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### A.9.9 Available Functions Window

Available Functions Window

This window displays all the functions (Java methods) which are available for use when writing algebraic expressions. This includes both the built-in expressions and any extended ones you might have added. You can find this window by using the Show Functions () button in the Synthetic Column or Algebraic Subset window toolbars.

On the left hand side of the window is a tree-like representation of the functions you can use. Each item in this tree is one of the following:

Folder
  A group of classes. There's only one of these, marked "Activation Functions", and it contains functions which are only available for use in Activation Actions. When defining a new synthetic columns or algebraic subsets they are not used.
Class
  A set of functions and/or constants; it doesn't matter what class a function is in when you use it, but since the functions in a class are usually related this makes it easier to find the one you're looking for in this window.
Function
  A function that you can use in an expression.
Constant
  A constant value which you can refer to by name in an expression (as long as it doesn't clash with a column name).

Of these, the Folder and Class items have a 'handle' (), which means that they contain other items (classes and functions/constants respectively). By clicking on the handle (or equivalently double-clicking on the name) you can toggle whether the item is open (so you can see its contents) or closed (so you can't). So to see the functions in a class, click on its handle and they will be revealed.

You can click on any of these items and information about it will appear in the right hand panel. In the case of functions this describes the function, its arguments, what it does, and how to use it. The explanations should be fairly self-explanatory; for instance the description in the figure above indicates that you could use the invocation `atan2(X_POS,Y_POS)` as the expression for a new table column which gives the angle from the X axis of a point whose position is given by columns with the names X_POS and Y_POS. Examples of a number of these functions are given in Section 7.8.

Using the Add button () you can specify the name of a class to add to those available. You should enter the fully-qualified class name (i.e. including the dot-separated package path). The class that you specify must be on the class path which was current when TOPCAT was started, as explained in Section 10.2.1. Note however it would be more usual to specify these using the system property `jel.classes` or `jel.classes.activation` at startup, as described in Section 7.9. Classes added in this way will be visible in the tree, but may not have proper documentation (clicking on them may not reveal a description in the right hand panel).

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@bristol.ac.uk