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 [