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MathGroup Archive 2004

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Re: Using "Sum" (i = 1 ... N) in a function definition

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49724] Re: Using "Sum" (i = 1 ... N) in a function definition
  • From: Bill Rowe <readnewsciv at earthlink.net>
  • Date: Thu, 29 Jul 2004 07:45:03 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

On 7/27/04 at 7:00 AM, wilhelm.rainer at gmx.net (Rainer) wrote:

>I was trying to define a function using the Sum command in the
>following way (simplified example):

> MyData = {3,2,4,5,6};
> MyFunc[i_] = Sum[MyData[[j]],{j,1,i}];

>As you can see, the idea is to use the length of the list (i) as a
>variable in the function definition, such that

> MyFunc[1] = 3
> MyFunc[2] = 3 + 2 = 5
> MyFunc[3] = 3 + 2 + 4 = 9

>etc. Mathematica does not like the abobe statements, and prints the
>following error message:

>"Part specification K$394 is neither an integer nor a list of
>integers."

>Does anybody know how to define (correctly) such a function. Should
>be possible ...? Thanks Rainer

Rather than use Sum I would define MyFunc as follows

MyFunc[n_]:=Total@Take[MyData, n]  (* using version 5.0 *)

or

MyFunc[n_]:=Tr@Take[MyData, n]

or

MyFunc[n_]:=Plus@@Take[MyData, n]
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