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Synthetic Column dialogue window
The Synthetic Column Window allows you to define a new "Synthetic" column,
that is one whose values are defined using an algebraic expression
based on the values of other columns in the same row.
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 column-oriented spreadsheet.
You can activate the dialogue using the
Add Column () or
Replace Column () buttons in the
Columns Window or from the
(right-click) popup menu in the Data Window.
The window consists of a number of fields you must fill in to
define the new 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 algebraic 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 the algebraic expression which defines the values that the
cells in the new column of the table will have. The rules for
writing algebraic expressions 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.
-
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.
-
Description
- A short textual description of what the values contained by this
column are. May be left blank.
-
UCD
- A Unified Content Descriptor
for the column; a UCD is a semantic label
attached to the column indicating what kind of quantity it contains
by picking one option from a list defined by the
CDS.
The list of known UCDs is available via a selection box, or you
can type a UCD in by hand. You may leave this blank if the you do not
wish to assign a UCD to the column.
A brief description of the UCD selected is visible below
selection box itself.
-
Index
- Determines the position in the displayed table at which the new
column will initially appear.
Of these, the Expression is the only one which must be
filled in.
Having filled in the form to your satisfaction, hit the
OK button at the bottom and the new column will be
added to the table. If you have made some mistake in filling in
the fields, a popup window will give you a message describing the
problem. This message may be a bit arcane - try not to panic and
see if you can rephrase the expression in a way that the parser might
be happier with. If you can't work out the problem, it's time to
consult your friendly local Java programmer (failing that, your
friendly local C programmer may be able to help) or, by all means,
contact the author.
If you wish to add more metadata items you can edit the appropriate
cells in the Columns Window.
You can edit the expression of an existing synthetic column in the same way.
Once created, a synthetic column is added to the
Apparent Table and behaves just like any other;
it can be moved, hidden/revealed, used in expressions for other
synthetic columns and so on.
If the table is saved the new column and its contents will
be written to the new output table.
Next Previous Up Contents
Next: Sky Coordinates Window
Up: Other Windows
Previous: New Parameter 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:
topcat-user@jiscmail.ac.uk