At the lowest level, the only defining aspect of a column in a table is its number, or position. But selecting columns purely by number is not very convenient and, especially when the tables are large it can be very frustrating and prone to errors. Hence, table file formats (for example see Recognized table formats) have ways to store additional information about the columns (meta-data). Some of the most common pieces of information about each column are its name, the units of data in the it, and a comment for longer/informal description of the column’s data.
To facilitate research with Gnuastro, you can select columns by matching, or searching in these three fields, besides the low-level column number. To view the full list of information on the columns in the table, you can use the Table program (see Table) with the command below (replace table-file with the filename of your table, if its FITS, you might also need to specify the HDU/extension which contains the table):
$ asttable --information table-file
Gnuastro’s programs need the columns for different purposes, for example in Crop, you specify the column containing the Right Ascension of the crop centers with the --racol option and the column containing the Declination with --deccol. Thus, there is no unified common option name to select columns for all programs. However, when the program expects the column for a specific context (like the RA and Dec example above), the option names end in the col suffix (for example --racol and --deccol). These options accept values in integer (column number), or string (metadata match/search) format.
If the value can be parsed as a positive integer, it will be seen as the
low-level column number. Note that column counting starts from 1, so if you
ask for column 0, the respective program will abort with an error. When the
value can’t be interpreted as an a integer number, it will be seen as a
string of characters which will be used to match/search in the table’s
meta-data. The meta-data field which the value will be compared with can be
selected through the --searchin option, see Input/Output options. --searchin can take three values:
comment. The matching will be done following this
-r/RA_/, or --racol=/RA_/), then it is assumed to be a regular expression with the same convention as GNU AWK. GNU AWK has a very well written chapter describing regular expressions, so we we will not continue discussing it here. Regular expressions are a very powerful tool in matching text and useful in many contexts. We thus strongly encourage reviewing this chapter for greatly improving the quality of your work in many cases, not just for searching column meta-data in Gnuastro.
Note that in both cases, you can ignore the case of alphabetic characters with the --ignorecase option, see Input/Output options. Also, in both cases, multiple columns may be selected with one call to this function. In this case, the order of the selected columns (with one call) will be the same order as they appear in the table.