StringQ

Status: Stable

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

Description

StringQ[expr]

gives True if expr is a string, and False otherwise. The empty

Notes

string "" gives True. Called with any number of arguments other than one it leaves the expression unevaluated (StringQ::argx).

Examples (17)

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

Basic examples (4)

In[1]:= StringQ["AbC"]
Out[1]= True

In[2]:= StringQ[""]
Out[2]= True

In[3]:= StringQ[123]
Out[3]= False

In[4]:= StringQ[] StringQ::argx: StringQ called with 0 arguments; 1 argument is expected.

Scope (6)

In[5]:= MachineNumberQ[Sin[1000.]]
Out[5]= True

Overflows to +inf

In[6]:= MachineNumberQ[Exp[1000.]]
Out[6]= False

In[7]:= MachineNumberQ[-29037945.290347]
Out[7]= True

MPFR, not machine

In[8]:= MachineNumberQ[N[Pi, 30]]
Out[8]= False

In[9]:= MachineNumberQ[1.0 + 2.0 I]
Out[9]= True

Exact Gaussian integer

In[10]:= MachineNumberQ[1 + 2 I]
Out[10]= False

Applications (7)

A string

In[11]:= StringQ["AbC"]
Out[11]= True

The empty string still counts

In[12]:= StringQ[""]
Out[12]= True

A number is not a string

In[13]:= StringQ[123]
Out[13]= False

Not Listable: a list is not a string

In[14]:= StringQ[{"a", "b"}]
Out[14]= False

Map it for an element-wise test

In[15]:= StringQ /@ {"a", 1, x, "b"}
Out[15]= {True, False, False, True}

As a predicate

In[16]:= Select[{1, "a", x, "bc"}, StringQ]
Out[16]= {"a", "bc"}

ToString always returns a String

In[17]:= StringQ[ToString[42]]
Out[17]= True

Implementation notes

Algorithm. builtin_stringq (src/core.c) is a one-argument type test: it returns True exactly when res->data.function.args[0]->type == EXPR_STRING, and False for every other leaf or compound. The empty string "" is still an EXPR_STRING, so it gives True. StringQ is not Listable, so a list of strings is tested as a single object (a List is not a string) and gives False rather than threading.

Data structures. A single tag check on the argument Expr; nothing is allocated on the common path beyond the True/False symbol returned.

Complexity / limits. O(1). Any arity other than one is a malformed call shape: the builtin routes a StringQ::argx diagnostic through builtin_arg_error (so Quiet[]/Check[] see it) and leaves the call unevaluated. Attributes Protected.

Attributes: Protected.

References

See also: AtomQ, NumberQ, IntegerQ, MachineNumberQ, Complex, ExactNumberQ

• Source: src/core.c
• Specification: docs/spec/builtins/expression-information.md
• Tests: tests/test_core.c
• Tests: tests/test_graphics.c
• Tests: tests/test_ml_classify.c
• Tests: tests/test_pred_compile.c

Notes & additional examples

Notes

• StringQ[expr] is True exactly when expr is a string, including "", and False for everything else. The test is a single type-tag check.
• It is deliberately not Listable, so a list is tested as a single object. Map it for an element-wise answer.
• Any arity other than one emits StringQ::argx through the message funnel and leaves the call unevaluated. Protected.
• See also: StringMatchQ, NumberQ, MatchQ with _String.