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Re: Zero or one


On 6 Dec 2005, at 14:03, Trevor Baca wrote:

> Matching "zero or one" of something is helpful sometimes. String
> patterns have the usual ? regex, but _, __ and ___ aren't quite the
> same for function definitions and MatchQ[ ]. For these occasions,
> there's the following bit of syntactic sugar.
>
> The pattern
>
>   ___ ? (Length[{#}] == 0&)
>
> is one way to say "zero of something". Wrapped into a function, we get
> something like
>
>   optional[expr_] := expr | ___ ? (Length[{#}] == 0&);
>
> as a way of saying "zero or one of expr". Examples:
>
>   MatchQ[{1,3}, {1, optional[2], 3}]
>   True
>
>   MatchQ[{1,2, 3}, {1, optional[2], 3}]
>   True
>
>   MatchQ[{1, 2, 2, 3}, {1, optional[2], 3}]
>   False
>
>   MatchQ[{1, "foo", 3}, {1, optional[_String | _Integer], 3}]
>   True
>
>   MatchQ[{1, "foo", "bar", 3}, {1, optional[_String | _Integer], 3}]
>   False
>
> Generalizations over a sequence of expressions, or to count between
> exactly m and n repetitions of an expression, are also possible.
>
> Trevor.
>

A nice idea. It may be a little more direct to define optional by:

optional[expr_] := expr | ___? (#==Null&);

Andrzej Kozlowski


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