Row Subsets

An useful feature of the TOPCAT is the ability to define Row Subsets. A Row Subset is effectively a table composed from some subset of the rows in a whole table being viewed by the TOPCAT tool. You can define these and use them in several different ways; the usefulness comes from defining them in one context and using them in another. The Subset Window displays the currently defined Row Subsets and permits some operations on them.

You can define a group of rows in the table in a number of ways (by writing a boolean expression, selecting rows in the browser, identifying points in a plot) and view this subset as a separate table, or display different subsets using different markers during plotting.

Defining subsets

You can define a Row Subset in one of the following ways:

Selecting rows in the browser: You can select a single row in the main table browser 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 items in the browser Subsets menu 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 field.
Defining an algebraic expression: If you select the New Subset Expression item from the Subsets menu in the main browser, you will be able to enter an algebraic expression. This is a java-like expression which can refer to the values of cells in various columns in a row to produce a boolean (true/false) result indicating whether the row should be included in that subset or not. The Expression Syntax section explains more about how to specify these.
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.
Visible plotted points: In the Plot Window you can plot columns against each other, and subsequently zoom in and out using the mouse. If you zoom to display only some of the plotted points and then use the New Subset From Visible menu item or toolbar button () then a new subset will be created containing only rows represented by points in the field of view of the plot at the time.

In all these cases you will be asked to assign a name for the subset. As for column names, it is a good idea to follow a few rules for these names. They should be:

Different from other subset names, 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

In the first two subset definition methods above, the newly-defined subset will be applied immediately in the main browser window.

Using Subsets

Once a Row Subset is defined it can be used in one of the following ways:

Browser display: The Apply Subset item in the Subsets menu of the main browser window allows you to choose a subset from the list of those defined. Only rows in the subset that is chosen will show up in the browser window.
Exporting Tables: If a subset has been applied (and hence is active in restricting the rows which are visible in the main browser window) then saving the table, or exporting it to a new viewer using the action, or launching Mirage from TOPCAT will all cause the restricted set of rows only to be exported.
Calculating Statistics: In the Statistics Window you can calculate statistics not just for the column values in the whole table, but for any defined subset of the rows. Use the subset selector button at the bottom of the window.
Plotting: In the Plot Window the plot is drawn with different markers for points appearing in different subsets. Use the Subsets to Plot item in the Subsets menu to select which subsets should be displayed. Markers are chosen for each subset automatically, and a key of which subset is represented by which kind of marker is shown to the right of the plot. In general markers from earlier subsets in the list are obscured by ones later in the list. In future versions more flexibility will be introduced into the display.