Synthetic Columns

As well as the columns that are present in the table as loaded into TOPCAT, it is possible to define additional "synthetic" columns, composed as a function of the other columns. The idea is that the value of the cells in a given row in this column will be calculated on demand as a function of the values of cells of other columns in that row. You can think of this as providing functionality like that of a very primitive column-oriented spreadsheet.

This is done using the Synthetic Column dialogue, which can be activated from the Columns menu of the Column Metadata window or a popup menu got by right clicking on one of the columns in the main browser window.

The Synthetic Column dialogue asks you to enter the following pieces of information for the new column you will create:

Column Name

The name of the new column. This should preferably be unique (different from all the other column names). It will be easier to use it in other expressions if it 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

Column Units

The units of the column. If the quantity it represents is dimensionless or you don't know the units, this can be left blank. It would be a good idea to use a similar format for the units to that used for the existing columns in the table.

Column Description

A short textual description of what the values contained by this column are. May be left blank.

Expression

This is the algebraic expression which defines the values that the cells in the new column of the table will have. It may be any legal expression which returns a primitive or object. Broadly speaking you can use mathematical operators to combine values taken from the same row in other columns which are referred to by the names or $ID identifiers of those columns. The rules are explained in the Expression Syntax section.

If the expression you enter doesn't make sense you will be shown an error trace and asked to re-enter the expression.

If you wish to add more metadata items you can edit the appropriate cells in the Column Metadata Window grid (though not all such cells are currently editable). The expression associated with a synthetic column cannot currently be edited once the column has been created.

Once created, a synthetic column behaves just like any other; it can be moved, deleted, used in expressions for other synthetic columns and so on. If the table is saved, this column and its contents will be written to the new output table.