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### 6.2 Specifying a Single Column

If an argument is specified in the help text for a command with the symbol `<col-id>` it means you must give a string which identifies one of the existing columns in a table.

There are several ways you can specify a column in this context:

Column Name
  The name of the column may be used if it contains no spaces. It is usually matched case insensitively. If multiple columns have the same name, the first one that matches is selected.
Column Index or $ID
  The index of the column may always be used; this is a useful fallback if the column name isn't suitable for some reason. The first column is '1', the second is '2' and so on. You may alternatively use the forms '$1', '$2' etc.

  Tip: if counting which column has which index is giving you a headache, running `tpipe` with `omode=meta` or `omode=stats` on the table may help.

Column 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$phot_mag_`". If multiple columns have UCDs which match the given identifer, the first one will be used.
Column utype$ 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$<utype-spec>`". Utypes may contain various punctuation marks such as colons and dots; for the purpose of this syntax these should all be represented as underscores ("`_`"). So to identify a column which has the Utype "`ssa:Access.Format`", you should use the identifier "`utype$ssa_Access_format`". Matching is not case-sensitive. If multiple columns have Utypes which match the given identifier, the first one will be used.

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Next: Specifying a List of Columns
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STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page: http://www.starlink.ac.uk/stilts/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk