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### A.9.8 Algebraic Subset Window

Algebraic Subset dialogue window

The Algebraic Subset Window allows you to define a new Row Subset which uses an algebraic expression to define which rows are included. The expression must be a boolean one, i.e. its value is either true or false for each row of the table. You can activate this dialogue using the Add Subset () button in the Subsets Window.

The window consists of two fields which must be filled in to define the new subset:

Subset Name
  The name of the new subset. This should preferably be unique (different from existing subset names). It will be easier to use it in other expressions if it is also:
  - Different from other columns even apart from upper/lower case distinctions
  - In the form of a java identifier (starts with a letter, continues with alphanumerics including underscore)
  - Not too long
Expression
  This is a boolean expression which defines the subset; it is a function of the values of any combination of the columns; only rows for which it evaluates to true will be included in the subset. The values of the other columns in the same row are referenced using their names or $ID identifiers, and other subsets may be referenced using their names or _ID identifiers. The rules for expression syntax are described in Section 7, and detailed documentation of the functions you can use can be seen in the Available Functions Window, which you can see by clicking the Show Functions () button on the toolbar.

Having filled in the form to your satisfaction, hit the OK button at the bottom and the new subset will be added to the list that can be seen in the Subsets Window where it behaves like any other. If you have made some mistake in filling in the fields, a popup window will give you a message describing the problem.

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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