Next Previous Up Contents
Next: Row Order
Up: Row Subsets
Previous: Row Subsets
You can define a Row Subset in one of the following ways:

Defining an algebraic expression
 From the Subset Window using the
Add New Subset () button will pop up
the Algebraic Subset Window
which allows you to define a new subset using an algebraic expression
based on the values of the cells in each row.
The format of such expressions is described in Section 7.
The Subsets Window also provides some variants on this option
for convenience, like selecting
the first N (),
last N (),
or every N^{th} () rows,
or the complement () of an existing subset.

Graphical selection
 There are several ways to
indicate a region graphically
in the plotting area of the
plotting windows.
which can be used to define subsets.
The options are
New Subset From Visible (),
Draw Subset Blob () and
Draw Subset Polygon (),
though not all are available for all plot types.

Classifying by value
 The Column Classification Window
lets you define multiple mutually exclusive subsets based on
the value in a given column (or other algebraic expression).

Boolean columns
 Any column which has a boolean (true/false) type value can be used
as a subset; rows in which it has a true value are in the subset
and others are not. Any boolean column in a table is made available
as a row subset with the same name when the table is imported.

Selecting rows in the browser
 You can select a single row in the
Data Window by
clicking on it, or select a group of adjacent rows by dragging
the mouse over them. You can add more rows to the selection by
keeping the <Control> button pressed while you do it.
Once you have a set of rows selected you can use the
Subset From Selected Rows () or
Subset From Unselected Rows ()
buttons to create a new subset based
on the set of highlighted rows or their complement.
Combining this with sorting
the rows in the table can be useful;
if you do a Sort Up on a given column and then drag out the
top few rows of the table you can easily create a subset consisting
of the highest values of a given column.
In all these cases you will be asked to assign a name for the subset.
As with column names, it is a good idea to follow a few rules for these
names so that they can be used in algebraic expressions.
They should be:
 Distinct from other subset and column names,
even apart from upper/lower case distinctions
 In the form of a java identifier (starts with a letter, continues with
alphanumerics including underscore, no spaces)
 Not too long
When you choose a name, you can either type one in, or select one from
the dropdown list, which gives the names of all the existing subsets.
This allows you to redefine existing subsets. Note if you do select
or type in one of the existing names,
any previous content of that subset will be lost.
In the first subset definition method above,
the current subset will be set immediately to the
newly created one. In other cases the new subset may be highlighted
appropriately in other windows, for instance by being plotted in
scatter plot windows.
Next Previous Up Contents
Next: Row Order
Up: Row Subsets
Previous: Row Subsets
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