1.1. Leader Matcher

Records can be filtered based on selected properties of the leader. The following properties can be checked:

• base_addr — Base address of data (position 12-16)

• encoding — Character coding scheme (position 09)

• length — Record length (position 00-04)

• status — Record status (position 05)

• type — Type of record (position 06)

A leader predicate always consists of the prefix ldr. followed by the property to which the predicate refers. This prefix is followed by a comparison operator (==, !=, >=, <=, >, or <), which specifies the type of comparison, and a reference value against the comparison is to be made. Note that the data type of the reference value must match the data type of the corresponding leader field; i.e., the base_addr and length field can only be compared with a 32-bit unsigned integer value, and the remaining fields can only be compared with a single character enclosed in either single or double quotes.

Suppose the following leader of the Ada Lovelace test record:

LDR 03612nz  a2200589nc 4500

Examples:

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc.gz"
>>>
>>> df = read_marc21(filename, "001", predicate="ldr.base_addr == 589")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="ldr.length >= 3600")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="ldr.status == 'n'")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="ldr.type == 'z'")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="ldr.encoding != ' '")
>>> assert df.height == 1