Re: OneIdentity
- To: mathgroup at smc.vnet.net
- Subject: [mg103601] Re: [mg103478] OneIdentity
- From: "Kurt TeKolste" <tekolste at fastmail.us>
- Date: Wed, 30 Sep 2009 04:58:24 -0400 (EDT)
- References: <200909240352.XAA13458@smc.vnet.net>
You have misread the definition of OneIdentity. OneIdentity declares
key to be what mathematicians call "idempotent," i.e. you get the
answer after one application and no change occurs with repeated
application. The usual example of an idempotent operator is projection
in linear algebra: the result of projecting to the x-axis a projection
to the x-axis is the same
xProjection[xProjection[{x,y}]] = xProjection[{x,y}] = {x,0}
Note, however that
xProjectionn[{x,y}] == {x,y} is false in general
On Wed, 23 Sep 2009 23:52 -0400, "Chris Osborn" <chrisosb at gmail.com>
wrote:
> I do not understand why I get "False" instead of "True" here:
>
>
> In[773]:= ClearAll[key];
> SetAttributes[key, OneIdentity];
>
> MatchQ[13, key[_]]
>
> Out[775]= False
>
>
> According to the docs:
>
> OneIdentity
> is an attribute that can be assigned to a symbol f to indicate that f
> [x], f[f[x]], etc. are all equivalent to x for the purpose of pattern
> matching.
>
>
> Thanks in advance,
> Chris
>
Regards,
Kurt Tekolste
- References:
- OneIdentity
- From: Chris Osborn <chrisosb@gmail.com>
- OneIdentity