Problems with Outer
- To: mathgroup at smc.vnet.net
- Subject: [mg2873] Problems with Outer
- From: "Ross Moore <ross at zeus.mpce.mq.edu.au>" <ross at zeus.mpce.mq.edu.au>
- Date: Tue, 2 Jan 1996 03:33:22 -0500
Hi there.
Can someone please explain the following behaviour.
I want to create the outer product of some lists, whose
elements themselves may be lists.
The obvious function to use is Outer .
(Zeus2.2.2) In[1]:= ?Outer
Outer[f, list1, list2, ...] gives the
generalized outer product of the listi.
This works fine...
(Zeus2.2.2) In[2]:= Outer[{#1,#2}&
, {aa[a,a,a],bb[b,b,b]}
, {cc[c,c,c],dd[d,d,d]}]//MatrixForm
(Zeus2.2.2) Out[2]//MatrixForm=
aa[a, a, a] aa[a, a, a]
cc[c, c, c] dd[d, d, d]
bb[b, b, b] bb[b, b, b]
cc[c, c, c] dd[d, d, d]
But when the heads of the elements are List , the result
is quite different...
(Zeus2.2.2) In[3]:= Outer[{#1,#2}&
, {List[a,a,a],List[b,b,b]}
, {List[c,c,c],List[d,d,d]}]//MatrixForm
(Zeus2.2.2) Out[3]//MatrixForm=
a a a a a a a a a
c c c c c c c c c
a a a a a a a a a
d d d d d d d d d
b b b b b b b b b
c c c c c c c c c
b b b b b b b b b
d d d d d d d d d
Without the MatrixForm there is a mess of extra bracketings.
Where do they come from, and why?
Of course I can get the answer I want using those
extraneous heads...
(Zeus2.2.2) In[4]:=
%1/.{aa->List,bb->List,cc->List,dd->List}//MatrixForm
(Zeus2.2.2) Out[4]//MatrixForm=
a a a a a a
c c c d d d
b b b b b b
c c c d d d
... or in a 1-liner ...
(Zeus2.2.2) In[9]:= Outer[{#1,#2}&
, aa@@#&/@{{a,a,a},{b,b,b}}
, aa@@#&/@{{c,c,c},{d,d,d}}]/.{aa->List}//MatrixForm
(Zeus2.2.2) Out[9]//MatrixForm=
a a a a a a
c c c d d d
b b b b b b
c c c d d d
... but it seems overly cumbersome to have to do it this way.
Explanations please?
...or alternative simple workarounds that you may already use
to overcome this bug(?).
Regards,
Ross
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Ross Moore Internet: ross at mpce.mq.edu.au
Mathematics Department Work: +61 2 850-8955
Macquarie University Home: please do not try
North Ryde, Sydney Fax: +61 2 850-8114
Australia 2109 http://www.mpce.mq.edu.au/~ross/
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