any error while running the calculation. The downside to this is that the output will also sometimes have the calculated column and sometimes not.

Another way around this can be to iterate through a table’s columns, like so:

[arg[name][0] for name in arg.column_names()]

Or always use for example the first column in the Table:

arg[arg.column_names()[0]]

Another way is to use conditional expressions. Here is an example of a calculation which tries to copy a column, but if it doesn’t exist it will instead create a column of zeros:

arg['My column'] if 'My column' in arg else np.zeros(arg.number_of_rows())

Calculation Attributes

Each calculated column can have any number of custom associated attributes. These are, at least for now, much more limited than calculations. Each attribute has a string for its name and another string for its value and both are treated as text and are not evaluated as python expressions. The use for these is being able to associate metadata to output columns created by calculations. For example:

Name

Value

unit

ms

will attach milliseconds for unit to a specific column.

Output

Each column of the output will have a calculation attribute with a string representation of the calculation used to create that column.

In the configuration, there is an option on how to handle exceptions (Action on calculation failure) produced by the node, for example missing signals or erroneous calculations.

In the list of calculations there is also the option to disable individual calculations, i.e., exclude them from the output. This makes it possible to make intermediary calculations that are not actually included in the output from the node. Such intermediary calculations don’t even need to have the same lengths as the the rest of the calculations.

Definition

Input ports

port0

[<a>]

Generic Input

Output ports

port1

[table]

Tables with results from the calculations.

Configuration:
List of calculations (calc_list)

List of calculations.

(no label) (calc_attrs_dict)

Calculation attributes as json dict-list-string!

Copy input (copy_input)

If enabled the incoming data will be copied to the output before running the calculations. This requires that the results will all have the same length. An exception will be raised if the lengths of the outgoing results differ.

Action on calculation failure (fail_strategy)

Decide how a failed calculation should be handled

class node_calculator.CalculatorGenericListNode[source]

Plugins

Related nodes

Example flows