Re: Testing the Head of List Elements

• To: mathgroup at smc.vnet.net
• Subject: [mg25240] Re: Testing the Head of List Elements
• From: "Allan Hayes" <hay at haystack.demon.co.uk>
• Date: Sun, 17 Sep 2000 04:47:18 -0400 (EDT)
• References: <8psf4k\$9n1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Marshall,

Cases[{"str", 1, -2,  1.2, 1 + 2 I, 2/3, A, aB, AA}, _Complex]

{1 + 2 \[ImaginaryI]}

Try the following in  place of  _Complex.

_String, _Integer, _Real, _Rational, _Symbol

Also we can use builtin tests, Positive, Negative or NonNegative and those
ending in Q. Please look these up individually on in the Help Browser.

?*Q

ArgumentCountQ       MatrixQ
AtomQ                MemberQ
DigitQ               NameQ
EllipticNomeQ        NumberQ
EvenQ                NumericQ
ExactNumberQ         OddQ
FreeQ                OptionQ
HypergeometricPFQ    OrderedQ
InexactNumberQ       PartitionsQ
IntegerQ             PolynomialQ
IntervalMemberQ      PrimeQ
InverseEllipticNomeQ SameQ
LegendreQ            StringMatchQ
LetterQ              StringQ
ListQ                UnsameQ
LowerCaseQ           UpperCaseQ
MachineNumberQ       ValueQ
MatchLocalNameQ      VectorQ
MatchQ

Cases[{"str", "a", 1, -2, Pi,  1.2, 1 + 2 I, 2/3, A, aB, AA}, _?NumericQ]

\!\({1, \(-2\), 1.2`, 1 + 2\ \[ImaginaryI], 2\/3}\)

For characters we can use

Cases[{"str", "a", 1, -2, Pi,  1.2, 1 + 2 I, 2/3, A, aB, AA},
x_String /; StringLength[x] == 1]

{"a"}

--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565

"Marshall Bartlett" <mgb_news at hotmail.com> wrote in message
news:8psf4k\$9n1 at smc.vnet.net...
> I'm something of a newbie with Mathematica; please excuse my ignorance.
>
> I am looking for a clever way to select elements from a list based on
their
> type (Head).  Specifically, I want to extract all the elements of type
> dtring from a list of elements of different types (string, number,
> character, etc.).
>
> Any suggestions?
>
> --
> --------------------------------------------------------------
> Marshall Bartlett
> Department of Geology and Geophysics
> University of Utah
> mgb_news at hotmail.com
> --------------------------------------------------------------
>
>
>

```

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