Re: Conditonal sum
- To: mathgroup at smc.vnet.net
- Subject: [mg50309] Re: [mg50263] Conditonal sum
- From: "David Annetts" <davidannetts at ihug.com.au>
- Date: Thu, 26 Aug 2004 06:51:01 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
Hi Christopher,
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
--------------------
I suggest looking at Select, then applying Plus to the results of Select.
mylist = Table[Random[Real, {-5, 5}], {100}];
Plus @@ Select[mylist, (# > 0 &)] (* Sum of +ve numbers *)
Plus @@ Select[mylist, (-2 < # < 2) &] (* Sum of numbers in range *)
Another way might be through NestList. Again, we can apply Plus to the
resulting list.
For example, the code below sums the first 8 powers of 2 ...
Plus @@ NestList[2 # &, 1, 8]
The downside to Select (& NestList, as well as a lot of other functional
programming) is that it is often less intuitive than the Fortran (or C++)
approach of doing things in a loop. The upside, as far as Mathematica is
concerned, is that the functional approach is often much faster.
Regards,
Dave.