Re: Question about thread
- Subject: [mg2642] Re: Question about thread
- From: wagner at bullwinkle.cs.Colorado.EDU (Dave Wagner)
- Date: Thu, 30 Nov 1995 21:02:40 -0500
- Approved: usenet@wri.com
- Distribution: local
- Newsgroups: wri.mathgroup
- Organization: Wolfram Research, Inc.
In article <49do38$8a6 at dragonfly.wri.com>,
Scott Herod <sherod at boussinesq.Colorado.EDU> wrote:
>I have a question about the way Thread behaves. From the description
>in the _Mathematica_ book, I would expect it to return lists. For
>example
>
>In[1]:= Thread[f[{a,b},x]]
>
>Out[1]= {f[a, x], f[b, x]}
>
>
>But that doesn't always seem to be the case.
>
>In[2]:= blat[p_, q_] := Module[{}, Plus @@ (Variables[p]^q)]
>
>In[3]:= blat[a+b,x]
>
> x x
>Out[3]= a + b
>
>In[4]:= Thread[blat[{a+c,b+d},x]]
>
> x x x x
>Out[4]= a + b + c + d
The problem you are having is that blat[{a+c,b+d}, x] is evalauting
before Thread does. Try this:
(Local) In[99]:=
Thread[Unevaluated[blat[{a+b, c+d}, x]]]
(Local) Out[99]=
x x x x
{a + b , c + d }
Or you could change blat to do some type-checking (e.g.,
blat[p_?PolynomailQ,...]).
Dave Wagner
Principia Consulting
(303) 786-8371
dbwagner at princon.com
http://www.princon.com/princon