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Re: Conditonal sum

• To: mathgroup at smc.vnet.net
• Subject: [mg50271] Re: Conditonal sum
• From: "Peter Pein" <petsie at arcor.de>
• Date: Tue, 24 Aug 2004 06:22:06 -0400 (EDT)
• References: <cgcigl$ep6$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

"Christopher Grinde" <christopher.grinde at hive.no> schrieb im Newsbeitrag
news:cgcigl$ep6$1 at smc.vnet.net...
> I am new to mathematica and could really need a tip on how to implement a
> conditional sum.  Pseudocode would be something like this
>
>
> EndSum=a;
> if j in Sm:
> EndSum=EndSum+f[j]
> end
>
>
> I have tried doing this by using an If-clause inside the sum-function, but
>
> this seems little elegant and is nor very suitable for symbolic
calculations.
>
> Any advice would be highly appreciated
>
> Christopher Grinde
>
Hi Christopher,

first apply f to each element of Sm:
 f/@ Sm
then change the head of the result from List to Plus, which results in:
Plus @@ (f /@ Sm), or - in most cases faster:
Tr[f/@Sm]

Peter

-- 
Peter Pein, Berlin
to write to me, start the subject with [