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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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Next: Multi Algebraic Subset Window
Up: Other Windows
Previous: Sky Coordinates Window
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:
topcatuser@jiscmail.ac.uk