MissingQ¶

Status: Stable

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

Description¶

MissingQ[expr]

Gives True if expr has head Missing (Missing[], Missing["reason"], Missing["KeyAbsent", k], ...), and False otherwise.

Examples (6)¶

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

Basic examples (3)¶

In[1]:= MissingQ[Missing["KeyAbsent", "z"]]
Out[1]= True

In[2]:= MissingQ[<|"a" -> 1|>["z"]]
Out[2]= True

In[3]:= Select[{1, Missing[], 3}, Not @* MissingQ]
Out[3]= {1, 3}

Applications (3)¶

In[4]:= MissingQ[Missing[]]
Out[4]= True

The absent-key result is Missing

In[5]:= MissingQ[Lookup[<|a -> 1|>, b]]
Out[5]= True

In[6]:= MissingQ[5]
Out[6]= False

Implementation notes¶

Algorithm. builtin_missingq returns True exactly when its single argument has head Missing — head_is(arg, SYM_Missing) — for any arity (Missing[], Missing["reason"], Missing["reason", data]). A call with other than one argument emits MissingQ::argx and is left unevaluated.

Data structures. None beyond the head comparison against the interned SYM_Missing.

Complexity / limits. O(1). It is purely a head test, so it says nothing about why the value is missing; pair it with DeleteMissing to remove such entries, or with Lookup's default to replace them.

Attributes: Protected.

References¶

See also: Missing

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

Notes & additional examples¶

Notes¶

MissingQ is a plain head test: True for any Missing[…] expression and False otherwise. It is the standard guard after a Lookup or key access that may not find its key.