       Re: Matching Or expressions

• To: mathgroup at smc.vnet.net
• Subject: [mg72259] Re: [mg72250] Matching Or expressions
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Sun, 17 Dec 2006 06:19:59 -0500 (EST)
• References: <200612161018.FAA12216@smc.vnet.net>

```On 16 Dec 2006, at 19:18, Johan Grönqvist wrote:

> The context of my problem is explained below.
>
> I wish to distinguish expressions of the form And[p,q] from those
> of the
> form Or[p,q], but the only thing I find is
> MatchQ[myExpr, Or[_]]
> and to my surprise I see that MatchQ[And[p,q],Or[_]] is True. Thus
> I can
> not use this to match only the Or-expressions.
>
> Thanks
>
> Johan
>
> The context is as follows:
>
> I wish to find the number of solutions to a  diophantine equation
> with a
> finite number of solutions.
>
> I do the following
> sols = Reduce[eqns, vars, Integers]
> and this gives me an expression that normally looks like Or[_,_,_] if
> there are several solutions, or something else if there are not.
>
> Now I wish to find the number of solutions using
> Which[MatchQ[sols, Or[_]], Length[sols], DoOtherStuff]
>
> The problem is that the MatchQ[ expr, Or[_] ] also matches e.g.
> (x==2 &&
> y==4), which is only one solution.
>

Note that:

Or[_]
_

So  MatchQ[And[p,q],Or[_]] evaluates to MatchQ[And[p,q],_] and, not
surprisingly, this in turn evaluates to True. All you need to do is:

MatchQ[And[p,q],HoldPattern[Or[_]]]

False