Re: Curiosity concerning transformation rules for List

• To: mathgroup at smc.vnet.net
• Subject: [mg70925] Re: Curiosity concerning transformation rules for List
• From: "dimitris" <dimmechan at yahoo.com>
• Date: Wed, 1 Nov 2006 03:56:42 -0500 (EST)
• References: <ei4ktg\$dbf\$1@smc.vnet.net>

```Why don't define a rule like below?

myrule = {x_, x_} -> {};

lst = Table[Random[Integer, {0, 5}], {20}, {2}]
{{1, 4}, {2, 1}, {1, 1}, {4, 3}, {1, 1}, {3, 2}, {5, 1}, {0, 1}, {4,
0}, {3, 4}, {1, 2}, {4, 4}, {3, 0}, {5, 2}, {0, 1},{4, 0}, {5, 1}, {2,
3}, {3, 0}, {5, 2}}

lst /. myrule

{{1, 4}, {2, 1}, {}, {4, 3}, {}, {3, 2}, {5, 1}, {0, 1}, {4, 0}, {3,
4}, {1, 2}, {}, {3, 0}, {5, 2}, {0, 1}, {4, 0}, {5, 1}, {2, 3}, {3, 0},
{5, 2}}

Or use ReplacePart?

repEmpty[lis_List] := ReplacePart[lis, {}, Position[lis, {x_, x_}]]

repEmpty[lst]
{{1, 4}, {2, 1}, {}, {4, 3}, {}, {3, 2}, {5, 1}, {0, 1}, {4, 0}, {3,
4}, {1, 2}, {}, {3, 0}, {5, 2}, {0, 1}, {4, 0}, {5, 1}, {2, 3}, {3, 0},
{5, 2}}

Regards
Dimitris

Andrew Moylan wrote:
> Since the List symbol is locked, I am curious about the possibility (or
> otherwise) of giving definitions for which the left-hand-side of the
> transformation rule contains only the List symbol. Here's an arbitrary,
> explicit example:
>
> Is it possible to make a definition such that: any list of two
> identical elements evaluates to the empty list? E.g. {x_, x_} -> {}.
>
> I can't see any way this transformation rule can be added. It's not
> possible to modify the DownValues for List; and there are no
> first-level symbols to which an UpValue can be added. Does anyone have
> any ideas?

```

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