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### A.11.10 Multi Algebraic Subset Window

The Multi Algebraic Subset Window is displayed when an action can create more than one subset with algebraic definitions at once. This happens on completion of the Draw Algebraic Subset () or Algebraic Subset From Visible () actions; more than one subset may result if more than one data set is plotted. If there is more than one subset to be added, they will be shown here in separate rows. Often however, only one subset will be presented.

Multi Algebraic Subset Window showing a subset expression to add

The Subset Name selector gives the name of the subset that will be added. You can type in a new name, or select one from the list of existing subsets if you wish to redefine one (re-use an existing subset name). Note: if you re-use an existing name, the expression must not reference that name directly or indirectly. If it does, an error will be displayed, and you will have to choose again.

Each row has the following entries:

Create
  Indicates whether the subset should be added; you can uncheck it if you don't want to add that subset.
Table
  Indicates which table the subset will be added to.
Expression
  Gives the algebraic expression corresponding to the region you have defined. You can edit this before accepting it if you like by double-clicking on the field in the usual way. If the expression contains a syntax error, it will be greyed out, and attempting to add it will cause an error; you can uncheck the Create checkbox if you want to exclude it.

Once you are happy with the configuration, hit the OK button and the subset(s) will be created.

An Expressions panel also appears at the top of the window. This will, if possible, show representations of the selected region: TOPCAT uses the expression language used elsewhere in the application, and ADQL uses ADQL syntax. The ADQL form can be cut and pasted for use in the WHERE clause of a query in the TAP window. The variable names in these expressions are generic (e.g. "`x`" and "`y`" for Cartesian axes), instead of the actual quantities plotted, which are used in the subset expressions below.

Note: the display of expression information in this window is somewhat experimental; better support for region descriptions may be provided elsewhere in the application in some future release.

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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@jiscmail.ac.uk