Comparator.md

How to compare two lines

Many cdx tools need to compare lines of text with each other, for sorting or just equality.

The default behavior is a simple textual match of the whole line - byte for byte.

These tools can take a Comparator of the form --key=Spec where the spec is :

Column,Method,Pattern

where Column is a column name or number. Leave this empty to look at the line as a whole, or for comparators like expr that do not need a separate column specification. Most comparators don't take a pattern, so you can simplify to simply Column,Method. The default method is plain, so for a plain comparison, you can simplify to Column. An empty string would compare the whole line, plain.

By default, a field that does not represent a valid value for the method sorts higher than any valid value, but by default, all field values are considered valid.

Method is one of :

• plain : sort the plain bytes of the column (the default)
• expr : sort the value of the arithmetic expression in the pattern.
• length (or 'len') : sort fields by their length
• float : best effort to convert field to 64-bit floating point value, and compare that
• numeric (or 'num') : Numeric compare, assuimg input is in the form [-]nnn.nnn, with no limit on length or value
• ip : Sort as IP address or 1.2.3 section numbers
• lower : Sort as lower case ascii
• equal : All lines are always equal to each other

Combined with :

• rev : reverse sort
• low : invalid values sort lower than any valid value
• trail : a valid value with trailing garbage is ok. That is, a value is invalid unless it has a valid prefix.
• strict : All bytes of the field must contain a valid value

For example

• --key 2 sorts by column 2 increasing.
• -k price,float.strict
• -k 3,rev.float sorts by column 3, by decreasing floating point value
• --key title,rev.len sorts by decreasing length of the title column
• -k author -k title sorts by author, and if the authors are the same, by title.