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### 7.1 Referencing Cell Values

To create a useful expression for a cell in a column, you will have to refer to other cells in different columns of the same table row. You can do this in three ways:

By Name
  The Name of the column may be used if it is unique (no other column in the table has the same name) and if it has a suitable form. This means that it must have the form of a Java variable - basically starting with a letter and continuing with letters or numbers. In particular it cannot have any spaces in it. The underscore and currency symbols count as letters for this purpose. Column names are treated case-insensitively.
By $ID
  The "$ID" identifier of the column may always be used to refer to it; this is a useful fallback if the column name isn't suitable for some reason (for instance it contains spaces or is not unique). This is just a "$" sign followed by a unique integer assigned by the program to each column when it is first encountered. You can find out the $ID identifier by looking in the Columns Window.
By ucd$ specifier
  If the column has a Unified Content Descriptor (this will usually only be the case for VOTable or possibly FITS format tables) you can refer to it using an identifier of the form "`ucd$<ucd-spec>`". Depending on the version of UCD scheme used, UCDs can contain various punctuation marks such as underscores, semicolons and dots; for the purpose of this syntax these should all be represented as underscores ("`_`"). So to identify a column which has the UCD "`phot.mag;em.opt.R`", you should use the identifier "`ucd$phot_mag_em_opt_r`". Matching is not case-sensitive. Futhermore, a trailing underscore acts as a wildcard, so that the above column could also be referenced using the identifier "`ucd$phot_mag_`". If multiple columns have UCDs which match the given identifer, the first one will be used.

  Note that the same syntax can be used for referencing table parameters (see Section 7.3); columns take preference so if a column and a parameter both match the requested UCD, the column value will be used.

By utype$ specifier
  If the column has a Utype (this will usually only be the case for VOTable or possibly FITS format tables) you can refer to it using an identifier of the form "`utype$<utype-spec>`". Utypes can contain various punctuation marks such as colons and dots; for the purpose of this syntax these should all be represented as underscores ("`_`"). So to identify a column which has the Utype "`ssa:Access.Format`", you should use the identifier "`utype$ssa_Access_Format`". Matching is not case-sensitive. If multiple columns have Utypes which match the given identifier, the first one will be used.

  Note that the same syntax can be used for referencing table parameters (see Section 7.3); columns take preference so if a column and a parameter both match the requested Utype, the column value will be used.

There is a special column whose name is "Index" and whose $ID is "$0". The value of this is the same as the row number in the unsorted table (the grey numbers on the left of the grid in the Data Window), so for the first column in the unsorted table it's 1, for the second it's 2, and so on.

The value of the variables so referenced will be a primitive (boolean, byte, short, char, int, long, float, double) if the column contains one of the corresponding types. Otherwise it will be an Object of the type held by the column, for instance a String. In practice this means: you can write the name of a column, and it will evaluate to the numeric (or string) value that that column contains in each row. You can then use this in normal algebraic expressions such as "`B_MAG-U_MAG`" as you'd expect.

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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@bristol.ac.uk