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MemberQ

Status: Stable

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

Description

MemberQ[list, form] returns True if an element of list matches form, and False otherwise.
MemberQ[list, form, levelspec] tests all parts of list specified by levelspec.
MemberQ[form] represents an operator form of MemberQ that can be applied to an expression.

Examples

All examples below are verified against the current Mathilda build.

In[1]:= MemberQ[{1, 3, 4, 1, 2}, 2]
Out[1]= True

In[2]:= MemberQ[{x^2, y^2, x^3}, x^_]
Out[2]= True

In[3]:= MemberQ[{{1, 1, 3, 0}, {2, 1, 2, 2}}, 0, 2]
Out[3]= True

In[4]:= MemberQ[{{1, 1, 3, 0}, {2, 1, 2, 2}}, 0]
Out[4]= False

Implementation notes

builtin_memberq (src/patterns.c) returns True/False for whether any part of the first argument matches the second (a pattern), tested by do_member_at_level which applies the pattern matcher match at each position within the level spec (default level 1 — immediate elements). It supports integer/{min,max}/All/Infinity level specs and the Heads option; the one-argument form returns an operator Function[MemberQ[#1, patt]].

  • Protected.
  • Default option: Heads -> False.
  • form can be a structural pattern.
  • The first argument of MemberQ can have any head, not necessarily List.
  • Returns immediately upon finding the first match.
  • Standard level specifications are supported. The default value for levelspec in MemberQ is {1}.

Attributes: Protected.

Implementation status

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

References

Notes & additional examples

Worked examples

In[1]:= MemberQ[{1, 2, 3}, 2]
Out[1]= True

In[1]:= MemberQ[{1, 2, 3}, _Integer]
Out[1]= True

In[1]:= MemberQ[{x^2, y^3, z}, _^_]
Out[1]= True

In[1]:= MemberQ[{{1, 2}, {3, 4}}, 3, {2}]
Out[1]= True

In[1]:= MemberQ[#, 0] & /@ {{1, 2}, {0, 3}}
Out[1]= {False, True}

Notes

MemberQ[list, form] tests whether any element of list matches form. The second argument is a full pattern, not just a literal value — MemberQ[list, _Integer] checks for any integer and MemberQ[{x^2, y^3, z}, _^_] finds the first element stored as a Power. The optional level specification controls how deep to look: MemberQ[{{1, 2}, {3, 4}}, 3, {2}] searches at level 2 (inside the sub-lists) and so detects the 3, whereas the default level 1 would not. Because MemberQ is an ordinary predicate, it maps cleanly over collections, as in the last example which flags which rows contain a zero.