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MathGroup Archive 1999

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Re: Options for a function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18148] Re: [mg18116] Options for a function
  • From: BobHanlon at aol.com
  • Date: Fri, 18 Jun 1999 00:51:47 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Virgil,

You need to use SameQ (===) instead of Equal (==)

x == y

x == y

It is unevaluated because the answer is unknown; whereas,

x === y

False

For the If statement this results in

If[x == y, a, b, c]

c

If[x === y, a, b, c]

b

Options[g] = {Opt1 -> BoxCar};
g[n_, opts___?OptionQ] :=
   Module[{opt1, ans},
  opt1 = Opt1 /. {opts} /. Options[g];
  If[opt1 === BoxCar, ans = yes, ans = no];
  {n, ans, opt1}
 ];

g[3]

{3, yes, BoxCar}

g[3, Opt1 -> TEST]

{3, no, TEST}


Bob Hanlon

In a message dated 6/17/99 3:31:59 PM, virgil.stokes at neuro.ki.se writes:

>The following simple function for testing
>options does not work the way I expect
>(Vers. 3.0.1).
>
>
>Options[g] = {Opt1->BoxCar};
>g[n_,opts___?OptionQ] :=
>   Module[{opt1,ans},
>  opt1 = Opt1 /. {opts} /. Options[g];
>  If[opt1 == BoxCar, ans = yes, ans = no];
>  {n,ans,opt1}
> ];
>
>If I use
>
>  g[3]
>
>then it returns
>
>{3,yes,BoxCar}
>
>which is of course correct. However, if I use
>
>g[3,Opt1->TEST]
>
>then it returns
>
>{3,ans$7,TEST}
>
>And, this is not what I would expect. In fact, I am
>unable to get the
>
>   ans = no
>
>part of the If statement to execute. Why?
>


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