Re: Sum pattern
- To: mathgroup at smc.vnet.net
- Subject: [mg128296] Re: Sum pattern
- From: Alexei Boulbitch <Alexei.Boulbitch at iee.lu>
- Date: Fri, 5 Oct 2012 02:50:47 -0400 (EDT)
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Hi,
I'm trying to put together a rule whose left hand side is a sum of arbitrar=
y
length whose elements all have the same head f.
For example:
In[4]:= x = f[a1, s] + f[a2, s] + f[a3, s]
Out[4]= f[a1, s] + f[a2, s] + f[a3, s]
In[6]:= y = f[First /@ x, s]
Out[6]= f[a1 + a2 + a3, s]
which is what I want.
However when I turn this into a rule
In[7]:= z = x /. (p : Plus[__f]) -> f[First /@ p, s]
Out[7]= f[f[a1, s], s] + f[f[a2, s], s] + f[f[a3, s], s]
Why isn't z equal to y?
How can I make this rule work?
Thanks in advance,
Dave Snead
Hi, Dave,
It may be done this way:
f[Table[x[[i, 1]], {i, 1, Length[x]}] /. List -> Plus, x[[1, 2]]]
f[a1 + a2 + a3, s]
or, somewhat shorter, like this:
f[First[List @@@ x], x[[1, 2]]]
f[a1 + a2 + a3, s]
Have fun, Alexei
Alexei BOULBITCH, Dr., habil.
IEE S.A.
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