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Re: Constructing Expressions from List Elements
*Subject*: [mg2227] Re: [mg2143] Constructing Expressions from List Elements
*From*: wagner at goober.cs.colorado.edu (Dave Wagner)
*Date*: Tue, 17 Oct 1995 06:34:45 GMT
*Approved*: usenet@wri.com
*Distribution*: local
*Newsgroups*: wri.mathgroup
*Organization*: University of Colorado, Boulder
*Sender*: daemon at wri.com ( )
In article <45d02k$5tc at ralph.vnet.net>,
Lou Talman <me at talmanl.mscd.edu> wrote:
>In[1]:=
> q = ##&
>Out[1]=
> ##1 &
>In[2]:=
> limits = {{x, 0, 1}, {y, -Infinity, Infinity}, {z, 0, 1}}
>Out[3]=
> {{x, 0, 1}, {y, -Infinity, Infinity}, {z, 0, 1}}
>In[4]:=
> Integrate[f[x, y, z], Apply[q, limits]]
>Out[5]=
> Integrate[f[x, y, z], {x, 0, 1}, {y, -Infinity, Infinity}, {z, 0, 1}]
>
>--Lou Talman
>
What Lou is doing is taking a List and turning it into a Sequence.
There's a more direct way to do this:
Integrate[f[x, y, z], Sequence @@ limits]
Another nice use of this trick is when you have a list of subscripts that
you want to use as a multiple-level index into a list:
s[[Sequence @@ subscripts]]
Note that s[[subscripts]] behaves quite differently.
You can think of Sequence@@ as an operator that obliterates the
head of the expression it is applied to. Actually the head List
is replaced by Sequence, which subsequently is flattened.
Dave Wagner
Principia Consulting
(303) 786-8371
dbwagner at princon.com
http://www.princon.com/princon
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