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Re: annoying Mathematica 3.0 behavior

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  • Subject: [mg10778] Re: annoying Mathematica 3.0 behavior
  • From: (Stonewall Ballard)
  • Date: Thu, 5 Feb 1998 00:58:24 -0500
  • Organization: StoneSoft, Inc.
  • References: <6b3p4h$>

In article <6b3p4h$>, wrote:

> 2) Is there an elegant way for a default (or optional) argument to a
> function to take a value which depends on another argument to the same
> function if it's not explicitly passed? e.g. in "foo" I would like to
> have "a" default to Length[lst].
>   foo[lst_List, a_Integer : Length[lst_]] := lst[[a]]
> but foo[{5,6,7,8}] returns "List", which implies that it's defaulting
> "a" to 0. I've tried definding an optional argument with
>   Options[foo] = {a -> Length[lst_]}
> with no luck....Obviously, I can pass a : Infinity and set it
> appropriately within the functions' body after a quick check, but it
> doesn't seem like an elegant solution.

I was unable to figure out definitively from the doc when the default
value is evaluated, but it appears to be evaluated at the time the
function foo is defined.

I would propose using a pair of functions:

foo[lst_List, a_Integer] := lst[[a]] foo[lst_List] := foo[lst,

This strikes me as "elegant".

  - Stoney

Stonewall Ballard                    StoneSoft, Inc. Remove ".nospam"
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