Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
1999
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 1999

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Multiple sum with iterators that cannot equal

  • To: mathgroup at smc.vnet.net
  • Subject: [mg19407] Re: [mg19365] Multiple sum with iterators that cannot equal
  • From: "David Park" <djmp at earthlink.net>
  • Date: Mon, 23 Aug 1999 13:57:18 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Eric,

Try this...

Sum[If[i != j, f[i, j], 0], {i, 1, 5}, {j, 1, 5}]

f[1, 2] + f[1, 3] + f[1, 4] + f[1, 5] + f[2, 1] + f[2, 3] + f[2, 4] + 
  f[2, 5] + f[3, 1] + f[3, 2] + f[3, 4] + f[3, 5] + f[4, 1] + f[4, 2] + 
  f[4, 3] + f[4, 5] + f[5, 1] + f[5, 2] + f[5, 3] + f[5, 4]

A second approach which is much faster if f[i,j] had defined numeric values is

mat = Array[f, {5, 5}];
Plus @@ Flatten[mat] - Tr[mat]

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/


>Using  multiple summmation signs with condition 'i "not
>equal" j', evaluation stops immediately as 'i' starts as
>1 as does 'j'.  My formula requires that the 'i's' and
>'j's' together:
>
> Sum[Sum[f,{j,1,n}],{i,1,n}] ignore cases where i = j.
>
> Or as copied from my notebook:
>
>\!\(\(\(\[Sum]\+\(i = 1\)\%n\[Sum]\+\(j = 1\)\%n\)\+\(i
>!= j\)\) \(w\_i\) 
>    w\_j\)
>
>How do I solve this without some cumbersome loops.
>
>Many Thanks
>
>Eric Spahr
>Cottage Technology rspahr at worldnet.att.net
>
>






  • Prev by Date: Re: Characteristic Polynomials and Eigenvalues
  • Next by Date: Re: Multiple sum with iterators that cannot equal
  • Previous by thread: Re: Multiple sum with iterators that cannot equal
  • Next by thread: Re: Multiple sum with iterators that cannot equal