Re: alternating sums
- To: mathgroup at christensen.cybernetics.net
- Subject: [mg317] Re: [mg307] alternating sums
- From: Jerry Keiper <keiper>
- Date: Mon, 12 Dec 1994 09:36:12 -0600
> I often have occasion to convert mylist = {a1,a2,a3,...}
> into an alternating sum, a1-a2+a3-a4+... .
One way to do it is to do
In[9]:= {a1, a2, a3, a4, a5, a6, a7}
Out[9]= {a1, a2, a3, a4, a5, a6, a7}
In[10]:= Fold[(#2-#1)&, 0, Reverse[%]]
Out[10]= a1 - a2 + a3 - a4 + a5 - a6 + a7
The Reverse is necessary to ensure that it works correctly with both
even length and odd length lists.
Jerry B. Keiper
keiper at wri.com
Wolfram Reearch, Inc.