Missing

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Missing[] | Missing["reason"] | Missing["reason", data]

Represents missing data. Lookup and key access give Missing["KeyAbsent", k] for an absent key; KeyUnion and JoinAcross fill gaps with Missing[...]. Test with MissingQ, remove with DeleteMissing.

Examples (6)

Every input below was run against the current Mathilda build and its output recorded.

Basic examples (3)

In[1]:= <|"a" -> 1|>["q"]
Out[1]= Missing["KeyAbsent", "q"]

In[2]:= Missing["NotAvailable"]
Out[2]= Missing["NotAvailable"]

In[3]:= DeleteMissing[{1, Missing["NotAvailable"], 3}]
Out[3]= {1, 3}

Applications (3)

An inert marker: stays as written

In[4]:= Missing["KeyAbsent", x]
Out[4]= Missing["KeyAbsent", x]

An absent key produces one

In[5]:= Lookup[<|a -> 1|>, b]
Out[5]= Missing["KeyAbsent", b]

Test with MissingQ

In[6]:= MissingQ[Missing["Unknown"]]
Out[6]= True

Implementation notes

Algorithm. Missing is an inert, Protected head that represents missing data; it has no evaluation builtin, so Missing[], Missing["reason"] and Missing["reason", data] persist unchanged. It is produced by the association machinery: Lookup and key access return Missing["KeyAbsent", key] for an absent key (make_missing builds it), KeyUnion fills padded gaps with it, and JoinAcross fills unmatched rows with Missing["Unmatched"].

Data structures. A plain Expr function node with head Missing; the second argument, when present, carries the key or reason that explains the absence.

Complexity / limits. None — it is a symbolic marker. Test for it with MissingQ and drop such entries from an association with DeleteMissing; Lookup's third-argument default is the usual way to supply a value in place of one.

Attributes: Protected.

References

See also: KeyUnion, JoinAcross, MissingQ, DeleteMissing

• Source: src/assoc_ops.c
• Specification: docs/spec/builtins/data-structures.md
• Tests: tests/test_assoc_forms.c
• Tests: tests/test_assoc_ops.c
• Tests: tests/test_assoc_read.c
• Tests: tests/test_association.c

Notes & additional examples

Notes

Missing[…] is a placeholder for data that is not there, not something that evaluates. Absent-key lookups yield Missing["KeyAbsent", key], and KeyUnion / JoinAcross fill gaps with Missing[…]. Detect it with MissingQ, strip it with DeleteMissing, or avoid it entirely by giving Lookup a default.