Re: Intersection[...,SameTest] ?
- To: mathgroup at smc.vnet.net
- Subject: [mg33068] Re: Intersection[...,SameTest] ?
- From: "Avraham Shinnar" <as1619 at columbia.edu>
- Date: Fri, 1 Mar 2002 06:51:10 -0500 (EST)
- Organization: Columbia University
- References: <a5htcj$hsk$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Apparently you have to test in both directions:
Intersection[a, b, SameTest -> ((#1 == 2*#2 || #2 == 2*#1 ) &)]
similarly,
a = {1, 2, 3, 4};
b = {2, 4, 7, 8};
Intersection[a, b, SameTest ->
((Print[#1, "=?=", #2] || True)
&& (#1 == 2*#2 || 2*#1 == #2)
&& (Print[True] || True) &)
which outputs (correctly)
1\[InvisibleSpace]"=?="\[InvisibleSpace]2
True
2\[InvisibleSpace]"=?="\[InvisibleSpace]2
2\[InvisibleSpace]"=?="\[InvisibleSpace]4
True
3\[InvisibleSpace]"=?="\[InvisibleSpace]2
3\[InvisibleSpace]"=?="\[InvisibleSpace]4
3\[InvisibleSpace]"=?="\[InvisibleSpace]7
3\[InvisibleSpace]"=?="\[InvisibleSpace]8
4\[InvisibleSpace]"=?="\[InvisibleSpace]2
True
Out[6]=
{1, 2, 4}
-- Avraham
"Konstantin L Kouptsov" <klk206 at is5.nyu.edu> wrote in message
news:a5htcj$hsk$1 at smc.vnet.net...
>
> Intersection[..., SameTest->test] seem to do just that. However:
>
> 1. It is not clear from the manual (book or help browser), what this
command
> is supposed to return.
>
> 2. It simply does not work:
>
> In[83]:=
> a={1,2,3,4};
> b={2,4,7,8};
> Intersection[a,b,SameTest->(#1==2*#2&)]
>
> Out[83]=
> {1}
>
> or more sophisticated:
>
> In[85]:=
> a={1,2,3,4};
> b={2,4,7,8};
> Intersection[a,b,SameTest->
> ((Print[#1,"=?=",#2]||True)
> &==2*#2
> &&(Print[True]||True)&)
> ]
>
> 2 =?= 1
> True
> 3 =?= 1
> 4 =?= 3
> 2 =?= 1
> True
> 4 =?= 3
> 4 =?= 4
>
> Out[85]=
> {1}
>
> which does not seem to enumerate all pairs.
>
> What idea stands behind this function? what is supposed to do?
>
> (well, for my task I sure could run double For[] cycle, but I am
> curious about Intersection[])
>