1.2.6. Count Matcher

The count matcher can be used to determine the count of one or more fields and compare it against a reference value. The expression begins with a # character and consists of a tag matcher an optional indicator matcher, an optional subfield matcher using the {}- notation, a comparison operator (==, !=, >=, >, <=, or <), and the comparison value (unsigned integer).

In its simplest form, all fields that match the specified tag matcher* and, if applicable, an *indicator matcher are counted:

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc"
>>>
>>> df = read_marc21(filename, "001", predicate="#035 > 5")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="#500/1# == 3")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="#400/* < 20")
>>> assert df.height == 1
>>>
>>> df = read_marc21(filename, "001", predicate="#099 == 0")
>>> assert df.height == 1

In the following example, the number of fields 400 is counted using the indicators 1 and #, for which a subfield a exists and a subfield 4 is set to the value nafr. The expression evaluates to true if this number is equal to 2.

>>> from marc21_learn.io import read_marc21
>>>
>>> filename = "../tests/data/ada.mrc"
>>> predicate = "#400/1#{ a? && 4 == 'nafr' } == 2"
>>>
>>> df = read_marc21(filename, "001", predicate=predicate)
>>> assert df.height == 1